7.a. Upper Pliensbachian – lower Toarcian

Organic carbon isotope and TOC_WR curves for the upper Pliensbachian – middle Toarcian of the Dove’s Nest core have been described by Trabucho-Alexandre et al. (Reference Trabucho-Alexandre, Gröcke, Atar, Herringshaw and Jarvis2022, fig. 2). These data are reviewed here to provide a stratigraphic framework and context for the elemental chemostratigraphy.

Trabucho-Alexandre et al. (Reference Trabucho-Alexandre, Gröcke, Atar, Herringshaw and Jarvis2022) demonstrated that the carbon isotope and TOC profiles, when combined with lithostratigraphy, provide a sound basis for the correlation of the Dove’s Nest core with the Yorkshire coastal succession (Fig. 2). The data from the Dove’s Nest core are fully compatible with previous high-resolution studies from the Yorkshire coast, confirm a consistent regional chemostratigraphy, and offer new higher-resolution reference profiles for the upper Pliensbachian A. margaritatus – P. spinatum zones and for the lower middle Toarcian H. bifrons Zone.

Carbon isotopes and organic carbon display large amplitude variation through the study interval at Dove’s Nest: −31.5‰ to −24.5‰ δ¹³C_org and 0.2 – 9.2% TOC_WR, respectively (Fig. 2). Employing the standard Yorkshire coast lithostratigraphy and biostratigraphy (Simms et al., Reference Simms, Chidlaw, Page, Morton and Gallois2004b; Hesselbo & King, Reference Hesselbo, King and Lord2019), key features of the long-term trends, from bottom to top, are:

1. (1) The upper Staithes Sandstone Formation and Cleveland Ironstone Penny Nab Member (A. margaritatus Zone) show a somewhat erratic plateau in the δ¹³C_org profile, with values of −25.7 ± 0.5‰ (Fig. 2). Maximum values for the section of ∼ −25‰ occur at the top of the Penny Nab Member in the A. gibbosus Subzone. This maximum corresponds to the level of the Late Pliensbachian Event (LPlE) positive CIE (Korte & Hesselbo, Reference Korte and Hesselbo2011; De Lena et al., Reference De Lena, Taylor, Guex, Bartolini, Adatte, van Acken, Spangenberg, Samankassou, Vennemann and Schaltegger2019; Cramer & Jarvis, Reference Cramer, Jarvis, Gradstein, Ogg and Ogg2020; Hollaar et al., Reference Hollaar, Hesselbo, Deconinck, Damaschke, Ullmann, Jiang and Belcher2023; Zhang et al., Reference Zhang, Kemp, Thibault, Jelby, Li, Huang, Sui, Wang, Liu and Jia2023), although the δ¹³C_org peak at Dove’s Nest is small and likely truncated by the disconformity at the base of the Pecten Seam. A much larger positive excursion of δ¹³C_wood values (>4‰) has been recorded from the upper A. margitatus Zone of Robin Hood’s Bay (Korte & Hesselbo, Reference Korte and Hesselbo2011). TOC_WR contents show a stepped rise from 0.7 ± 0.3% in the Staithes Sandstone to 1.7 ± 0.6% at the top of the Penny Nab.

2. (2) Carbon isotope values decline from −24.8‰ to −26.9‰ δ¹³C_org through the Kettleness Member (P. spinatum Zone) and a marked double negative excursion with values falling to −29.6‰ spans the Cleveland Ironstone – Whitby Mudstone formation boundary. This is the PlToBE negative CIE (Littler et al., Reference Littler, Hesselbo and Jenkyns2010; Fantasia et al., Reference Fantasia, Adatte, Spangenberg, Font, Duarte and Föllmi2019; Ruebsam et al., Reference Ruebsam, Mayer and Schwark2019; Cramer & Jarvis, Reference Cramer, Jarvis, Gradstein, Ogg and Ogg2020; Al-Suwaidi et al., Reference Al-Suwaidi, Ruhl, Jenkyns, Damborenea, Manceñido, Condon, Angelozzi, Kamo, Storm, Riccardi and Hesselbo2022; Ullmann et al., Reference Ullmann, Szȕcs, Jiang, Hudson and Hesselbo2022; Bodin et al., Reference Bodin, Fantasia, Krencker, Nebsbjerg, Christiansen and Andrieu2023). TOC_WR contents display a concave pattern (Fig. 2), falling from 1.8% at the base to a minimum of 0.6% in the middle of the Kettleness Member and then rising back to 1.5% at the top.

3. (3) δ¹³C_org values plateau at 25.8 ± 0.3‰ in the Grey Shale Member (upper P. paltum – lower D. tenuicostatum subzones) above the PlToBE, accompanied by a TOC_WR rise from 1.2% to 2.2%.

4. (4) A sharp change in the geochemical profiles occurs with the appearance of black laminated carbonaceous mudstones (‘black shales’) in the middle of ‘bed’ 31 in the mid-D. semicelatum Subzone (Fig. 2). This marks the base of a broad large negative δ¹³C_org excursion in the upper Grey Shale and Jet Rock beds of the lower Mulgrave Shale Member, with values falling to −31.5‰, accompanied by a large TOC_WR peak and values up to 9.2% (Fig. 2). This is an expression of the T-OAE (Section 3), which terminates at the top of the C. exaratum Subzone.

5. (5) δ¹³C_org values rise and plateau at −27.4 ± 0.4‰ in the Bituminous Shales (H. falciferum Subzone), continuing upward into the lower Alum Shale (H. bifrons Zone) and are accompanied by generally falling TOC_WR from 3.9% to 1.8%. A coincident small, stepped rise in δ¹³C_org and fall in TOC_WR occurs in the upper Bituminous Shales above ‘bed’ 44 (Peak Stones) in the mid-H. falciferum Subzone (Fig. 2).

6. (6) An interval of elevated TOC values bounded at the bottom and top by peaks of up to 4% occurs between ‘beds’ 48 (Ovatum Band) and 50, the interval of the Hard Shales (lower D. commune Subzone) at the base of the Alum Shale Member.

The most prominent features of the chemostratigraphy, therefore, are the presence of two intervals with negative δ¹³C_org excursions (Fig. 2): the PlToBE and the T-OAE, the latter accompanied by a thick interval of laminated carbonaceous mudstones with high TOC contents. Additionally, a large step down to lower δ¹³C_org and step up to higher TOC contents in the long-term profiles occur at the base of the T-OAE interval. It is notable that δ¹³C_org values below the T-OAE are consistently ∼1‰ higher than the average Phanerozoic black shale value of −27‰ (Meyers, Reference Meyers2014), while the succession above yields marginally lower values than this average.

TOC_WR values in the lower section lie close to an average shale value of 0.8% (Law, Reference Law, Beaumont and Foster1999), while those above are significantly higher, at >2% (characteristic of a very good petroleum source rock). Average black shale TOC_WR values of 3% (Vine & Tourtelot, Reference Vine and Tourtelot1970) are exceeded in the three laminated mudstones of the Sulphur Bands (SB1 – 3) in the Grey Shale (Fig. 2), throughout the upper Grey Shales to mid-Mulgrave Shale, and at the bottom and top of the Hard Shales.

7.b. Pliensbachian – Toarcian boundary

The amplitude of the PlToBE negative CIE approaches 4‰ δ¹³C_org at Dove’s Nest (Fig. 2). The Sulphur Band (SB1) is not obvious in the core, probably due to the lack of weathering. However, a distinctive bioturbated layer with abundant Chondrites burrows at ∼185 m is similar to that developed above the Sulphur Band in the top of ‘bed’ 43 at Kettleness and Runswick Bay (Trabucho-Alexandre et al., Reference Trabucho-Alexandre, Gröcke, Atar, Herringshaw and Jarvis2022, fig. 4). The Pliensbachian–Toarcian boundary is therefore placed below the Chondrites interval, at 185.5 m in the core. This placement of the boundary is consistent with the δ¹³C_org record (Fig. 2): a large negative excursion is developed below the boundary (minimum −29.6‰ at 185.75 m) and a second negative excursion above it (minimum −27.8‰ at 184.41 m).

The δ¹³C_org minimum identified at Dove’s Nest occurs in an Fe-rich (12% Fe₂O₃) calcareous (8.9% CaCO₃^e) sample at 185.75 m that is only moderately enriched in TOC_WR (1.5%). Coincident enrichment in Fe, Mg, Ca and Mn indicates the presence of siderite. Comparison to the succession at Hawsker Bottoms (Littler et al., Reference Littler, Hesselbo and Jenkyns2010, fig. 2) suggests that our sampling missed the Sulphur Band in the core, while sampling the sideritic ironstone a short distance below (Hawsker Bottoms ’bed’ 42), or the successions differ.

7.c. Toarcian Oceanic Anoxic Event (T-OAE)

The thickness (9.5 m) of the lower Toarcian δ¹³C_org negative excursion constituting the T-OAE in the Dove’s Nest core, which occurs between 175.1 and 165.2 m with δ¹³C_org values of −31.5‰ to −25.8‰ (Fig. 2), is very similar to that in the curves of Hesselbo et al. (Reference Hesselbo, Gröcke, Jenkyns, Bjerrum, Farrimond, Bell and Green2000) and DB Kemp et al. (Reference Kemp, Coe, Cohen and Schwark2005), which were derived from the lower Toarcian outcrops at Hawsker Bottoms and Port Mulgrave (=mid-‘bed’ 31 to top ‘bed’ 38) on the Yorkshire coast. The shape of the excursion, which extends from the mid-D. semicelatum to upper C. exaratum subzones and the δ¹³C_org values, are most similar to those of Hesselbo et al. (Reference Hesselbo, Gröcke, Jenkyns, Bjerrum, Farrimond, Bell and Green2000); values reported by DB Kemp et al. (Reference Kemp, Coe, Cohen and Schwark2005) are more ¹³C-depleted, down to −33.2‰.

The Dove’s Nest δ¹³C_org profile is overlain on the Yorkshire coast composite curve and correlated to a compilation of published upper Pliensbachian – middle Toarcian carbon isotope data (Oliveira et al., Reference Oliveira, Rodriguez, Duarte and Lemos2006; Hesselbo et al., Reference Hesselbo, Jenkyns, Duarte and Oliveira2007; Hermoso et al., Reference Hermoso, Le Callonnec, Minoletti, Renard and Hesselbo2009a, Reference Hermoso, Minoletti, Le Callonnec, Jenkyns, Hesselbo, Rickaby, Renard, de Rafelis and Emmanuel2009b, Reference Hermoso, Minoletti, Rickaby, Hesselbo, Baudin and Jenkyns2012, Reference Hermoso, Minoletti and Pellenard2013; Silva et al., Reference Silva, Duarte, Comas-Rengifo, Mendonça Filho and Azerêdo2011; Peti et al., Reference Peti, Thibault, Clemence, Korte, Dommergues, Bougeault, Pellenard, Jelby and Ullmann2017; Xu et al., Reference Xu, Ruhl, Jenkyns, Leng, Huggett, Minisini, Ullmann, Riding, Weijers, Storm, Percival, Tosca, Idiz, Tegelaar and Hesselbo2018; Fantasia et al., Reference Fantasia, Adatte, Spangenberg, Font, Duarte and Föllmi2019; Storm et al., Reference Storm, Hesselbo, Jenkyns, Ruhl, Ullmann, Xu, Leng, Riding and Gorbanenko2020; Ullmann et al., Reference Ullmann, Szȕcs, Jiang, Hudson and Hesselbo2022) for three well-characterized European sections in Figure 3. This exemplifies how the PlToBE and T-OAE offer robust tie points for correlation that complement the biostratigraphy.

The δ¹³C_org excursion interval at Dove’s Nest corresponds to a large TOC_WR peak with values of up to 9.2% (Fig. 2), which extends from the base of the T-OAE interval to the top of the Jet Rock at 164.0 m, the top calcareous beds of which (Top Jet Dogger and Millstones, ‘beds’ 39, 40) exhibit small δ¹³C_org and TOC_WR peaks.

8.a. Unit I Staithes Sandstone and Cleveland Ironstone

The lowest beds sampled during this study are assigned to the uppermost Staithes Sandstone Formation (A. stokesi Subzone, Fig. 2; Powell, Reference Powell1984, Reference Powell2010; Howard, Reference Howard1985; Hesselbo & Jenkyns, Reference Hesselbo, Jenkyns and Taylor1995; Hesselbo & King, Reference Hesselbo, King and Lord2019). In the coastal sections, the formation consists of bioturbated fossiliferous, fine- to medium-grained, micaceous, calcareous sandstones and sandy siltstones. At outcrop, colours range from blue-grey when fresh to yellow-brown when weathered. Thicker sandstones are commonly cross-bedded, including hummocky cross-stratification (HCS). Thinner sandstone sheets have erosional bases and fine-upward to mudstone. Parallel lamination, low-angle cross-lamination and wave-ripple lamination are common in the sandstones where not destroyed by bioturbation. Sideritic and calcitic concretions are present in the more argillaceous beds. The sandstones were deposited from sediment-laden, storm-surge and ebb currents in inner- and mid-shelf settings (Powell, Reference Powell2010). The formation is 28.6 m thick in the type section at Staithes and 25.7 m at Hawsker Bottoms (Howard, Reference Howard1985).

The Staithes Sandstone is interpreted to have accumulated predominantly in a shoreface environment above storm wave-base, likely 25 – 30 m depth in an epeiric setting (Walker & Plint, Reference Walker, Plint, Walker and James1992; Immenhauser, Reference Immenhauser2009), followed by long-term deepening through the Cleveland Ironstone, which was deposited mostly below storm wave-base (Van Buchem & Knox, Reference Van Buchem, Knox, Graciansky, Hardenbol, Jacquin and Vail1998). Following Howard (Reference Howard1985), the base of the Cleveland Ironstone Formation at Hawsker Bottoms is placed at the base of ‘bed’ 17 of Howarth (Reference Howarth1955), where hard ferruginous and calcareous sandstone (‘bed’ 16) passes up into mudstone (‘bed’ 17). This lies towards the summit of the A. stokesi Subzone (A. margaritatus Zone) of the lowest upper Pliensbachian (Hesselbo & Jenkyns, Reference Hesselbo, Jenkyns and Taylor1995).

Blue-grey coarse mudstones and very fine sandstones predominate from the base of the Dove’s Nest core study interval at 223.64 m. Cross-lamination and lenticular bedding are common. A lithological change to very thin – thin beds of grey medium- to coarse-grained mudstone at 209.7 m is correlated to the base of ‘bed’ 17 at Hawsker Bottoms (Fig. 2), marking the base of the Cleveland Ironstone Formation (Howard, Reference Howard1985). The Staithes Sandstone in the core contains more mud than at the coast and the sandstones are finer and thinner; the medium to thick HCS beds of the coastal exposures are absent. By comparison to the coast, the 13.5 m thick section studied at Dove’s Nest represents the upper half of the formation.

8.b. Unit II Whitby Mudstone lower – middle Grey Shale

The Grey Shale (Powell, Reference Powell1984) is the lowest member of the Whitby Mudstone Formation in Yorkshire (Fig. 2), which is the type area for the D. tenuicostatum Zone, the lowest zone of the Toarcian (Howarth, Reference Howarth1973; Hesselbo et al., Reference Hesselbo, Ogg, Ruhl, Hinnov, Huang, Gradstein, Ogg, Schmitz and Ogg2020b). Hawsker Bottoms ‘bed’ 43 contains the Pliensbachian – Toarcian boundary at the base of the Sulphur Band (SB1), 33 cm above the base of the Grey Shale (= base of Kettleness ‘bed’ 25). The member consists of 13.6 m of grey mudstone containing 15 rows of calcareous concretions. The lower beds include micaceous mudstones and ripple-laminated coarse-grained mudstones indicative of current activity, but these pass up into laminated mudstones in the upper part of the member. Evidence for storm-influenced deposition includes normally graded layers with silt lags interpreted as tempestites, as well as ripples, gutter casts and HCS (Kemp et al., Reference Kemp, Fraser and Izumi2018), indicative of deposition above storm wave-base and likely < 30 m water depth (Section 8a; cf. Walker & Plint, Reference Walker, Plint, Walker and James1992; Immenhauser, Reference Immenhauser2009).

The mudstones of the Grey Shale in the Dove’s Nest core are similar to those of the Cleveland Ironstone Formation, but they are a much darker grey and are the least fossiliferous interval of the studied section. By contrast, the Cleveland Ironstone and the Bituminous Shales are the two most fossiliferous intervals albeit with very different assemblages.

The intervals containing the Sulphur Band marker bed (SB1) and overlying Sulphur Band 2 (SB2), with the twin δ¹³C_org minima of the Pliensbachian – Toarcian Boundary Event, are clearly picked out by TOC, Fe₂O₃, Fe/Al (Figs 4, 5, S1) and S (not determined here, but well displayed in the coastal succession (e.g. Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018, fig. 8) peaks.

Figure 8. Box plot summary of stratigraphic trends in major- and trace-element contents and Al-ratio data of chemostratigraphic units through the upper Pliensbachian – middle Toarcian of the Dove’s Nest core. Boxes represent 25–75% quartiles with median values shown by the vertical line inside the box. Whiskers are drawn from the top of the box up to the largest data point less than 1.5 times the box height from the box (the ‘upper inner fence’) and similarly below the box (Hammer et al., Reference Hammer, Harper and Ryan2001). Outlier values outside the inner fences are shown as circles, values further than 3 times the box height from the box (the ‘outer fences’) are shown as stars.

8.c. Unit III (T-OAE) Whitby Mudstone, Grey Shale – Mulgrave Shale

Excellent agreement exists between the high-resolution chemostratigraphic records published for the interval of the T-OAE on the Yorkshire coast (Figs 6, 7; Hesselbo et al., Reference Hesselbo, Gröcke, Jenkyns, Bjerrum, Farrimond, Bell and Green2000; DB Kemp et al., Reference Kemp, Coe, Cohen and Schwark2005, Reference Kemp, Coe, Cohen and Weedon2011; Ruvalcaba Baroni et al., Reference Ruvalcaba Baroni, Pohl, van Helmond, Papadomanolaki, Coe, Cohen, van de Schootbrugge, Donnadieu and Slomp2018; Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018; Wang, Reference Wang2022) and the new data from Dove’s Nest (Fig. 7).

8.d. Unit IV Whitby Mudstone, middle Mulgrave Shale

The Bituminous Shales, ‘beds’ 41 – 48, which constitute the middle and upper portions of the Mulgrave Shale Member, overlie the Jet Rock and are 22.9 m thick at Saltwick Nab (Howarth, Reference Howarth1962). These mainly laminated grey carbonaceous mudstones, contain few carbonate concretions but more abundant pyrite and crushed pyritized fossils than the underlying Jet Rock. Belemnites and fossilized wood are common, the latter including logs up to 3.7 m long at the outcrop. In the Dove’s Nest core, the base of the Bituminous Shales comprises thin beds of coarser mudstone with fossiliferous bedding planes characterized by monospecific assemblages of Pseudomytiloides dubius (Sowerby). Cross-lamination reappears in these mudstones (Trabucho-Alexandre et al., Reference Trabucho-Alexandre, Gröcke, Atar, Herringshaw and Jarvis2022), which include abrupt alternations of thin beds of fine and coarse mudstone (striped beds).

Three beds in the Bituminous Shales provide markers in the coastal sections (e.g. Rawson & Wright, Reference Rawson and Wright2018): ‘bed’ 42, a row of scattered oval doggers with pyritic skins and Pseudomytiloides; ‘bed’ 44, a row of scattered doggers with pyritic aggregates; ‘bed’ 46, a red-weathering sideritic mudstone (Fig. 2). A distinctive 36 cm interval comprising a double row of red-weathering pyritic doggers at the top of the H. falciferum Subzone (‘bed’ 48) yields a unique ammonite fauna that includes Ovaticeras ovatum (Young and Bird). Following Simms et al. (Reference Simms, Chidlaw, Page, Morton and Gallois2004b), we place this ‘bed’, the Ovatum Band, at the top of the Bituminous Shales, although Howarth (Reference Howarth1962) listed it separately and did not assign it to a specific member. The Bituminous Shales (including the Ovatum Band) lie entirely within the H. falciferum Subzone, H. serpentinum Zone (Hesselbo & Jenkyns, Reference Hesselbo, Jenkyns and Taylor1995).

Geochemical profiles (e.g. Figs 2, S1) demonstrate that a major facies change occurs at ‘bed’ 44. A sharp step fall in TOC_WR occurs above this bed, from 3.4 ± 0.5% below to 2.4 ± 0.3% above (Fig. 2), with coincident marked falls documented for redox-sensitive trace elements in the coastal sections, including total S, Cd, Co, Mo and Mn (McArthur et al., Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008; McArthur, Reference McArthur2019). The boundary between chemostratigraphic Units IV and V based on this major redox-proxy shift (following Remírez & Algeo, Reference Remírez and Algeo2020), is placed at the base of ‘bed’ 45. A more subtle compositional change occurs within the lower part of the Bituminous Shales around the level of ‘bed’ 42, which marks a change from falling TOC_WR values below, to a plateau above (Fig. 2) and coincident breakpoints occur in most redox-sensitive trace-element profiles (McArthur et al., Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008; McArthur, Reference McArthur2019). We subdivide these beds into chemostratigraphic Subunits IVa below and IVb above.

8.e. Unit V Whitby Mudstone, Mulgrave Shale – Alum Shale

The chemostratigraphic boundary between Units IV and V at ‘bed’ 44 (Peak Stones) within the Bituminous Shales previously observed on the Yorkshire coast (Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018; McArthur, Reference McArthur2019; Remírez & Algeo, Reference Remírez and Algeo2020) is clearly developed in the Dove’s Nest core above 151.9 m, which is correlated to ‘bed’ 44.

8.f. Elemental chemostratigraphy overview

The general characteristics of the chemostratigraphic units described above are summarized in Figure 8, showing log-scaled box plots of the elemental and Al-ratio data for all major and trace elements determined in the Dove’s Nest core. Elements showing an association with the coarser detrital fraction (e.g. Si, Ti, Zr) or phyllosilicates (e.g. K, Rb, Sc, Th) commonly show significantly different trends for the Al-ratio data, while those associated strongly with organic matter content and sediment redox (TOC, P, Mn, Mo, U, V) are less affected by normalization. The figure illustrates the geochemical integrity of the different units.

9.a. Pearson r correlation

Inter-element relationships were assessed using Pearson correlation of the high-resolution sample set, and the low-resolution samples with additional trace-element data. Very similar coefficients were obtained for elements present in both sample suites. Results for the low-resolution samples are provided in Supplementary Material Table S3. Correlations between major elements are attributed to their partitioning in the dominant minerals in the rocks. Mineralogical analyses were not performed as part of the present study, but the interpretation is informed by comparison to published mineralogical data from equivalent intervals on the Yorkshire coast (e.g. Gad et al., Reference Gad, Catt and Le Riche1969; Pye & Krinsley, Reference Pye and Krinsley1986). Quartz and clay mineral contents for 21 samples from the Dove’s Nest core were provided by de Vos (Reference de Vos2017).

High correlation coefficients (r >0.85) are observed between Al₂O₃ and other aluminosilicate-associated elements TiO₂, K₂O, Cs and Rb (Table S3). The highest correlation coefficients (r > 0.9) are between K₂O, Rb and Cs, typically associated with phyllosilicates, especially illite (e.g. González López et al., Reference González López, Bauluz, Yuste, Mayayo and Fernández-Nieto2005) and mica. Lower correlation coefficients for Al₂O₃ (r = 0.5 – 0.8) with SiO₂, Cr, Sc and Th indicate additional mineralogical controls (e.g. quartz, heavy minerals) on these elements. SiO₂ shows the highest correlation with TiO₂ (r = 0.8) and lower values (r = 0.5 – 0.8) for Al₂O₃ Na₂O, Cr, Sc, Th and Zr, an element suite likely associated with quartz, Na-plagioclase and heavy minerals (e.g. rutile, ilmenite, titanite and zircon) in a coarser-grained terrigenous clastic fraction.

CaO, principally controlled by carbonate, predictably has a high negative correlation (r = < −0.8) with most of the above elements (Table S3) but shows modest positive correlations with MnO and P₂O₅ (r = 0.52) attributable to Mn substitution in calcite and the presence of carbonate fluorapatite. Fe₂O₃ is positively correlated only with MnO and MgO (r = ≥0.6) and V (r = 0.4) reflecting its disparate mineral associations that include siderite but also Fe–Mn oxyhydroxides, pyrite and berthierine.

9.b. PCA results

Loading coefficients of the different variables of the variance-covariance matrix for the first three principal components of the PCA for selected Dove’s Nest data (96 samples with a full element suite) are listed in Supplementary Material Table S4. The first two principal components together account for 71.3% of the variance in the data. PC1 (48.4% of the variance) shows positive loadings for siliciclastic mineral-associated (quartz, feldspars, phyllosilicates, heavy minerals) elements, SiO₂, TiO₂, Al₂O₃, Na₂O, K₂O, Cs, Cr, Rb, Th, Zr and a large negative loading for carbonate (CaO). This negative loading highlights the significance of carbonate impacting the geochemical composition of samples and that this is not solely an artefact of the closed-sum nature of the original dataset. Further discrimination between samples is offered by PC2 (22.9% of the variance) with large negative values for Mo* and TOC_WR but positive loadings for MnO and Fe₂O₃, amongst others. PC3 accounts for a further 9.8% of the variance and is distinguished particularly by its high positive loading for Mo*.

A biplot of the first two principal components (Fig. 9) illustrates clear inter-element associations that can be interpreted as reflecting mineralogical controls on the bulk geochemistry of the rocks. It further demonstrates how these relate to the stratigraphic units comprising the upper Pliensbachian – middle Toarcian succession in the Cleveland Basin.

Figure 9. Principal component analysis biplot of PC1 vs PC2 for geochemical data from the upper Pliensbachian – middle Toarcian of the Dove’s Nest core. Compositional data for samples (n = 96) having a full major- and trace-element dataset were transformed using a Centre Log-Ratio to remove closure effects prior to PCA. The first two principal components account for 48.4% and 22.9% of the variance, respectively (Table S4).

The close proximity of vectors for individual constituents on the biplot indicates associations between (1) SiO₂, Na₂O, Zr and V, together with Cr, Sc and Th, representing siliciclastic detritus; (2) Al₂O₃, K₂O, Rb and Cs is a clay-mineral association; (3) Fe₂O₃ and Mn reflect enrichment in a Fe–Mn component – likely including siderite, goethite, other Fe–Mn oxyhydroxides and berthierine; (4) CaO and P₂O₅ represent carbonate cements and phosphate; (5) TOC_WR, Mo* and U are associated with organic matter. It is notable that TiO₂ falls between the main siliciclastic and clay-mineral association vectors, MgO and Y lie between the siliciclastic and Fe–Mn associations and Sr plots between carbonate cements and organic matter. This pattern indicates the presence of multiple mineralogical and/or geochemical controls on these elements.

The distribution of samples on the biplot (Fig. 9) shows good separation of the stratigraphic units, reflecting the changing proportions of mineralogical constituents and geochemical environments through the succession. Outliers are caused principally by Fe – Mn enrichment in ironstone intervals within the Staithes Sandstone and Cleveland Ironstone, and extreme carbonate cementation of individual samples in the Jet Rock (Whale Stones and Top Jet Dogger). It is worth noting that the Hard Shales (Subunit Vb) cluster with samples displaying strong organic matter enrichment (upper Grey Shales – lower Bituminous Shales, Units II and IV).

11.a. Clay minerals

Gad et al. (Reference Gad, Catt and Le Riche1969) reported that clay fractions in the Whitby Mudstone contain approximately equal amounts of kaolinite, mica (illite) and swelling clays (vermiculite and/or smectite) with minor chlorite (<5%). The silt fraction includes clay-mineral aggregates. Houben et al. (Reference Houben, Barnhoorn, Lie-A-Fat, Ravestein, Peach and Drury2016a, b) recorded 31 – 54% illite–smectite, 17 – 27% illite, 26 – 40% kaolinite and 3 – 4% chlorite in four samples from the formation.

The clay mineralogy of the 21 samples from the Dove’s Nest core was determined by de Vos (Reference de Vos2017). Quantitative clay mineral analysis employing the XRD peak-area integration with peak-height ratio method showed kaolinite (27 ± 5%), illite (48 ± 4%) and vermiculite (25 ± 5%), with a dominance of irregularly interstratified illite–vermiculite (I/V), to be the principal components of the clay fraction (<4 μm). Muscovite and chlorite are present in minor amounts, the latter only in the lower part of the succession below 175 m (Staithes Sandstone – Grey Shale; Units I and II).

The results of de Vos (Reference de Vos2017) are broadly consistent with earlier clay mineral studies of Pliensbachian – Toarcian sedimentary rocks in the Cleveland Basin by Morris (Reference Morris1980), Sellwood & Sladen (Reference Sellwood and Sladen1981), Jeans (Reference Jeans2006) and Houben et al. (Reference Houben, Barnhoorn, Lie-A-Fat, Ravestein, Peach and Drury2016a, b). Those authors emphasized the dominance of illite and kaolinite with mixed-layer clays, although they did not specify the presence of a I/V phase. Kemp & McKervey (Reference Kemp and McKervey2001) and SJ Kemp et al. (Reference Kemp, Merriman and Bouch2005) reported illite–smectite as the interstratified species but acknowledged difficulties in the interpretation of the XRD spectra. Precise comparison between the studies is hampered by differences in analytical methodology and clay mineral terminology.

Most authors have regarded the Lias Group clay mineral assemblages in the Cleveland Basin as being principally detrital (Sellwood & Sladen, Reference Sellwood and Sladen1981; Jeans, Reference Jeans2006; Dera et al., Reference Dera, Pellenard, Neige, Deconinck, Pucéat and Dommergues2009) despite a maximum burial depth of 3 – 4 km and temperatures of 100 – 120° C (SJ Kemp et al., Reference Kemp, Merriman and Bouch2005), although evidence exists for the development of some authigenic kaolinite (Pye & Krinsley, Reference Pye and Krinsley1986).

The relatively high abundance of kaolinite in the Upper Lias throughout northern Europe suggests warm temperatures and near-constant annual humid conditions on adjacent landmasses (Singer, Reference Singer1984; Chamley, Reference Chamley1989; Fagel, Reference Fagel, Hillaire-Marcel and De Vernal2007; Dera et al., Reference Dera, Pellenard, Neige, Deconinck, Pucéat and Dommergues2009). Local factors may also have played a role. For example, in Scotland, the sand and clay mineralogy of Jurassic sedimentary rocks suggests that they were derived from erosion of Devonian and Carboniferous strata (Hurst, Reference Hurst1985; Hall & Bishop, Reference Hall, Bishop, Doré, Cartwright, Stoker, Turner and White2002). The high detrital kaolinite content of Rhaetian to Sinemurian mudstones in northeast Scotland has been attributed to the reworking of Carboniferous regoliths, which were uplifted and eroded at the time of formation of the Viking Graben (Hurst, Reference Hurst1985).

The kaolinite content in the Whitby Mudstone of the Dove’s Nest core (de Vos, Reference de Vos2017) is lower (21 – 30%) than in the underlying Staithes Sandstone and Cleveland Ironstone (24 – 40%) which might imply a transition to cooler and dryer climate. However, this is contrary to trends elsewhere, including at Mochras in the Cardigan Bay Basin (Fig. 1a), that show widespread kaolinite enrichment in the lower Toarcian (Raucsik & Varga, Reference Raucsik and Varga2008; Dera et al., Reference Dera, Pellenard, Neige, Deconinck, Pucéat and Dommergues2009; Brański, Reference Brański2012; Hermoso & Pellenard, Reference Hermoso and Pellenard2014; Xu et al., Reference Xu, Ruhl, Jenkyns, Leng, Huggett, Minisini, Ullmann, Riding, Weijers, Storm, Percival, Tosca, Idiz, Tegelaar and Hesselbo2018) and palaeotemperature records (Section 18) that indicate generally cool climate in the late Pliensbachian, a short-lived hyperthermal during the T-OAE, with persistence of warm climate conditions thereafter (Suan et al., Reference Suan, Mattioli, Pittet, Lecuyer, Sucheras-Marx, Duarte, Philippe, Reggiani and Martineau2010; Korte et al., Reference Korte, Hesselbo, Ullmann, Dietl, Ruhl, Schweigert and Thibault2015; Bougeault et al., Reference Bougeault, Pellenard, Deconinck, Hesselbo, Dommergues, Bruneau, Cocquerez, Laffont, Huret and Thibault2017; Ruebsam & Schwark, Reference Ruebsam, Schwark, Reolid, Duarte, Mattioli and Ruebsam2021).

The lower kaolinite content in the lower Toarcian at Dove’s Nest may reflect clay mineral fractionation with increasing distance to shore (Gibbs, Reference Gibbs1977; Godet et al., Reference Godet, Bodin, Adatte and Föllmi2008; Deconinck et al., Reference Deconinck, Hesselbo and Pellenard2019), driven by relative sea-level rise accompanying the early Toarcian transgression (Hallam, Reference Hallam1997, Reference Hallam2001; Hesselbo, Reference Hesselbo2008) and the impact of extreme seasonal aridity during the T-OAE (Slater et al., Reference Slater, Twitchett, Danise and Vajda2019). Similarly, a pulse of kaolinite at the base of the Kettleness Member (Fig. 10) with falling values above, may reflect nearer-shore conditions associated with the depositional hiatus that resulted in the regional disconformity at the base of the P. spinatum Zone, followed by sea-level rise. However, changes in sediment provenance cannot be discounted as a mechanism to explain the mineralogical trends.

Illite abundance is associated with weak weathering intensities and increased levels of mechanical weathering, implying terrestrial erosion (Singer, Reference Singer1984). Illite contents at Dove’s Nest, ascribed to a I/V mixed layer component (de Vos, Reference de Vos2017), are relatively constant throughout the succession except for the T-OAE interval (Subunits IIa, b), which shows a marked rise to 54 – 64% illite from an average of 46% below, before falling back to an average of 47% above the event (Fig. 10). The increase in illite in the I/V fraction has been ascribed to higher terrestrial erosion rates leading to deeper erosion of podzol soil profiles (de Vos, Reference de Vos2017). The implied elevated levels of continental erosion during the earlier phases of the T-OAE match the increase of continental weathering flux of >40% indicated by the increase in ¹⁸⁷Os/¹⁸⁸Os ratio (Section 16.b.4) from ∼0.4 to ∼1.0 in the top Grey Shale and Jet Rock on the coast (Fig. 10; Cohen et al., Reference Cohen, Coe, Harding and Schwark2004), and coincident increases in the Mochras borehole (Percival et al., Reference Percival, Cohen, Davies, Dickson, Hesselbo, Jenkyns, Leng, Mather, Storm and Xu2016), in western North America (Them et al., Reference Them, Gill, Selby, Gröcke, Friedman and Owens2017b) and in Japan (Kemp et al., Reference Kemp, Selby and Izumi2020).

11.b. Detrital proxies in the silt fraction

Mineralogical controls on bulk-rock geochemistry may be investigated using element-ratio Pearson correlations. Detrital proxy element ratios Si/Al, Ti/Al, Na/Al and Zr/Al show high positive correlations with each other of r = ≥0.62 (p = 0.0001), most >0.75 and are negatively correlated to K/Al (r = <−0.46, p = 0.0001). Of these, Ti/Al has the highest correlation with mean grain size (r = 0.73) and particularly with the medium silt fraction (r = 0.76), although all other proxies show similar relationships.

Figure 11 shows example element-ratio cross-plots of (a) Si/Al vs Ti/Al; (b) Na/Al vs K/Al; and (c) Ti/Al vs quartz mean grain size for Dove’s Nest. The geochemical units and their constituent lithostratigraphic divisions display discrete clusters. Si/Al in the Staithes Sandstone (Subunit Ia) shows the greatest scatter (Fig. 11a) attributable to large sample-to-sample differences in quartz abundance in the coarser fractions. The Cleveland Ironstone (Subunits Ib, Ic) shows a tight positive array on the Si/Al vs Ti/Al plot attributable to a close association between Ti-bearing heavy minerals (rutile, anatase) and quartz in a changing silt fraction. Claystones (Fig. 11c) of the Bituminous Shales (Unit IV, Subunit Va) display low and constant Si/Al and Na/Al ratios (Fig. 11a, b) but significant variation in Ti/Al and K/Al reflecting variable clay mineralogy.

The remarkable similarity between the Si/Al and Na/Al profiles (Fig. 4) and a high positive correlation between Si/Al and Na/Al (r = 0.93) point to Na-rich plagioclase as a dominant mineralogical control, likely present as both a primary detrital fraction and as a replacement of K-feldspar (cf. Min et al., Reference Min, Zhang, Li, Zhao, Li, Lin and Wang2019). The negative correlation between Na/Al and K/Al (Fig. 11b) is attributed to the presence of Na-plagioclase in the silt fractions and dominance of illite in the clay fraction, with scatter generated particularly by the presence of variable K-feldspar and micas. The large scatter and high values of K/Al in the mudstones from the T-OAE are particularly significant and are explained by pulses of illitic clay during the event (cf. Fig. 10).

Zr/Al shows the strongest correlation to Si/Al but a more irregular profile (Fig. 4) which may be explained by the nugget effect of zircon grains present in the small sample sizes analysed. The resulting element cross-plots (not shown) are like those of Si/Al.

11.c. Chemical Index of Alteration - CIA

The CIA (Nesbitt & Young, Reference Nesbitt and Young1982; McLennan, Reference McLennan1993) is a widely applied chemical weathering proxy and has been used in many recent T-OAE studies (e.g. Fu et al., Reference Fu, Wang, Zeng, Feng, Wang and Song2017; Fantasia et al., Reference Fantasia, Föllmi, Adatte, Bernárdez, Spangenberg and Mattioli2018a, b; Bomou et al., Reference Bomou, Suan, Schlögl, Grosjean, Suchéras-Marx, Adatte, Spangenberg, Fouché, Zacaï, Gibert, Brazier, Perrier, Vincent, Janneau, Martin, Reolid, Duarte, Mattioli and Ruebsam2021; Fu et al., Reference Fu, Wang, Wen, Song, Wang, Zeng, Feng and Wei2021; Alnazghah et al., Reference Alnazghah, Koeshidayatullah, Al-Hussaini, Amao, Song and Al-Ramadan2022; Liu et al., Reference Liu, Cao, He, Liang, Pu and Wang2022). A ternary plot of Al₂O₃ – CaO*+Na₂O – K₂O for the Dove’s Nest data is presented in Figure 12 with the corresponding CIA values. These are plotted stratigraphically in Figure 10. Due to the large variation in carbonate content between samples, particularly in the T-OAE interval (Unit III, Fig. 7), Figure 10 plots CIA values calculated using CaO* = Na₂O (blue filled circles and blue line) for all samples and using CaO* = phosphate corrected CaO (open blue circles) for samples where the remaining number of moles was less than that of Na₂O (following McLennan, Reference McLennan1993). It is recognized that the CIA profile based on the former method is identical to that for Ln(Al₂O₃/Na₂O) in molar proportions (not plotted) which is an alternative weathering proxy applicable for moderate to high carbonate samples (Von Eynatten et al., Reference Von Eynatten, Barceló-Vidal and Pawlowsky-Glahn2003; Xia & Mansour, Reference Xia and Mansour2022). The CIX proxy of Garzanti et al. (Reference Garzanti, Padoan, Setti, López-Galindo and Villa2014) that uses a modification of the CIA formula excluding CaO generates the same trend.

The CIA data for very low carbonate samples that enable the use of CaO* values for calculation generally display higher CIA values and lower amplitude change through the succession but show the same stratigraphic trends as the Na₂O substituted values (Fig. 10). The reduced amplitude is due primarily to the high Na₂O content of the upper Pliensbachian sediments (Unit I, Figs 4, S1). The long-term trend shows increasing upwards CIA values representing weak to moderate chemical weathering of <70 (Na₂O-substituted values) in the Staithes Sandstone (Subunit Ia), moderate chemical weathering ∼70 – 75 in the Cleveland Ironstone (Subunits Ib, Ic), rising to 75 – 80 in the basal Toarcian (Units II, III) including the T-OAE, reaching high and constant values of >80, indicative of intense chemical weathering, through the remainder of the lower Toarcian – middle Toarcian (Unit IV, V) section.

The CIA profile for the upper Pliensbachian displays a cyclicity that is inversely correlated to Si/Al, Ti/Al and Zr/Al and positively correlated to K/Al and Rb/Al (compare Figs 4 and 10), indicating a strong grain-size control. For comparison, Dinis et al. (Reference Dinis, Garzanti, Vermeesch and Huvi2017, fig. 3) documented a 10-unit offset to lower CIX values in river sands compared to associated river muds in their study of weathering proxies along the modern SW African margin, although parallel trends that broadly followed the regional weathering trends were observed for the two size fractions.

For Dove’s Nest, coarser-grained Pliensbachian sedimentary rocks have lower CIA values due to lower clay and higher plagioclase contents that might conceivably represent cycles of chemical weathering intensity but are attributed principally to grain-size variation linked to cyclic changes in bottom energy conditions and shoreline proximity. The sequence stratigraphy (Section 8.a.2) indicates that these were driven by changes in relative sea level (Howard, Reference Howard1985; Young et al., Reference Young, Aggett, Howard and Young1990; Macquaker & Taylor, Reference Macquaker and Taylor1996).

The general anticorrelation of CIA with Ti/Al and the other detrital proxies continues through the Toarcian at Dove’s Nest but the positive correlation to K/Al and Rb/Al is reversed in T-OAE Subunits IIIb – c where increasing K/Al ratios are linked to a pulse of illitic clay (Fig. 10). Notably, a Th/K minimum at this level is a feature of the field spectral gamma-ray stratigraphic profiles generated for the Yorkshire coast Pliensbachian – Toarcian by Myers & Wignall (Reference Myers, Wignall, Leggett and Zuffa1987) and Parkinson (Reference Parkinson1996). The latter considered Th/K to be a likely proxy for the kaolinite/illite ratio. The illite pulse correlates to a major global weathering event documented by a large increase in ¹⁸⁷Os/¹⁸⁸Os values in the coastal succession. Si/Al and K/Al profiles obtained for the T-OAE interval and adjacent beds by Thibault et al. (Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018) and Wang (Reference Wang2022) on the coast display identical trends to those seen in the core (Fig. 7) but those authors were unable to calculate CIA values due to an absence of Na₂O data.

The continental weathering pulse indicated by the positive Os excursion coincides with a negative excursion in CIA values at Dove’s Nest (Fig. 10). This is the opposite of what might be expected since increased chemical weathering would lead to higher CIA values. The T-OAE interval elsewhere is commonly characterized by a prominent increase in CIA values and other chemical weathering proxies (Fantasia et al., Reference Fantasia, Föllmi, Adatte, Bernárdez, Spangenberg and Mattioli2018a, Reference Fantasia, Adatte, Spangenberg, Font, Duarte and Föllmi2019; Bomou et al., Reference Bomou, Suan, Schlögl, Grosjean, Suchéras-Marx, Adatte, Spangenberg, Fouché, Zacaï, Gibert, Brazier, Perrier, Vincent, Janneau, Martin, Reolid, Duarte, Mattioli and Ruebsam2021; Wang, Reference Wang2022) attributable to a warmer and wetter climate during the Toarcian hyperthermal.

In the Cleveland Basin, a pulse of detrital illite combined with a short-term upward increase in sediment grain size drove falling CIA values through the δ¹³C minimum of the T-OAE (Subunit IIIb), despite evidence of an increasingly hot climate with extreme wet/dry seasonality based on terrestrial palynomorph assemblages (Slater et al., Reference Slater, Twitchett, Danise and Vajda2019). Seasonality peaked during the later phases of the T-OAE (Subunits IIIc – d), associated with a sharp increase in CIA and lower illite content (Fig. 10). A peak of charcoal abundance at this level in the Peniche and Mochras successions (Baker et al., Reference Baker, Hesselbo, Lenton, Duarte and Belcher2017) is consistent with dryer climate conditions (increased wildfires) in the European region following a wetter phase during the initial stages of the T-OAE.

The charcoal maximum correlates to a coincident peak in the relative abundance of Cerebropollenites. The proliferation of Cerebropollenites during multiple past hyperthermal events suggests that the source plants were adapted to hot, arid climates able to survive in warm, drought-like conditions (Slater et al., Reference Slater, Twitchett, Danise and Vajda2019). In the case of the Cleveland Basin, therefore, it is suggested that a wetter climate during the interval of the δ¹³C minimum led to increased detrital input that overprinted the normal chemical weathering proxies. A change from dominantly wet to dominantly dry conditions indicated by the charcoal and palynological proxy records may reflect a temporal shift in the relative importance of dry vs wet in a strongly seasonal wet/dry cycle.

11.d. Fe–Mn enrichment and carbonate cements

The Pearson correlation and PCA results for Dove’s Nest show a close association between Fe, Mn and Mg, with r values of >0.6 (Table S3) and with Ca and P falling in the same sector of negative PC1 and positive PC2 scores (Fig. 9). The relationship between Fe–Mn enrichment and carbonate may be illustrated using a Fe+Mn–Ca–Mg ternary plot (Fig. 13). As with the detrital proxy elements (Fig. 11), the chemostratigraphic units display distinctive clusters and arrays on the Fe+Mn–Ca–Mg plot. The Pliensbachian formations of Unit I form a low-Ca cluster adjacent to muscovite and kaolinite mineral analyses from the Cleveland Ironstone (Aggett, Reference Aggett1990); Ca values lower than average shale (PASS) indicate negligible carbonate in most samples. Individual analyses falling outside the cluster incorporate calcite grains and/or cements, generating an array of scattered points towards the Ca vertex (Fig. 13) or are siderite-rich, exemplified by the Avicula Seam Ironstone (sample 200.39) the bulk chemistry of which displays high Fe+Mn and falls close to the siderite analyses of Aggett (Reference Aggett1990). Generally low Si values of Fe-rich samples indicate that berthierine is not a significant mineral component in the sample suite analysed here.

Figure 12. Ternary diagram of Al₂O₃–(CaO*+Na₂O)–K₂O in Dove’s Nest rock samples. Values are molecular proportions. (a) CIA = Chemical Index of Alteration (Nesbitt & Young, Reference Nesbitt and Young1982) with CaO* moles assumed to be equivalent to Na₂O (McLennan, Reference McLennan1993; see text). Tonalite, granodiorite and granite compositions after Condie (Reference Condie1993). (b) Enlargement of plotted data in the top sector of the diagram (red outline in a) with selected sample details. Grey shading represents the field of the Subunit IIIb illite and weathering pulse with high K/Al and anomalous low CIA values.

Figure 13. Ternary plot of iron and carbonate-associated elements Fe+Mn–Ca–Mg in Dove’s Nest rock samples compared to constituent mineral compositions. Stars are electron microprobe determinations of mineral fractions from the Cleveland Ironstone of Staithes (Aggett, Reference Aggett1990). Open triangles are calculated pure mineral values (webmineral.com). Average shale (PASS) composition after Taylor & McLennan (Reference Taylor, McLennan and Meyers2001). Plot is scaled based on the maximum and minimum values of the three components (cf. de Lange et al., Reference de Lange, Jarvis and Kuijpers1987).

Unit II, the lower Grey Shale, forms a tight cluster close to average shale (Fig. 13). The carbonaceous mudstones of Units III and IV form long scattered arrays between moderate Fe+Mn–Mg contents and the Ca vertex. The very high carbonate content of the Whale Stones sample (167.89), interpreted to represent a carbonate concretion, is demonstrated by its proximity to the calcite mineral analysis (Aggett, Reference Aggett1990). Samples from Units III and IV fall along a mixing line between calcite, ankerite and kaolinite. This may be fortuitous since the scatter towards the Fe+Mn vertex is likely attributable in part to the presence of pyrite which is abundant in these units (e.g. Houben et al., Reference Houben, Barnhoorn, Wasch, Trabucho-Alexandre, Peach and Drury2016b) but was not quantified in this study. Nonetheless, ankerite has been recorded as a minor mineral phase in these intervals (Houben et al., Reference Houben, Barnhoorn, Wasch, Trabucho-Alexandre, Peach and Drury2016b). Offset of Unit III samples to higher Mg values is attributed to the presence of dolomite (cf. Pye, Reference Pye1985).

Unit V samples cluster in the same low-Ca area of the Fe+Mn–Ca–Mg plot as those from the Cleveland Ironstone (Subunits Ib, Ic) indicating a return to less Fe–Mn enrichment and generally low carbonate contents (compare Fig. 9). Samples from the Hard Shales (Subunit Vb) are variably displaced to higher Ca values (Fig. 13), reflecting the presence of calcite cements.

12.a. TOC/P_T molar ratio

The TOC/P_T ratio is a redox proxy based on the differing processes and rates of organic carbon and phosphorus remineralization under varying redox conditions (Ingall et al., Reference Ingall, Bustin and Van Cappellen1993; Van Cappellen & Ingall, Reference Van Cappellen and Ingall1994; Algeo & Ingall, Reference Algeo and Ingall2007; Algeo & Li, Reference Algeo and Li2020; Papadomanolaki et al., Reference Papadomanolaki, Lenstra, Wolthers and Slomp2022). Marine plankton biomass has a remarkably constant TOC/P molar ratio of 106:1, the Redfield ratio (Redfield et al., Reference Redfield, Ketchum, Richards and Hill1963), and lacustrine particulates have a comparable composition (Hecky et al., Reference Hecky, Campbell and Hendzel1993). In modern oxic marine bottom waters and surficial sediments, organic carbon is mostly returned to the water column during bacterial respiration, but P is trapped by adsorption onto Fe-oxyhydroxides and the precipitation of carbonate-fluorapatite or its precursor in the sediment during iron reduction (Jarvis et al., Reference Jarvis, Burnett, Nathan, Almbaydin, Attia, Castro, Flicoteaux, Hilmy, Husain, Qutawnah, Serjani and Zanin1994, fig. 3; März et al., Reference März, Riedinger, Sena and Kasten2018). These processes lead to a fall in the sediment TOC/P_T molar ratio with values typically < 50 in oxic facies (Algeo & Ingall, Reference Algeo and Ingall2007). Under anoxic conditions the preservation of organic matter is enhanced and, in the absence of Fe-oxyhydroxides, reactive P is released back to seawater leading to an increased TOC/P_T molar ratio of > 106. Modern anoxic sediments generally display ratios of 100 – 200, with values exceeding 1000 in some ancient black shales (Algeo & Ingall, Reference Algeo and Ingall2007; Papadomanolaki et al., Reference Papadomanolaki, Lenstra, Wolthers and Slomp2022).

TOC/P_T molar ratios in the upper Pliensbachian Unit I at Dove’s Nest are indicative of oxic facies with a mean value of 52 ± 26 (Fig. 14). A small upward increase in the average ratio is seen from the Staithes Sandstone to the Kettleness Member of the Cleveland Ironstone, between three Subunits Ia – c. A mean TOC/P_T molar ratio of 57 ± 24 in lower Grey Shale Unit II indicates predominantly oxic conditions with the notable exceptions of Sulphur Bands 2 and 3 that exceed 120 (SB1 was not sampled in the core but displays a TOC/P_T ratio of >200 on the coast), evidencing short-lived episodes of anoxia – euxinia.

High TOC/P_T molar ratios that consistently exceed the Redfield ratio characterize T-OAE Unit III of the top Grey Shale and Jet Rock (Fig. 14), with the highest values occurring in Subunit IIIb (230 ± 69). These indicate that significant quantities of P must have been released back to anoxic bottom waters during the peak of the T-OAE. TOC/P_T ratios fall sharply in Subunit IIId but remain higher in Units IV and V than in the pre-T-OAE section (Units I and II) indicating continuing oxygen depletion in bottom waters during the later early Toarcian and early middle Toarcian, fluctuating within the dysoxic field with a molar ratio of 85 ± 18 (Fig. 14).

An identical pattern is displayed by Yorkshire coast data but with higher TOC/P_T molar ratios recorded by the ultra-high-resolution data obtained for T-OAE Unit III (Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018) with values of 390 ± 104 for Subunit IIIb (Figs 14, 15).

Figure 15. Bottom-water redox proxy interpretation for the upper Pliensbachian – lower Toarcian of the Cleveland Basin derived from TOC, TOC/P, DOP and Fe speciation. δ¹³C_org profile for Dove’s Nest (Trabucho-Alexandre et al., Reference Trabucho-Alexandre, Gröcke, Atar, Herringshaw and Jarvis2022, black) rescaled to match coast composite data (see Fig. 2 for sources). Rescaled whole-rock TOC profile for Dove’s Nest (Trabucho-Alexandre et al., Reference Trabucho-Alexandre, Gröcke, Atar, Herringshaw and Jarvis2022, dark green) with coast composite data of Kemp et al. (Reference Kemp, Coe, Cohen and Weedon2011; thin yellow-green high-resolution curve) and trend of the low-resolution coast datasets of Ruvalcaba Baroni et al. (Reference Ruvalcaba Baroni, Pohl, van Helmond, Papadomanolaki, Coe, Cohen, van de Schootbrugge, Donnadieu and Slomp2018, open triangles) and McArthur (Reference McArthur2019) (thin pale green low-resolution curve; see Fig. 2). Average shale and black shale values as in Figure 2; ‘anoxic threshold’ of TOC_WR = 2.5 wt% follows Algeo & Maynard (Reference Algeo and Maynard2004). Low-resolution TOC/P_T (dark green) and DOP_T (dark orange) curves from McArthur et al. (Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008) and Remírez & Algeo (Reference Remírez and Algeo2020) with high-resolution data (thin pale curves) from Thibault et al. (Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018); see Figure 14 for further information. Iron speciation data from Salem (Reference Salem2013, cream-filled circles) and Houben et al. (Reference Houben, Goldberg and Slomp2021, yellow-filled circles) with redox field boundaries after Raiswell et al. (Reference Raiswell, Hardisty, Lyons, Canfield, Owens, Planavsky, Poulton and Reinhard2018). Extinction levels (i)– (iii) after Caswell et al. (Reference Caswell, Coe and Cohen2009). Pliensb. = Pliensbachian; Bitumin. Sh. = Bituminous Shales; D. semic. = Dactylioceras semicelatum; Dt = D. tenuicostatum; Dc = D. clevelandicum; P. hawk. = Pleuroceras hawskerense; Pa = P. apyrenum.

12.b. Degree of pyritization DOP_T

The ‘degree of pyritization’ (DOP, Raiswell & Berner, Reference Raiswell and Berner1985) is an established proxy for bottom-water redox conditions (Raiswell et al., Reference Raiswell, Buckley, Berner and Anderson1988, Reference Raiswell, Hardisty, Lyons, Canfield, Owens, Planavsky, Poulton and Reinhard2018). DOP values exceeding 0.5 have been suggested to provide evidence of euxinia (free H₂S in the water column) driving the addition of syngenetic pyrite precipitated in the water column to the sediment (Raiswell et al., Reference Raiswell, Newton and Wignall2001; Algeo & Maynard, Reference Algeo and Maynard2004), although more conservative threshold DOP values of <0.45 for oxic or dysoxic depositional environments and >0.75 for a euxinic environment were proposed by Raiswell et al. (Reference Raiswell, Hardisty, Lyons, Canfield, Owens, Planavsky, Poulton and Reinhard2018).

Although originally defined as DOP = pyrite iron/total reactive iron (Berner, Reference Berner1970; Raiswell et al., Reference Raiswell, Buckley, Berner and Anderson1988), values may be closely approximated using whole-rock total iron and total sulfur data. Algeo & Li (Reference Algeo and Li2020) adopted the equation:

$${\rm{DO}}{{\rm{P}}_{\rm{T}}} = {{\rm{S}}_{\rm{T}}} \times \left( {{\rm{55}}{\rm{.85/64}}{\rm{.12}}} \right){\rm{/F}}{{\rm{e}}_{\rm{T}}}$$

where total sulfur (S_T) and total iron (Fe_T) and the coefficient 55.85/64.12 represent the weight ratio of Fe/S in stoichiometric pyrite. The use of S_T and Fe_T results in the inclusion of some non-pyrite sulfur (e.g. organic sulfur) and some non-reactive iron (e.g. silicate- or carbonate-bound Fe) in the calculation of the DOP_T ratio. These amounts are typically small but, nonetheless, they may result in different values of DOP and DOP_T for a given sample (e.g. Algeo et al., Reference Algeo, Rowe, Hower, Schwark, Merrmann and Heckel2008; Algeo & Liu, Reference Algeo and Liu2020). DOP vs DOP_T correlations are formation-specific, so DOP_T palaeoredox thresholds determined in the literature cannot be applied universally without validation.

Several studies have been undertaken on the Yorkshire coastal succession that include DOP data. DOP values of 0.84 ± 0.03 were reported for the Jet Rock (C. exaratum Subzone) by Raiswell & Berner (Reference Raiswell and Berner1985) and between 0.8 and 0.9 by Pearce et al. (Reference Pearce, Cohen, Coe and Burton2008). Recent studies on the Yorkshire coastal succession have used S_T and Fe_T data, where either DOP_T = 0.95 × S_T/Fe_T, the decrease in S content incorporated to allow for the presence of organic S (McArthur et al., Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008) or simply DOP_T = S_T/Fe_T (Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018). DOP_T values calculated by McArthur et al. (Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008) and Thibault et al. (Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018) are comparable (0.84 ± 0.09 for Subunit IIIb) to published DOP results. Such high values significantly exceed the 0.75 threshold for euxinic environments (Figs 14, 15; Raiswell et al., Reference Raiswell, Hardisty, Lyons, Canfield, Owens, Planavsky, Poulton and Reinhard2018) and offer very strong evidence for long-term euxinic bottom-water conditions in the Cleveland Basin during the T-OAE (Raiswell & Berner, Reference Raiswell and Berner1985; Raiswell et al., Reference Raiswell, Bottrell, Al-Biatty and Tan1993; Wignall et al., Reference Wignall, Newton and Little2005; McArthur et al., Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008; Pearce et al., Reference Pearce, Cohen, Coe and Burton2008; Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018). Nonetheless, petrographic evidence shows levels of fine-scale bioturbation even during this interval, despite an absence of visible trace fossils (Caswell & Herringshaw, Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023).

Sulfur contents and Fe speciation were not determined in the Dove’s Nest samples. Additionally, it should be noted that the presence of significant siderite and berthierine in parts of the Yorkshire succession, particularly in the Cleveland Ironstone, will be particularly significant for raising Fe_T values (e.g. Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018, fig. 7A) and will generate lower than true DOP values in samples containing these minerals.

DOP_T profiles for the coastal succession from McArthur et al. (Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008) and Thibault et al. (Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018) are illustrated in Figures 14 and 15. The DOP_T curve morphology and redox interpretation are very similar to those provided by TOC/P_T. In both cases, largely oxic conditions with possible minor dysoxic intervals characterized the late Pliensbachian (Unit I). Oxic conditions prevailed during the earliest Toarcian but brief intervals of anoxia – euxinia first appeared at the start of the Toarcian (Sulphur Band 1) and reoccurred during the deposition of Sulphur Bands 2 and 3. A general drift towards persistent dysoxia preceded the onset of T-OAE which was marked by a rapid shift to anoxia – euxinia that prevailed in the area, with varying intensity, throughout the event.

T-OAE Unit III is the only interval where DOP_T values consistently fluctuate above the threshold of >0.75 for euxinic environments proposed by Raiswell et al. (Reference Raiswell, Hardisty, Lyons, Canfield, Owens, Planavsky, Poulton and Reinhard2018), with the highest values present in Subunits IIIb and c. A decrease in pyrite framboid mean size to ≤5 μm accompanying rising TOC and TOC/P_T trends in Subunit IIIa is indicative of a fine fraction component precipitated from a euxinic water column (Wignall & Newton, Reference Wignall and Newton1998; Wignall et al., Reference Wignall, Newton and Little2005). Song et al. (Reference Song, Littke and Weniger2017) reported >95% of <7 μm syngenetic framboidal pyrite in T-OAE carbonaceous mudstones from Runswick Bay.

The TOC/P_T and DOP_T proxies show different trends following the OAE (Fig. 14): the former indicates a rapid return to fluctuating dysoxic conditions that prevailed through the remainder of the early Toarcian into the early middle Toarcian. The DOP_T proxy implies that anoxia persisted through most of the H. falciferum Subzone (Unit IV) prior to a shift to dysoxia in late H. falciferum Subzone time (Unit V). This interval of divergence coincides with a period of weaker basin restriction indicated by Mo vs TOC proxy (Section 13a) but also occurs with a step increase in δ³⁴S_CAS values (Section 16.a.2) interpreted to reflect the impact of global pyrite burial and indicate a significant fall in oceanic sulfate content.

12.c. Iron speciation Fe_HR/Fe_T and Fe_py/Fe_HR

Bottom-water redox interpretations for the Cleveland Basin derived from DOP_T values and pyrite framboid size may be further assessed using sedimentary rock Fe-speciation data obtained from the Yorkshire coast succession by Salem (Reference Salem2013) and Houben et al. (Reference Houben, Goldberg and Slomp2021). These proxies are based on the presence or absence of enrichments in key iron minerals (Raiswell & Canfield, Reference Raiswell and Canfield1998; Raiswell et al., Reference Raiswell, Hardisty, Lyons, Canfield, Owens, Planavsky, Poulton and Reinhard2018) with data derived from chemical extracts of bulk rock samples. Highly reactive iron (Fe_HR) comprises ferric oxides, iron carbonates, magnetite and pyrite. A highly reactive iron to total iron ratio (Fe_HR/Fe_T) of below 0.38 is indicative of oxic – dysoxic bottom waters with values <0.22 unambiguously oxic. Values >0.38 provide strong evidence for anoxic depositional conditions (Fig. 15).

The extent of pyritization of the highly reactive Fe pool (Fe_py/Fe_HR) indicates whether the bottom water was anoxic (i.e. oxygen depleted and ferruginous) or euxinic (Fe-free and sulfidic), provided that the Fe_HR/Fe_T ratio is above 0.38. Fe_py/Fe_HR values of <0.7 reflect ferruginous water column conditions. The threshold between anoxic and euxinic bottom-water conditions is less well-defined but is generally placed between 0.7 and 0.8 (Fig. 15; Raiswell et al., Reference Raiswell, Hardisty, Lyons, Canfield, Owens, Planavsky, Poulton and Reinhard2018). In cases where Fe_py/Fe_HR ratios of >0.7 indicate sulfidic conditions but Fe_HR/Fe_T is <0.38 (i.e. oxic – dysoxic bottom water) H₂S production is thought to occur close to but below the sediment/water interface.

The use of Fe-speciation proxies needs to be assessed critically (Raiswell et al., Reference Raiswell, Hardisty, Lyons, Canfield, Owens, Planavsky, Poulton and Reinhard2018). For example, carbonate-rich samples may include additional Fe²⁺ incorporated into diagenetic carbonates, which may lead to an overestimation of anoxia when Fe-speciation is applied to sediments with low Fe_T or low TOC contents (<0.5%). In this study, the Fe content of upper Pliensbachian – middle Toarcian rocks from Dove’s Nest, for example, fluctuates around the average shale value of 5.1% (Fig. S1), with the lowest value of 1.7% Fe (with 3.9% TOC) obtained from the Whale Stones concretion sample, so palaeoredox interpretations should be robust.

Fe_HR/Fe_T and Fe_py/Fe_HR data from the Yorkshire coast (Fig. 15) support interpretations derived from the TOC/P_T and DOP_T profiles. Highly variable redox conditions during the earliest Toarcian are indicated by coincident high Fe_HR/Fe_T and Fe_py/Fe_HR values within the three Sulphur Bands (SB1 – 3), indicating three major episodes of bottom-water euxinia. Additionally, cm-scale sampling through the Sulphur Bands at Port Mulgrave and Hawsker Bottoms by Salem (Reference Salem2013) evidences shorter-term fluctuating palaeoredox conditions during their deposition, with cycling between euxinic and dysoxic conditions. The bioturbated mudstones below, between and above the Sulphur Bands show less enrichment of TOC, reactive iron and trace metals, with lower TOC/P_T and DOP_T values (Fig. 15), but nonetheless yield Fe_HR/Fe_T ratios that fluctuate around the Fe-proxy threshold characteristic of anoxia (0.38), equating to the dysoxic intervals of the TOC/P_T and DOP_T proxies. These observations demonstrate that short periods of anoxia – euxinia, beginning at the Pliensbachian – Toarcian boundary, preceded the T-OAE. This is consistent with thallium isotope data from western Canada that indicates that the onset of global deoxygenation of ocean water occurred at this time (Them et al., Reference Them, Gill, Caruthers, Gerhardt, Gröcke, Lyons, Marroquin, Nielsen, Alexandre and Owens2018).

Samples through T-OAE Unit III into the base of the Bituminous Shales Subunit IVa yield Fe_HR/Fe_T values of >0.38 and generally ∼0.6 (Salem, Reference Salem2013; Houben et al., Reference Houben, Goldberg and Slomp2021), which point to persisting anoxic bottom-water conditions (Fig. 15). However, short-lived oxic intervals on a seasonal or annual scale would not be resolved by the current sampling resolution. Brief oxygenation events are indicated by petrographic evidence of fine-scale bioturbation in the laminated mudstones. On the other hand, it is striking that there is a total absence of macroscopic trace fossils throughout the T-OAE in the Cleveland Basin. Elsewhere in the Toarcian of Europe, there are usually at least a few bioturbated intervals, reflecting geologically brief oxygenation events, but there is no ichnological evidence of this in the Yorkshire succession.

Fe_py/Fe_HR ratios of 0.7 to 0.8, lying at the threshold to euxinic conditions, indicate that H₂S was present in bottom waters, albeit not permanently.

12.d. Fe_EF, Mn_EF and P_EF

Profiles for Fe, Mn and P for Dove’s Nest and the Yorkshire coast successions plotted as enrichment factors show excellent agreement between the sections (Fig. 14). Fe_EF and Mn_EF display highly coherent profiles that indicate common mineralogical and environmental controls on their distributions. The two elements are strongly associated by their co-occurrence in Fe–Mn oxyhydroxides, carbonates (calcite, dolomite and siderite) and berthierine (e.g. Aggett, Reference Aggett1990). Iron has an additional association with pyrite, and this is reflected by the relatively elevated values of the Fe_EF relative to the Mn_EF profile through Subunit IIIb, the interval of high DOP_T values indicative of high pyrite contents.

Highest Fe_EF and Mn_EF values occur in (1) ferruginous bands of the Staithes Sandstone where Fe–Mn oxyhydroxides (e.g. goethite) are likely the dominant minerals; (2) ironstones in the Cleveland Ironstone associated principally with high siderite and low phyllosilicate contents; (3) the Sulphur Bands of the Grey Shale (siderite); (4) concretionary carbonates (calcite, dolomite) in the Jet Rock – the Whale Stones and Top Jet Dogger; (5) concretionary calcitic (Peak Stones, Ovatum Band) or sideritic (‘beds’ 46, 50) levels in the Bituminous Shales and Hard Shales.

With the exception of the beds noted above, Fe_EF values generally fluctuate around average shale values through Units I, II and IV, are consistently enriched relative to PASS in Unit III (average 1.5), reflecting the addition of authigenic pyrite and show a plateau of low Fe_EF values (average 0.75) through Unit V (Fig. 14). By contrast, Mn is significantly depleted throughout the succession (Mn_EF ∼0.2 – 0.3) and Mn_EF values of >1 occur only in ironstones and concretionary carbonates. The generally low Mn values are consistent with the presence of oxygen-depleted bottom waters that prevented the deposition, redox cycling and concentration of Mn in the sediments (Calvert & Pedersen, Reference Calvert and Pedersen1996) which retained Fe in carbonates, silicates and sulfides. The carbonaceous mudstones correspond to Thermodynamic Zone III (low Mn, high Fe, low V, sulfidic) facies of Quinby-Hunt & Wilde (Reference Quinby-Hunt and Wilde1994), typically associated with anoxic bottom waters.

The P_EF profiles from Dove’s Nest and the Yorkshire coast show excellent agreement (Fig. 14). A cyclic pattern is particularly apparent in the ultra-high-resolution P data of Thibault et al. (Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018) from the coast, with regular P_EF peaks of >2 and intervening troughs with a typical minimum of <0.6. These cycles coincide with opposing shifts in the TOC/P_T profile that correspond to a transition to oxic bottom waters driving P enrichment and periods of anoxic bottom water leading to P depletion. A matching pattern is observed in the lower resolution data from Dove’s Nest. This is consistent with the differing processes and rates of organic carbon and phosphorus remineralization under varying redox conditions (Section 12a) and evidences a cyclic pattern of bottom-water oxygenation in the Cleveland Basin during the early Toarcian.

Five P_EF cycles are recognized between the bases of the D. clevelandicum († symbol in Fig. 14) and H. falciferum subzones dated at 183.94 Ma and 182.06 Ma (Gradstein et al., Reference Gradstein, Ogg, Schmitz and Ogg2020), an interval of 1.88 Ma. This yields a periodicity approaching the Milankovitch ∼405 ka long eccentricity cycle. Thibault et al. (Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018) postulated a ∼405 ka eccentricity cycle for detrital input in the D. tenuicostatum – C. exaratum subzones (see Fig. 4) in the Cleveland Basin based on spectral analysis of Zr/Al data from the coast (Hawsker Bottoms), but these do not show a consistent relationship to the P_EF cycles (compare Figs 4 and 14). By contrast, taking the base D. tenuicostatum and H. bifrons zone ages of 184.20 Ma and 181.17 Ma (3.03 Ma; Gradstein et al., Reference Gradstein, Ogg, Schmitz and Ogg2020) with 13 P_EF cycles (Fig. 14) yields a 250 ka periodicity. Applying other subzone ages generates a similar range of values. There is, therefore, no evidence for a consistent periodicity with potential orbital forcing of the P_EF cycles, but it is acknowledged that changes in sedimentation rate and/or hiatuses in the section (e.g. Suan et al., Reference Suan, Pittet, Bour, Mattioli, Duarte and Mailliot2008b; McArthur et al., Reference McArthur, Steuber, Page and Landman2016) make spectral analysis challenging (Boulila et al., Reference Boulila, Galbrun, Huret, Hinnov, Rouget, Gardin and Bartolini2014, Reference Boulila, Galbrun, Sadki, Gardin and Bartolini2019; Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018).

13.a. Molybdenum

A plot of Mo vs Al of upper Pliensbachian – middle Toarcian samples from Dove’s Nest and published data from the Yorkshire coastal succession (Fig. 17a) illustrates the significant enrichment in Mo (> 10 – 60 ppm) that characterizes the lower Bituminous Shales of Unit IV and to a lesser extent (> 3 ppm) the upper Grey Shale – Jet Rock interval of T-OAE Unit III, compared to other parts of the succession (≤ 2 ppm) and to average shale (1 ppm). Elevated Mo values also occur in the organic-rich intervals of the Sulphur Bands (SB1 – 3, 3 – 25 ppm) at the base of Subunit IIa, and at the bottom and top of the Hard Shales of Subunit Vb (up to 22 ppm). The Mo vs Al plot shows the lack of a clear correlation between the two elements, suggesting no significant association of Mo with the aluminosilicate fraction of the rocks. The Pearson correlation and PCA (Tables S3, S4) results both display a weak negative correlation (r = -0.3) between the two constituents.

Figure 17. Cross-plots for key redox-sensitive trace metals. (a) Mo vs Al. Mo shows no clear relationship with Al. (b) Mo vs TOC. The steep upper regression line (small grey dots) for Units IV and V (left) derived from the Dove’s Nest data, displaying a positive correlation between Mo and TOC, contrasts to the shallow lines (small grey dots) derived for T-OAE Unit III (right – upper line and statistics is for Dove’s Nest samples, lower line is for Yorkshire coast samples of McArthur et al. Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008). Regression lines from selected modern anoxic silled basins representing increasing deep water renewal times of <10 – 650 ka (Algeo & Rowe, Reference Algeo and Rowe2012) are Mo/TOC (ppm/%) ∼45 Saanich Inlet (purple); ∼25 Cariaco Basin (green); ∼9 Framvaren Fjord (red); ∼4.5 Black Sea (blue). Bottom water restriction trends after Algeo & Lyons (Reference Algeo and Lyons2006). (c) U vs Al. Lower regression line is for Pliensbachian Subunits Ia–c; upper line is for Toarcian Units IV and V (all samples ≥3 ppm U). (d) U vs TOC. Lower regression line (left) for Subunits Ia–c; upper regression line (right) for Units IV and V. (e) V vs Al. Regression line is for Pliensbachian Subunits Ia–c, excluding the three Fe-rich flyers. (f) V vs TOC. Regression lines are for Pliensbachian Subunits Ia–c (left, Dove’s Nest) and Toarcian Units II–V (right, coast samples; Ruvalcaba Baroni et al., Reference Ruvalcaba Baroni, Pohl, van Helmond, Papadomanolaki, Coe, Cohen, van de Schootbrugge, Donnadieu and Slomp2018). TOC-based anoxic (2.5%, vertical dashed green line marking boundary between oxic and anoxic non-sulfidic conditions) and euxinic (10%, upper limit of x-axis) thresholds after Algeo & Maynard (Reference Algeo and Maynard2004). Solid symbols are from the Dove’s Nest core (this study), faded symbols are for Yorkshire coastal outcrop samples (McArthur et al., Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008; Ruvalcaba Baroni et al., Reference Ruvalcaba Baroni, Pohl, van Helmond, Papadomanolaki, Coe, Cohen, van de Schootbrugge, Donnadieu and Slomp2018; McArthur, Reference McArthur2019; Remírez & Algeo, Reference Remírez and Algeo2020). Dove’s Nest TOC data are whole-rock values. Average shale (PASS) composition after Taylor & McLennan (Reference Taylor, McLennan and Meyers2001). Note that samples with 10 – 20% TOC reported from Unit III of the coastal outcrops fall outside the plot area of (B), (D) and (F) but lie on the trends of the regression lines shown.

The Mo vs TOC plot (Fig. 17b) illustrates that high Mo contents are associated with TOC enrichment, with Mo values increasing substantially in samples containing ≥2.5% TOC, the ‘anoxic threshold’ of Algeo & Maynard (Reference Algeo and Maynard2004), but with substantial scatter. Relatively few samples exceed the ‘intermittent euxinia’ threshold of 25 ppm Mo proposed by Scott & Lyons (Reference Scott and Lyons2012), all of which occur in Unit IV (Fig. 16). Positive correlations between Mo and TOC are observed for Unit III and IV sample suites but with very different slopes. In addition to the data plotted in Figure 17, samples with 10 – 20% TOC (exceeding the threshold for euxinic bottom waters) have been recorded from Unit III of the coastal outcrop (Fig. 2). These data fall outside the plot area of Figure 17a, d and f but lie on the trend of the Unit III lower regression line shown in Figure 17b.

The redox behaviour of Mo in marine environments has been reviewed by Tribovillard et al. (Reference Tribovillard, Algeo, Lyons and Riboulleau2006), Algeo & Tribovillard (Reference Algeo and Tribovillard2009), Scott & Lyons (Reference Scott and Lyons2012), Hardisty et al. (Reference Hardisty, Lyons, Riedinger, Isson, Owens, Aller, Rye, Planavsky, Reinhard, Gill, Masterson, Asael and Johnston2018), Bennett & Canfield (Reference Bennett and Canfield2020), Them et al. (Reference Them, Owens, Marroquín, Caruthers, Trabucho-Alexandre and Gill2022) and Paul et al. (Reference Paul, van Helmond, Slomp, Jokinen, Virtasalo, Filipsson and Jilbert2023), amongst others. Molybdenum is present in seawater principally in the form of molybdate (MoO₄²⁻). Molybdenum is not concentrated by plankton and displays little affinity for the surfaces of clay minerals, CaCO₃ and, generally, Fe-oxyhydroxides (but see Scholz et al., Reference Scholz, Siebert, Dale and Frank2017) at marine pH values but may be captured by Mn-oxyhydroxides, typically at the sediment surface. Molybdenum accumulation in these deposits is very slow, but the overwhelming dominance of oxic environments in the modern oceans results in an estimated 35 – 50% of Mo being removed via adsorption onto Mn-oxyhydroxides.

Reduction of oxyhydroxides during diagenesis in organic-rich sediments liberates adsorbed Mo to pore waters. Molybdenum fixation is believed to take place principally by reduction to thiomolybdate complexes (MoO_xS_4−x , x = 0 – 3) in the presence of free H₂S and subsequent sequestration in organic matter and/or iron sulfides. Molybdenum fixation may also occur directly in a euxinic water column. Here, Mo is scavenged from seawater by settling the organic matter and sulfide particles, leading to substantial Mo enrichment (reaching 100 ppm) in the underlying organic-rich sediment. It is estimated that 50 – 65% of Mo is removed in the oceans via these processes today because even though only a very small proportion (≪1%) of the modern seafloor is overlain by euxinic bottom waters, burial efficiency is very high.

McArthur et al. (Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008) argued that the development of weak hydrographic restriction in the Cleveland Basin during the early H. falciferum Subzone (lower Bituminous Shales of Unit IV) was responsible for substantial Mo enrichment from sulfidic bottom waters. However, samples from Unit III (T-OAE) also display a significant positive correlation with TOC yet these show up to two orders of magnitude less enrichment. By analogy to modern anoxic basin environments (Algeo & Lyons, Reference Algeo and Lyons2006; Algeo & Rowe, Reference Algeo and Rowe2012), the low Mo values of the T-OAE black shales (Unit III) relative to their high TOC contents have been previously interpreted to reflect the presence of an extremely restricted water mass in the Cleveland Basin during the T-OAE (McArthur et al., Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008; McArthur, Reference McArthur2019; Remírez & Algeo, Reference Remírez and Algeo2020), with local Mo drawdown limiting uptake of the element into the sediment.

The impact of water mass restriction on the Mo/TOC (ppm/%) ratio of sediments in modern basins may be illustrated by data from weakly restricted (e.g. Saanich Inlet, ∼45) to very restricted (e.g. Black Sea, ∼4.5) silled basins (Fig. 17). It should be noted, however, that a low Mo/TOC ratio of ∼6 also characterizes sediments from strong upwelling systems, such as modern offshore Namibia (Algeo & Lyons, Reference Algeo and Lyons2006). Here, very high productivity induced by coastal upwelling leads to a high rate of TOC deposition that overwhelms the supply of Mo from upwelling water.

TOC whole-rock concentrations determined in the Dove’s Nest samples all fall below 10% (Fig. 17b) but values exceeding 15% have been recorded from the upper beds of Subunit IIIb on the Yorkshire coast (Figs 2, 7; Kemp et al., Reference Kemp, Coe, Cohen and Weedon2011; Ruvalcaba Baroni et al., Reference Ruvalcaba Baroni, Pohl, van Helmond, Papadomanolaki, Coe, Cohen, van de Schootbrugge, Donnadieu and Slomp2018; McArthur, Reference McArthur2019; Houben et al., Reference Houben, Goldberg and Slomp2021). The coast data form an array that continues the trend of the shallowest regression line plotted in Figure 17b, interpreted by McArthur et al. (Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008) to represent the most extreme water mass restriction associated with the highest TOC deposits. However, Cleveland Basin T-OAE samples define trends that show considerably greater Mo depletion (Mo/TOC 0.4 – 2, Fig. 17) than the modern Black Sea.

Thibault et al. (Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018) argued that comparable trends in Mo across the T-OAE interval in both Yorkshire and the Paris Basin (Hermoso et al., Reference Hermoso, Minoletti and Pellenard2013) suggest a similar oceanic drawdown of this element accompanying widespread anoxia in both basins. Furthermore, clear evidence of a transgressive trend in the Cleveland Basin at the time of maximum Mo drawdown (Figs 10, 16) conflicts with the basin restriction model for the euxinic conditions that characterize the T-OAE interval. Data from a deep Panthalassic Ocean site in Japan (Inuyama area; Kemp et al., Reference Kemp, Chen, Cho, Algeo, Shen and Ikeda2022a) also evidences drawdown of Mo and U accompanying the T-OAE. Here also, a marked expansion of anoxic and possibly euxinic conditions to the seafloor occurred during both the PlToBE and the T-OAE, accompanied by increased organic carbon burial, but Mo drawdown cannot be related to hydrographic restriction.

Them et al. (Reference Them, Owens, Marroquín, Caruthers, Trabucho-Alexandre and Gill2022) documented comparable low Mo/TOC values (1 – 2) through the T-OAE intervals in a transect of 3 sites from the Western Canada Sedimentary Basin, an area that is interpreted to have maintained strong connections with Panthalassa open-ocean deep waters throughout the Early Jurassic. By contrast to the Cleveland Basin and Japan sites, this area experienced relatively stable anoxic and euxinic conditions throughout late Pliensbachian – middle Toarcian (Them et al., Reference Them, Gill, Caruthers, Gerhardt, Gröcke, Lyons, Marroquin, Nielsen, Alexandre and Owens2018), and there is no geological evidence to suggest that basin restriction played any role in the observed Mo/TOC trends. Sustained anoxic/euxinic conditions at a site are essential to successfully isolate global changes in the marine Mo reservoir from local factors.

The results of Them et al. (Reference Them, Owens, Marroquín, Caruthers, Trabucho-Alexandre and Gill2022) confirm previous work (Pearce et al., Reference Pearce, Cohen, Coe and Burton2008; Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018; Kemp et al., Reference Kemp, Chen, Cho, Algeo, Shen and Ikeda2022a) indicating that global drawdown of oceanic Mo accompanied the T-OAE. In the upper Pliensbachian, the average Mo/TOC value from the three Western Canada sites is ∼25 (range of ∼22 – 30), which is similar to the modern euxinic Cariaco Basin (∼25, Fig. 17; cf. Algeo & Lyons, Reference Algeo and Lyons2006), suggesting a large global marine Mo inventory similar to today. The average Mo/TOC value of ∼2 for anoxic/euxinic sediments deposited during the T-OAE in Western Canada is identical to that obtained for Dove’s Nest, with an even lower value of 0.4 being derived from the larger Yorkshire coast dataset (Fig. 17b). These very low values suggest additional local Mo drawdown in the Cleveland Basin from already highly depleted ocean water. Post-T-OAE Mo/TOC values in the Canadian sites typically range from 4 to 10, like those seen in the lower Bituminous Shales, Unit IV of Yorkshire, indicative of a return to higher Mo contents in seawater (and contraction in the global area of euxinic bottom waters), but at lower concentrations than those that preceded the T-OAE.

Local basin restriction with the development of highly euxinic bottom waters, coincident with a global drawdown of Mo, has been interpreted for the T-OAE interval in the Dutch Central Graben, West Netherlands Basin, SW German Basin and Paris Basin (Fernández-Martínez et al., Reference Fernández-Martínez, Martínez Ruiz, Rodríquez-Tovar, Piñuela, García-Ramos and Algeo2023). Other areas in northern Europe including the Cardigan Bay Basin, together with Tethyan and Panthalassa Ocean sites, remained unrestricted but with evidence of the presence of euxinic pore waters at the sediment/water interface at some localities. The small global marine Mo reservoir during the T-OAE in combination with variable basin restriction explains the large regional differences in sediment Mo (and inferred water mass) concentrations and isotopes documented from European basins.

13.b. Uranium

The U vs Al plot (Fig. 17c) illustrates a strong positive correlation (R² = 0.73) between the two elements for the Pliensbachian samples of Subunits Ia – c and Toarcian Unit V that indicates an association between U and the aluminosilicate fraction (principally clay minerals and a heavy mineral suite) of the rocks, as generally seen in marine black shales (Swanson, Reference Swanson1961). Mudstones with the highest Al contents in the array, cluster around the average shale value of 3 ppm.

Low U concentrations of 2 – 4 ppm are generally found in mudstones (Swanson, Reference Swanson1961; Spirakis, Reference Spirakis1996) and U contents of 1 – 3 ppm are typical throughout the Earth’s crust (Hazen et al., Reference Hazen, Ewing and Sverjensky2009). Scatter towards lower values on the U vs Al plot (Fig. 17c) is driven principally by increasing proportions of U-poor carbonates (calcite, siderite) and quartz diluting the bulk-rock content. Anomalously high U contents, up to 6.5 ppm, characterize Units III and IV and these show a strong negative correlation (R² = 0.73) with Al attributable to a positive correlation with TOC (Fig. 17d). The positive correlation between U vs Al and U vs TOC for Subunits Ia – c (Fig. 17c, d) contrasts to the absence of correlation displayed in the corresponding Mo plots. The low U and low TOC contents in the sideritic ironstone of the Avicula Seam are notable.

Uranium occurs in nature in two main redox states, U(IV) and U(VI). In oxygenated solutions, the hexavalent uranyl (U^VIO₂²⁺) species forms highly soluble and non-reactive Ca and Mg complexes with UO₂(CO₃)₃⁴⁻; the dominant aqueous U(VI) species in seawater is neutral [Ca₂UO₂(CO₃)₃](aq) (Endrizzi & Rao, Reference Endrizzi and Rao2014). Under dysoxic – anoxic conditions, U is reduced and forms insoluble tetravalent U compounds and is ultimately fixed in sediments principally as uraninite (UO₂). This reduction, which is likely mediated by microorganisms, leads to preferential uptake of U by anoxic – euxinic sediments (Barnes & Cochran, Reference Barnes and Cochran1990; Klinkhammer & Palmer, Reference Klinkhammer and Palmer1991; Khaustova et al., Reference Khaustova, Tikhomirova, Korost, Poludetkina, Voropaev, Mironenko and Spasennykh2021). Unlike Mo, uranium reduction has not been observed in the marine water column but occurs predominantly in association with near-surface iron reduction in organic-rich sediments with diffusion of U into the sediment from overlying bottom waters, favoured by lowered sedimentation rates (Algeo & Maynard, Reference Algeo and Maynard2004).

Toarcian high-U samples plot consistently within the sulfidic field beyond the ‘anoxic threshold’ of Algeo & Maynard (Reference Algeo and Maynard2004) on the U vs TOC plot (Fig. 17d), evidencing the formation of authigenic U under anoxic conditions during sedimentation, as uranium was removed from solution. However, the mechanisms involved in this removal are complex (Cumberland et al., Reference Cumberland, Douglas, Grice and Moreau2016) and the phase partitioning in the sediment remains uncertain. Unit III samples of the T-OAE display substantial scatter due in part to variable dilution by carbonates in the concretion beds of the Jet Rock (Fig. 17d), while Unit IV samples from the lower Bituminous Shales define a tighter array with a steeper positive correlation between U and TOC than that defined by Subunits Ia – c.

The offset to lower U/TOC ratios for Unit III T-OAE samples compared to lower Bituminous Shales Unit IV is consistent with global drawdown of U accompanying the T-OAE, as discussed above for Mo (Section 13.a) and interpreted for other episodes of expanded anoxic seafloor area accompanying OAEs, based on uranium isotope studies (e.g. Montoya-Pino et al., Reference Montoya-Pino, Weyer, Anbar, Pross, Oschmann, van de Schootbrugge and Arz2010; Clarkson et al., Reference Clarkson, Stirling, Jenkyns, Dickson, Porcelli, Moy, Pogge von Strandman, Cooke and Lenton2018).

13.c. Vanadium

Pliensbachian Subunits Ia – c samples display a significant positive correlation between V and Al (Fig. 17e) with some Unit III and most Unit IV samples lying on the same trend. By contrast, Unit V samples form a distinct tight cluster that lies below the line. This offset reflects the lower V/Al ratio that differentiates Unit V from the older beds (Figs 5, 8). Samples of T-OAE Unit III from the Yorkshire coast form a linear array above the line with a negative trend between V and Al, attributable to the diluting effect of the high TOC content in V-rich samples.

Three Fe-rich samples display anomalous high V contents of up to 521 ppm (sample 195.18, Fig. 17e). Moderate enrichment of V in sideritic ironstones (average 114 ppm) from the Cleveland Ironstone was noted by Gad (Reference Gad1966), with distinctive higher values (average 560 ppm) recorded in berthierine (chamosite)-bearing samples. Vanadium enrichment in ironstones is well displayed in the V/Al stratigraphic profile for Dove’s Nest (Fig. 5). The highest V samples also display anomalous high Th contents (Fig. S2) as observed elsewhere in berthierine (Yang et al., Reference Yang, Shen, Qin, Jin, Zhang, Tong and Liu2021), consistent with the presence of the mineral in these ironstones. Thorium enrichment in these beds is responsible for their distinctive low U/Th ratio on the trace-element ratio profile (Fig. 5).

Positive correlations are observed between V and TOC with a steeper regression line generated by Subunit Ia – c samples compared to Units II – V (Fig. 17f). This steepening may be attributed to the additional presence of Fe-mineral (berthierine/chamosite, siderite and goethite) associated V and the greater influence of quartz and feldspar dilution effects in the coarser grained, TOC-poor, oxic facies of the Pliensbachian, compared to predominantly phyllosilicate and TOC-associated V in the anoxic – euxinic Toarcian mudstones. The Toarcian sample V vs TOC regression line, derived from the V data from the Yorkshire coast of Ruvalcaba Baroni et al. (Reference Ruvalcaba Baroni, Pohl, van Helmond, Papadomanolaki, Coe, Cohen, van de Schootbrugge, Donnadieu and Slomp2018), has an intercept within error of average shale (PAAS = 140 ppm). Unit IV samples of the lower Bituminous Shales cluster above the Units II – V regression line indicate elevated V/TOC ratios compared to the full Toarcian sample suite, like, but less pronounced than, the offsets seen for Mo/TOC and U/TOC (Fig. 17b, d; also compare Figs 5 and S3).

Vanadium behaves as a conservative or near-conservative element in the modern oceans (Wu et al., Reference Wu, Owens, Huang, Sarafian, Huang, Sen, Horner, Blusztajn, Mprton and Nielsen2019). The early diagenetic behaviour of V in marine sedimentary environments is poorly studied. In oxic environments, it is considered that V is adsorbed on particulate organic matter and Fe–Mn oxyhydroxides, transferred from the water column to the sediment and released to pore waters during Mn and Fe reduction (e.g. Bennett & Canfield, Reference Bennett and Canfield2020). Dissolved V is reduced to the strongly adsorbed VO(OH)₃⁻ under anoxic (= suboxic) conditions and is retained by the sediment during burial. Free sulfide in close proximity to the Mn and Fe reduction zone likely favours the reduction and capture of V from the pore water as authigenic V(III) hydroxide. Under anoxic bottom waters, a hydrogenous fraction may be added to the sediment directly through inorganic redox reactions. The element is therefore commonly enriched in carbonaceous mudstones where it is initially bound to organic compounds but may also partition into clay minerals (Breit & Wanty, Reference Breit and Wanty1991; Tribovillard et al., Reference Tribovillard, Algeo, Lyons and Riboulleau2006).

The largest V enrichments are observed in modern sediments being deposited within the core of perennial oxygen-minimum zones on continental margins. Hydrographically restricted euxinic basins typically do not show high levels of V enrichment due to low rates of sedimentation limiting the particulate V flux and low rates of deep-water renewal limiting hydrogenous V uptake (Scholz et al., Reference Scholz, Siebert, Dale and Frank2017; Bennett & Canfield, Reference Bennett and Canfield2020). Importantly, V drawdown under reducing conditions does not require the presence of sulfide in the water column.

A study of Late Cretaceous (Cenomanian – Turonian) OAE2 indicates that drawdown of V accompanied the global expansion of oxygen-deficient but non-sulfidic waters, fingerprinting an expansion of anoxia prior to the development of euxinia evidence by subsequent drawdown of Mo (Owens et al., Reference Owens, Reinhard, Rohrssen, Love and Lyons2016). The offset to lower V/TOC ratios that characterize Unit III compared to higher values in Unit IV (Fig. 17f) is consistent with a drawdown and reduced inventory of V during the period of maximum oxygen depletion accompanying the T-OAE, although the additional impact of basin restriction cannot be excluded.

13.d. Molybdenum – Uranium covariation

Algeo & Tribovillard (Reference Algeo and Tribovillard2009) and Tribovillard et al. (Reference Tribovillard, Algeo, Baudin and Riboulleau2012) advocated the use of Mo_EF vs U_EF covariation diagrams for marine sediments and rocks to interpret bottom-water redox conditions, the operation of metal-oxyhydroxide particulate shuttles in the water column, and the degree of water mass restriction driving the evolution of water chemistry.

Molybdenum and U exhibit conservative behaviour under oxic conditions but show progressive uptake by the sediment from seawater where the bottom water mass and/or surface sediments are anoxic (Algeo & Tribovillard, Reference Algeo and Tribovillard2009; Tribovillard et al., Reference Tribovillard, Algeo, Baudin and Riboulleau2012). Aqueous Mo and U are both removed from seawater across the sediment–water interface in reducing facies, but Mo requires a lower redox potential for sedimentary uptake than U: uptake of U begins at the Fe(II)–Fe(III) redox boundary and therefore exceeds that of Mo when redox conditions at the sediment/water interface are anoxic, while authigenic Mo enrichment requires the presence of H₂S and is favoured over U when benthic redox conditions are euxinic.

In unrestricted open-ocean environments such as continental margin upwelling systems, typified by those of the modern eastern tropical Pacific, it is argued that a general shift from oxic through anoxic to sulfidic benthic redox conditions enhances sediment uptake of both Mo and U (Fig. 17) by diffusion downwards across the sediment/seawater interface, but the change in redox state drives a progressive increase in Mo/U ratios from 0.1 – 0.3 to 1 – 3 times the seawater molar ratio (Algeo & Tribovillard, Reference Algeo and Tribovillard2009).

In moderately restricted basins, such as the modern Cariaco Basin, with a deep or highly variable chemocline, the operation of a particulate Fe–Mn oxyhydroxide particulate shuttle may enhance the transfer of aqueous Mo to the sediment/water interface. Molybdenum is vigorously scavenged from seawater by Fe–Mn oxyhydroxide particles in the oxic water column. On reaching the sediment/water interface, these particles are reductively dissolved, releasing Mo to diffuse back into the bottom water or available to be scavenged by other phases within the sediment. Aqueous U is unaffected by this process, resulting in elevated Mo/U ratios that are typically 3 to 10 times those of seawater (Fig. 18).

Figure 18. Mo_EF vs U_EF cross-plot for stratigraphic units comprising the upper Pliensbachian – middle Toarcian of Dove’s Nest core. The three diagonal lines represent multiples (0.3, 1, 3) of the Mo:U ratio of present-day seawater (SW) converted to an average weight ratio of 3.1 for the purpose of comparison with sediment Mo:U weight ratios (Tribovillard et al., Reference Tribovillard, Algeo, Baudin and Riboulleau2012). General patterns of Mo_EF vs U_EF covariation in modern marine environments modified from Tribovillard et al. (Reference Tribovillard, Algeo, Baudin and Riboulleau2012) and Yano et al. (Reference Yano, Yasukawa, Nakamura, Ikehara and Kato2020): unrestricted open ocean field based on the eastern tropical Pacific; particulate shuttle field based on the Cariaco Basin and Saanich Inlet. Trend lines show deoxygenation trends in modern marine environments (Tribovillard et al., Reference Tribovillard, Algeo, Baudin and Riboulleau2012) with positions based on data from restricted basins and coastal settings (Paul et al., Reference Paul, van Helmond, Slomp, Jokinen, Virtasalo, Filipsson and Jilbert2023). The anomalous high EF values of Whale Stones sample 167.89, interpreted as sampling a carbonate concretion, are likely an artifact of the high carbonate content (81%).

In more restricted basins like the Black Sea, which are characterized by a shallow and stable chemocline, relative rates of authigenic Mo–U enrichment will depend on the degree of evolution of the water chemistry in the deep water mass. Initially, when the aqueous Mo/U molar ratio is close to that of seawater, sediment Mo/U ratios will be high (Fig. 18), but if aqueous Mo is depleted through sedimentation without compensatory resupply, the sediment Mo/U ratios will decline.

More recently, sequestration of Mo and U to particulate matter has been shown to occur at the seafloor in some open-ocean upwelling environments, exemplified by the Benguela upwelling system off Namibia, due to the intermittent presence of sulfide, either in bottom water or in pore water immediately at the sediment/water interface (He et al., Reference He, Clarkson, Andersen, Archer, Sweere, Kraal, Guthauser, Huang and Vance2021). Mo and U are sequestered to the sediment during temporarily more reducing conditions. They subsequently undergo oxidative dissolution under less reducing conditions and diffuse away from local Mo and U pore-water concentration peaks. Mo and U are taken up again into reduced authigenic phases (sulfide-related) at depth, setting the final Mo and U content and isotope characteristics of the sediment. In this way, early diagenetic enrichment of Mo and U may be governed by temporal redox fluctuations.

He et al. (Reference He, Clarkson, Andersen, Archer, Sweere, Kraal, Guthauser, Huang and Vance2021) have argued that early diagenetic processes in upwelling environments can produce patterns similar to those observed for coupled Mo–U covariation and isotope systematics in restricted and semi-restricted basins (see above) but via a different set of processes. Unambiguous identification of a restricted basin system therefore requires critical assessment of multiple proxies.

Samples from the upper Pliensbachian – lower Toarcian succession at Dove’s Nest fall within 3 different areas on a Mo_EF vs U_EF cross-plot (Fig. 18). Samples from Units I (upper Pliensbachian), II (basal lower Toarcian) and V (upper lower to middle Toarcian) form a broad array with U_EF values close to or less than PAAS (i.e., no U enrichment) but Mo_EF values range from ∼1 to ∼10 (moderate Mo enrichment). The pattern of U–Mo covariation does not fall along the established unrestricted open ocean trend but rises upward towards the particulate shuttle field. Significant outliers include Avicula Seam sample 200.39 which exhibits greater Mo and U enrichment than other samples from the Cleveland Ironstone (Fig. 18), possibly an artefact of a very low Al₂O₃ content (2.9%) and Sulphur Band 3 sample 180.91, which displays U enrichment typical of anoxic unrestricted water masses. Samples from the Hard Shales at the base of Unit V are characterized by variable high Mo_EF and U_EF values.

Following the approach of Algeo & Tribovillard (Reference Algeo and Tribovillard2009) and Tribovillard et al. (Reference Tribovillard, Algeo, Baudin and Riboulleau2012), Yorkshire Pliensbachian samples lie within a poorly defined field of dominantly oxic – anoxic conditions (cf. Fig. 17) but with the levels of Mo enrichment potentially indicating periods of bottom water restriction and/or uptake under sulfidic conditions in the near-surface sediment. However, interpretation of Mo_EF values in these beds must be treated with caution since Mo concentrations in many samples were below the limit of quantification (2 ppm) for the Dove’s Nest study (Table S2) and were allocated a nominal value of 1 ppm (=PAAS) for plotting purposes, equivalent to the average Mo value (1.2 ± 1.0 ppm) obtained for samples from this part of the succession on the coast by McArthur (Reference McArthur2019).

It is observed that most Pliensbachian samples, together with those from Toarcian Units II and V, exhibit U_EF values of <1 (Fig. 18). The Lower Jurassic in the central Cleveland Basin has been buried into the oil window (∼95° C) (Barnard & Cooper, Reference Barnard, Cooper and Brooks1983; Salem, Reference Salem2013; French et al., Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014; Song et al., Reference Song, Littke, Weniger, Ostertag-Henning and Nelskamp2015; Song et al., Reference Song, Littke and Weniger2017), and low U_EF values have previously been attributed to U loss accompanying hydrocarbon generation and migration (Wignall in Tribovillard et al., Reference Tribovillard, Algeo, Baudin and Riboulleau2012). However, a study of lower Toarcian black shales from the Alum Shale Formation of Sweden indicates that U remains immobile during catagenesis (Lecomte et al., Reference Lecomte, Cathelineau, Michels, Peiffert and Brouand2017), while U/TOC ratios increase with maturity in the lower Toarcian Posidonia Shale of the Lower Saxony Basin, northern Germany (Dickson et al., Reference Dickson, Idiz, Porcelli, Murphy, Celestino, Jenkyns, Poulton, Hesselbo, Hooker, Ruhl and van der Boorn2022b), due to loss of organic matter with a low metal content during secondary migration. Low U_EF values in the Cleveland Basin are therefore judged to be a primary feature.

Samples from T-OAE Unit III, the top Grey Shale and Jet Rock, form an extended array of high U_EF (∼1 – ∼2) and high Mo_EF (∼4 – ∼13) values with a relatively constant U/Mo ratio that parallels the seawater trend (Fig. 18). A sample from the Top Jet Dogger (164.91 m) falls within the particulate shuttle field, another from the Whale Stones (167.89 m) displays very high Mo_EF and U_EF values of 42 and 7.6, respectively, associated with the high carbonate content (81%) in this concretion sample. Unit II samples generally fall on the lower segment of the Black Sea trend of Yano et al. (Reference Yano, Yasukawa, Nakamura, Ikehara and Kato2020).

High Mo_EF values of ∼10 – ∼35 in lower Bituminous Shales Unit IV are accompanied by moderate U enrichment (U_EF ∼1 – ∼2) and Mo/U ratios that exceed 3× modern seawater. These values imply the operation of a particulate shuttle (Fig. 18), although one that was less active than that in the modern Saanich Inlet and Cariaco Basin (compare Yano et al., Reference Yano, Yasukawa, Nakamura, Ikehara and Kato2020, fig. 6) where, in the latter, Mo_EF and U_EF reach 200 and >8, respectively. Lower Mo_EF values paired with Mo/U ratios of ∼1 – 2× seawater in Unit III have been interpreted to reflect the drawdown of aqueous Mo as a consequence of transfer to the sediment in a highly restricted Cleveland Basin water mass during the T-OAE (McArthur et al., Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008), the ‘basin reservoir effect’ (Algeo, Reference Algeo2004; Algeo & Lyons, Reference Algeo and Lyons2006). Elevated U_EF values are consistent with the development of more reducing conditions in bottom waters.

Restriction of Cleveland Basin bottom waters has been linked to the development of a ‘deep’ pycnocline (a total water depth of 50 – 100 m was proposed for the Mulgrave Shale by Hallam, Reference Hallam1997) caused by increased freshwater runoff generating low-salinity surface waters that limited mixing between surface and bottom waters and exchange with the adjacent seas and open ocean (Küspert, Reference Küspert, Einsele and Seilacher1982; Sælen et al., Reference Sælen, Doyle and Talbot1996, Reference Sælen, Tyson, Talbot and Telnaes1998, Reference Sælen, Tyson, Telnaes and Talbot2000; Bailey et al., Reference Bailey, Rosenthal, McArthur, van de Schootbrugge and Thirlwall2003; van de Schootbrugge et al., Reference van de Schootbrugge, McArthur, Bailey, Rosenthal, Wright and Miller2005; Wignall et al., Reference Wignall, Newton and Little2005; McArthur et al., Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008; McArthur, Reference McArthur2019; Remírez & Algeo, Reference Remírez and Algeo2020). However, the amplitude of any salinity decrease is hotly contested (Hesselbo et al., Reference Hesselbo, Little, Ruhl, Thibault and Ullmann2020a) and palaeontological evidence and salinity proxies derived from the organic geochemical data (Section 15.b) indicate normal marine salinity throughout the T-OAE in the Cleveland Basin (Song et al., Reference Song, Littke and Weniger2017).

Reinterpretation of the Mo proxy (Section 13.a) weakens the geochemical argument for extreme basin restriction during the T-OAE. Like Mo, the oceans would have been subject to drawdown of U accompanying the global expansion of anoxic-euxinic bottom waters and seafloor area during the T-OAE. However, the differing redox sensitivities of the two elements would impose a trend of changing global Mo/U seawater ratios that compromise a simplistic interpretation of water mass restriction based on observations from modern basins.

14.a. Arsenic

Arsenic displays similarities to the distribution to Mo when plotted stratigraphically as As/TOC ratios but As contents of the Pliensbachian section (Unit I) overlap more strongly with those in the higher beds (Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018). The As_EF profile shows a very similar pattern to DOP_T (Fig. 14) that likely reflects deposition of As with Fe–Mn oxyhydroxides and subsequent incorporation by organic matter and diagenetic pyrite (Maher & Butler, Reference Maher and Butler1988; Large et al., Reference Large, Bull and Maslennikov2011). Arsenic has been used as a redox proxy in some previous palaeoenvironmental studies (e.g. Bodin et al., Reference Bodin, Godet, Matera, Steinmann, Vermeulen, Gardin, Adatte, Coccioni and Föllmi2007; Atar et al., Reference Atar, Marz, Aplin, Dellwig, Herringshaw, Lamoureux-Var, Leng, Schnetger and Wagner2019a, Reference Atar, Marz, Schnetger, Wagner and Aplin2019b; Benamara et al., Reference Benamara, Charbonnier, Adatte, Spangenberg and Föllmi2020). However, critical analyses by Algeo & Liu (Reference Algeo and Liu2020) and Tribovillard (Reference Tribovillard2020, Reference Tribovillard2021) have concluded that arsenic is not a reliable proxy.

Arsenic shows marked enrichments in sediments from settings where dissolved reactive iron has been captured from seawater via shuttling by Fe–Mn oxyhydroxides and/or associated with cold seeps and is trapped in the sediment under reducing conditions (Tribovillard, Reference Tribovillard2020, Reference Tribovillard2021). Enrichment in arsenic indicates that reactive iron was present in sufficient abundance to keep free sulfide ions to low levels, thus preventing As solubilization, migration and loss from the sediment. However, when reactive iron availability is low, the development of high-sulfide and low-iron conditions prevent As capture and enrichment. In this case, protracted contact between free sulfide and organic matter induces sulfurization of the latter (Abubakar et al., Reference Abubakar, Taylor, Coker, Wogelius and van Dongen2022), producing a high-TOC, high-Mo and low-As association (Tribovillard, Reference Tribovillard2020).

The good correlation between As vs Mo and U in the Yorkshire successions is consistent with a high reactive-Fe environment and supports the operation of a particulate shuttle during the deposition of the anoxic facies of the lower Bituminous Shales, Unit IV, evidenced by Mo vs U covariation (Fig. 18). However, it should be noted that Kemp et al. (Reference Kemp, Chen, Cho, Algeo, Shen and Ikeda2022a) proposed a coincident global drawdown of Mo, U and As from seawater accompanying the severe deep-ocean deoxygenation accompanying the T-OAE.

14.b. Copper

Copper is regarded as having a weak euxinic affinity (Algeo & Maynard, Reference Algeo and Maynard2004) and correlates strongly with TOC in the Pliensbachian – Toarcian boundary succession of the Cleveland Basin. This association is demonstrated by the near-identical chemostratigraphic profiles for the two constituents illustrated by Ruvalcaba Baroni et al. (Reference Ruvalcaba Baroni, Pohl, van Helmond, Papadomanolaki, Coe, Cohen, van de Schootbrugge, Donnadieu and Slomp2018, fig. 2) and Thibault et al. (Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018, fig. 2), with a high correlation coefficient (R² = 0.7) in both studies, but Cu shows no additional trends that may be considered to have palaeoenvironmental significance.

Cu_EF values are generally <1 throughout the succession, rise to >1 in samples with >7% TOC in T-OAE Subunit IIIb, reaching a maximum of 2.5 in the highest-TOC (>12%) samples at the level of Whale Stones ’bed’ 35. Low Cu_EF values (<3) similarly characterize oxic – euxinic settings in the modern Cariaco Basin. High Cu_EF values of 3 – 7 recorded in the euxinic sediments of the modern Black Sea and upwelling zones of the Peru margin (Little et al., Reference Little, Vance, Lyons and McManus2015) have not been documented in the Yorkshire succession.

14.c. Cadmium

McArthur (Reference McArthur2019) presented Cd and Co data from the Yorkshire coastal succession and combined these with Mo and Mn results to apply the trace-metal palaeoenvironmental proxies proposed by Sweere et al. (Reference Sweere, van den Boorn, Dickson and Reichart2016), and assessed their concordance with the observations of Little et al. (Reference Little, Vance, Lyons and McManus2015).

Cadmium and Mo stand out as the two trace metals that typically show some of the highest enrichment factors in organic-rich sediments (Brumsack, Reference Brumsack2006, fig. 2). Cadmium is enriched in plankton (35 ppm) but has a low concentration in terrigenous detritus, reflected by an average shale content of 0.3 ppm (Brumsack, Reference Brumsack1989; Little et al., Reference Little, Vance, Lyons and McManus2015). Similar to C and P, Cd is rapidly regenerated in the ocean water column and shows a typical nutrient profile, but undergoes a dramatic solubility decrease at the O₂/H₂S boundary of anoxic basins. The Cd curve for the Pliensbachian – Toarcian boundary succession (McArthur, Reference McArthur2019, fig. 2) generally follows the S and DOP_T trends (Fig. 14), consistent with an association between Cd and sulfides (Cd vs S, R² = 0.40). However, low Cd contents (0.01 – 0.88 ppm) in the Yorkshire succession indicate an absence of significant CdS addition from euxinic bottom waters (cf. Sweere et al., Reference Sweere, Dickson, Jenkyns, Porcelli, Ruhl, Murphy, Idiz, van der Boorn, Eldrett and Henderson2020) or a restricted Cd supply to the basin.

Unlike Cd, Mo does not bioaccumulate in phytoplankton (2 ppm), which have a Cd/Mo mass ratio of ≫1. As a result, sediments in upwelling regions, which are subject to high export production of organic matter and Cd but not Mo, are characterized by high Cd/Mo ratios. In restricted environments, although anoxia/euxinia promote uptake of both Cd and Mo to the sediments, lower productivity limits plankton-derived export of Cd, leading to Cd/Mo ratios of ≪1 that can approach the seawater value of 0.008. The hydrographically restricted and upwelling fields on the Cd/Mo discrimination diagrams of Sweere et al. (Reference Sweere, van den Boorn, Dickson and Reichart2016) are separated by a Cd/Mo value of 0.1.

The Cd/Mo profile from the Yorkshire succession (McArthur, Reference McArthur2019, fig. 3) generally follows the TOC and TOC/P trends (Figs 14, 16), consistent with a close association between Cd and TOC. However, despite a positive correlation (R² = 0.50) between Cd vs TOC (McArthur, Reference McArthur2019, fig. 4), low Cd/Mo ratios (generally <0.1) throughout the succession place the overwhelming majority of samples in the restricted water mass field of Sweere et al. (Reference Sweere, van den Boorn, Dickson and Reichart2016, fig. 4) and are indicative of a low productivity setting.

However, interpretation of the Cd/Mo proxy based on direct comparison to values from modern basins is again compromised by evidence of significant stratigraphic changes in the global Mo content of ocean water through the late Pliensbachian – middle Toarcian, with the global drawdown of Mo accompanying the T-OAE (Section 13.a). This would change the threshold values between the restricted vs open basin fields, potentially placing samples from the T-OAE interval into the unrestricted high-productivity field.

14.d. Cobalt

Downward-decreasing vertical profiles of Co and Mn in the ocean water column demonstrate that these elements are scavenged by settling particulates (terrigenous detritus and organic matter) and are taken up by bottom sediments, principally as oxyhydroxides (e.g. van Hulten et al., Reference van Hulten, Middag, Dutay, De Baar, Roy-Barman, Gehlen, Tagliabue and Sterl2017; Hawco et al., Reference Hawco, Lam, Lee, Ohnemus, Noble, Wyatt, Lohan and Saito2018). In upwelling regions, the hydrogenous supply is limited by the depletion of both elements in upwelled water. By contrast, restricted basins commonly have abundant Co and Mn supplied by rivers, providing a greater hydrogenous supply.

In oxic sediments, Co and Mn are buried as oxyhydroxides but may be remobilized by bacterial reduction driving anoxia with subsequent refixation, typically Co in pyrite and organic matter and Mn in rhodochrosite. However, where the oxic – anoxic boundary lies at or above the sediment/water interface, remobilized elements can escape from the sediment and be returned to bottom waters. In open-ocean (unrestricted) settings, such as coast upwelling systems, these elements can then be lost by lateral advection of bottom waters. Oxyhydroxide particulates settling from the surface waters into the underlying oxygen-minimum zone dissolve, releasing Mn and Co, which can be transported oceanward by a ‘Mn-conveyor belt’ (Brumsack, Reference Brumsack2006), leading to low metal enrichments in the sediments.

In restricted settings, remobilized elements cannot escape and are returned to the sediment via redox cycling, causing significant enrichment over the original particulate supply. Sweere et al. (Reference Sweere, van den Boorn, Dickson and Reichart2016) determined that modern restricted environments have Co (ppm) × Mn (%) values > 0.4 while unrestricted environment values are < 0.4. McArthur (Reference McArthur2019) proposed the same threshold value for a Co_EF × Mn_EF proxy and demonstrated the use of shale-normalized data offered no advantage over a concentration-based ratio.

Cobalt and Mn display similar profiles through the Yorkshire upper Pliensbachian – middle Toarcian succession (McArthur, Reference McArthur2019, fig. 2), excluding large Mn peaks associated with sideritic mudstone and calcite concretion horizons in Unit V (‘beds’ 44, 46, 48 and 50; Fig. 14) which are Co depleted. Other levels containing significant siderite and/or calcite contents similarly show relatively lower Co contents reflecting the presence of carbonate-bound Mn and, in the upper beds, Fe–Mn oxyhydroxides at omission surfaces in condensed intervals (McArthur, Reference McArthur2019).

The shape of the Co × Mn profile (McArthur, Reference McArthur2019, fig. 3) is similar to those of Fe_EF and Mn_EF (Fig. 14), with high Co (ppm) × Mn (%) values of >0.4 in the Pliensbachian section (Unit I) interpreted by McArthur (Reference McArthur2019) to indicate restricted circulation that pre-dated the onset of black shale deposition. Maximum Co × Mn contents are recorded in the three Sulphur Bands and high values occur consistently through the T-OAE interval, Unit III, which features the highest average Mn content and Mn/Al ratio of the units in the succession (Fig. 8). A sharp decline in Mn and Mn_EF (Figs 8, 14), with Co (ppm) × Mn (wt%) values falling to <0.4, occurs in the upper Bituminous Shales at the base of Unit V, consistent with a change to more open conditions. The subsequent decreasing stratigraphically upward trend implies increasing open circulation accompanied by sea-level rise from the late early Toarcian (cf. Fig. 10).

McArthur (Reference McArthur2019) concluded that the Cd/Mo, Co × Mn and Mo/TOC proxies together confirm that hydrographic restriction was a defining feature of black shale deposition in the early Toarcian of the Cleveland Basin. However, evidence of global depletion of Mo and other trace metals in the oceans during the T-OAE requires the application of Mo-based ratios to be re-evaluated.

14.e. Rhenium

The principal focus of work on Re and Os in the lower Toarcian has been the generation of ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁷Re/¹⁸⁸Os ratios for isochron construction and Re-Os dating of black shales, and the generation of ¹⁸⁷Os/¹⁸⁸Os_i seawater time series (Fig. 18, Section 16.b.4) to examine changes in the balance between continental weathering and hydrothermal input in the oceans (e.g. Cohen et al., Reference Cohen, Coe, Bartlett and Hawkesworth1999, Reference Cohen, Coe, Harding and Schwark2004; Percival et al., Reference Percival, Cohen, Davies, Dickson, Hesselbo, Jenkyns, Leng, Mather, Storm and Xu2016; Them et al., Reference Them, Gill, Selby, Gröcke, Friedman and Owens2017b; van Acken et al., Reference van Acken, Tütken, Daly, Schmid-Röhl and Orr2019; Kemp et al., Reference Kemp, Selby and Izumi2020). Rhenium abundances for the Yorkshire Toarcian have been presented by Pearce et al. (Reference Pearce, Cohen, Coe and Burton2008).

Rhenium has the largest enrichment factor of all redox-sensitive metals in anoxic sediments relative to the detrital background (Helz & Adelson, Reference Helz and Adelson2013). The average Re concentration of 66 ppb in modern anoxic sediments compares to an upper crust value of 0.2 – 0.4 ppb, an enrichment factor of >200 (Sheen et al., Reference Sheen, Kendall, Reinhard, Creaser, Lyons, Bekker, Poulton and Anbar2018). Rhenium occurs as the soluble Re(VII)O₄⁻ oxyanion in oxic seawater and is a conservative element that occurs at low concentrations (average 7 parts per trillion in the modern ocean; Anbar et al., Reference Anbar, Creaser, Papanastassiou and Wasserburg1992; Colodner et al., Reference Colodner, Sachs, Ravizza, Turekian, Edmond and Boyle1993b; Morford et al., Reference Morford, Martin and Carney2012; Dickson et al., Reference Dickson, Hsieh and Bryan2020). The estimated residence time of Re in the ocean is 130 – 750 ka (Colodner et al., Reference Colodner, Boyle and Edmond1993a; Miller et al., Reference Miller, Peucker-Ehrenbrink, Walker and Marcantonio2011), likely marginally shorter than for Mo at 440 – 800 ka (Morford & Emerson, Reference Morford and Emerson1999; Miller et al., Reference Miller, Peucker-Ehrenbrink, Walker and Marcantonio2011) and U at 400 ka (Ku et al., Reference Ku, Knauss and Mathieu1977) but longer than V at 50 –100 ka (Wu et al., Reference Wu, Owens, Huang, Sarafian, Huang, Sen, Horner, Blusztajn, Mprton and Nielsen2019).

Under anoxic conditions, Re(VII) is reduced to Re(IV), becomes insoluble and is removed to the sediment either through complexation with organic matter and/or incorporated into sulfides. Unlike Mo, Re can be efficiently removed from anoxic sediments at low dissolved H₂S levels when bottom waters are dysoxic or anoxic (Crusius et al., Reference Crusius, Calvert, Pedersen and Sage1996; Calvert & Pedersen, Reference Calvert, Pedersen, Hillaire-Marcel and de Vernal2007). The ratio of Re/Mo in anoxic sediments is as much as 2.5 times greater than the seawater value, while the ratio is close to that of seawater in euxinic (sulfidic) sediments. Under oxic bottom waters the ratio may be less than that in seawater (Re/Mo 0.74 × 10⁻³; Bruland & Lohan, Reference Bruland, Lohan and Elderfield2003) due to fixation of Mo by Mn-oxyhydroxides but no uptake of Re.

Sediment Re concentrations and Re/Mo ratios have been used as local redox proxies by Turgeon & Brumsack (Reference Turgeon and Brumsack2006), Pearce et al. (Reference Pearce, Cohen, Coe and Burton2008) and Kunert & Kendall (Reference Kunert and Kendall2023), amongst others. Rhenium offers a potential proxy for tracking general ocean oxygen depletion (i.e. combined euxinic and non-euxinic anoxia) because the magnitude of authigenic Re enrichment in anoxic marine sediments is significantly higher than the detrital background compared with U, for example, as reflected by higher Re enrichment factors in organic-rich sediments.

A geochemical study of a lower Toarcian succession in western Canada by Kunert & Kendall (Reference Kunert and Kendall2023) indicates that Re and Mo oceanic mass balance models can be used to infer the extent of seafloor total anoxia and euxinia, respectively, using concentrations of these metals in locally anoxic or euxinic carbonaceous mudstones deposited in an unrestricted marine setting. They calculated an expansion of up to ∼7% of total global seafloor anoxia – euxinia, dominated by euxinia, during the early stages of the T-OAE, followed by a contraction before the end of the event.

A Re threshold of 5 ppb was used by Kunert & Kendall (Reference Kunert and Kendall2023) to identify samples where the sediment column was anoxic. Values rise consistently above this threshold from the base of the T-OAE interval Unit III in Yorkshire (Fig. 20), with a large step increase to ∼40 ppb at the base of the laminated black shale facies in ‘bed’ 31 of Subunit IIIa interpreted to reflect bottom water anoxia and decreasing basin restriction.

Figure 19. Stratigraphic variation in selected isotope geochemistry in the upper Pliensbachian – middle Toarcian of Yorkshire. δ¹³C_org profile for Dove’s Nest (Trabucho-Alexandre et al., Reference Trabucho-Alexandre, Gröcke, Atar, Herringshaw and Jarvis2022, black) rescaled to coastal succession, with compiled high-resolution coast curve (grey, see Fig. 2; Cohen et al., Reference Cohen, Coe, Harding and Schwark2004; DB Kemp et al., Reference Kemp, Coe, Cohen and Schwark2005; Littler et al., Reference Littler, Hesselbo and Jenkyns2010). Stratigraphic framework as in Figure 2. Rescaled whole-rock TOC profile for Dove’s Nest (Trabucho-Alexandre et al., Reference Trabucho-Alexandre, Gröcke, Atar, Herringshaw and Jarvis2022, dark green) with coast composite data of Kemp et al. (Reference Kemp, Coe, Cohen and Weedon2011; thin yellow-green high-resolution curve), McArthur (Reference McArthur2019; thin pale green low-resolution curve) and Ruvalcaba Baroni et al. (Reference Ruvalcaba Baroni, Pohl, van Helmond, Papadomanolaki, Coe, Cohen, van de Schootbrugge, Donnadieu and Slomp2018; green-filled triangles). Yorkshire coast Mo profile of McArthur (Reference McArthur2019; thick pink line) with elemental results for Mo-isotope samples of Pearce et al. (Reference Pearce, Cohen, Coe and Burton2008; red filled triangles) and high-resolution data of Thibault et al. (Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018; thin dark red line, see Fig. 16). δ^98/95Mo coast profile from Pearce et al. (Reference Pearce, Cohen, Coe and Burton2008). Details of the Mo concentration and isotope curves within Unit III (T-OAE) are presented in Figure 20. Belemnite carbonate-associated sulfur isotope profile (δ³⁴S_CAS) from Gill et al. (Reference Gill, Lyons and Jenkyns2011; white filled squares) incorporating the data of Newton et al. (Reference Newton, Reeves, Kafousia, Wignall, Bottrell and Sha2011; orange squares). Belemnite ⁸⁷Sr/⁸⁶Sr coast curve after McArthur et al. (Reference McArthur, Donovan, Thirlwall, Fouke and Mattey2000). ¹⁸⁷Os/¹⁸⁸Os_i profile of Cohen et al. (Reference Cohen, Coe, Bartlett and Hawkesworth1999; Reference Cohen, Coe, Harding and Schwark2004). Belemnite carbonate oxygen-isotope (δ¹⁸O, blue dots) and Mg/Ca ratios (blue circles) after McArthur et al. (Reference McArthur, Donovan, Thirlwall, Fouke and Mattey2000).

Figure 20. Stratigraphic profiles of δ¹³C_org, TOC and selected trace-metal isotopes within Unit III, the T-OAE interval of the Yorkshire coast. Stratigraphy as in Figures 6, 7. Bulk rock δ¹³C_org and TOC profiles of the composite section from Hesselbo et al. (Reference Hesselbo, Gröcke, Jenkyns, Bjerrum, Farrimond, Bell and Green2000, pale coloured lines) and DB Kemp et al. (Reference Kemp, Coe, Cohen and Schwark2005, dark lines), Kemp et al. (Reference Kemp, Coe, Cohen and Weedon2011, dark lines) – see Figure 7. Shaded grey bands indicate Unit III Subunits a – c (see Section 8.c); shaded blue band is the interval of the carbonate maximum, Subunit IIId. A – D mark coincident sharp falls in δ¹³C_org (after DB Kemp et al., Reference Kemp, Coe, Cohen and Schwark2005; Cohen et al., Reference Cohen, Coe and Kemp2007) and δ⁹⁸Mo, with increased Mo, as noted by Kemp et al. (Reference Kemp, Coe, Cohen and Weedon2011). Biotic extinction levels ii and iii after Caswell et al. (Reference Caswell, Coe and Cohen2009); base of trace fossil absent interval follows Caswell & Herringshaw (Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023). Thallium isotopes (ε²⁰⁵Tl) from Nielsen et al. (Reference Nielsen, Goff, Hesselbo, Jenkyns, LaRowe and Lee2011). Rhenium, Mo, Re/Mo and δ⁹⁸Mo profiles from Pearce et al. (Reference Pearce, Cohen, Coe and Burton2008). Note that Nielsen et al.’s (Reference Nielsen, Goff, Hesselbo, Jenkyns, LaRowe and Lee2011, fig. 5) comparison figure of ε²⁰⁵Tl vs δ⁹⁸Mo at Port Mulgrave incorrectly plotted the position of the Mo dataset relative to the stratigraphy, as presented by Pearce et al. (Reference Pearce, Cohen, Coe and Burton2008, fig. 2). The replotted data in our figure do not support an anti-correlation between these two isotope systems, as proposed by Nielsen et al. (Reference Nielsen, Goff, Hesselbo, Jenkyns, LaRowe and Lee2011) and modelled by Owens et al. (Reference Owens, Nielsen, Horner, Ostrander and Peterson2017).

Euxinic environments are predicted to be characterized by low Re/Mo ratios approaching those of seawater. Normal oxic environments show very variable but generally higher Re/Mo values. Re/Mo (ppb/ppm) thresholds of 10 – 15 for the oxic/dysoxic – anoxic boundary and 2 – 4 for the anoxic – euxinic boundary have been proposed by Turgeon & Brumsack (Reference Turgeon and Brumsack2006) and Kunert & Kendall (Reference Kunert and Kendall2023). The latter authors reported Re/Mo ratios of 11 ± 5, 6 ± 2 and 2 ± 1 for Toarcian carbonaceous mudstones assigned to dysoxic, anoxic and euxinic redox conditions based on independent criteria, respectively.

The Re/Mo profile spanning the T-OAE interval of the Yorkshire coast (Fig. 20) indicates: (1) a progressive transition from oxic – dysoxic conditions immediately preceding the onset of the T-OAE (top Subunit IIb and lower IIIa); (2) sharply falling ratios through ‘bed’ 32 at the top of Subunit IIIa, indicative of increasing local anoxia – euxinia; (3) consistently low values indicative of euxinic conditions throughout the δ¹³C_org minimum of Subunit IIIb; and (4) variable higher values representing euxinic –anoxic conditions during the later stages and immediately following the T-OAE (Subunits IIIc – IVa). The Re/Mo ratio of modern euxinic sediments approaches that of seawater, so the values recorded in Yorkshire T-OAE Subunit IIIb likely reflect the composition of Toarcian seawater (Fig. 20). The higher Re/Mo ratio compared to modern seawater would reflect a higher proportion of euxinic : anoxic seafloor during the Toarcian compared to the modern ocean.

14.f. I/Ca ratios

Iodine/calcium ratios (I/Ca) in marine carbonate have been proposed as a geochemical proxy to constrain global seawater redox change (e.g. Lu et al., Reference Lu, Jenkyns and Rickaby2010; Zhou et al., Reference Zhou, Jenkyns, Owens, Junium, Zheng, Sageman, Hardisty, Lyons, Ridgwell and Lu2015), and I/Ca ratios have been reported for belemnites collected from the Jet Rock (T-OAE Subunits IIIb – d) of the Yorkshire coast (Lu et al., Reference Lu, Jenkyns and Rickaby2010). Iodine has a residence time of ∼300 ka in the modern oceans resulting in a uniform total iodine concentration of 0.45 μM in most places, although the distribution of iodine species is complex (Wadley et al., Reference Wadley, Stevens, Jickells, Hughes, Chance, Hepach, Tinerl and Carpenter2020; Luther, Reference Luther2023). Low I/Ca ratios in the T-OAE and OAE2 intervals of Italy and England, respectively, have been interpreted to reflect the reduction of iodate (IO₃⁻) to iodide (I⁻) due to deoxygenation and global iodine drawdown by organic carbon burial (Lu et al., Reference Lu, Jenkyns and Rickaby2010).

A fall to very low I/Ca ratios in bulk carbonate has been documented in a Toarcian shallow-water carbonate platform section in southern Italy (Monte Sorgenza; Lu et al., Reference Lu, Jenkyns and Rickaby2010). The onset of falling I/Ca values begins at a peak of ∼8 μM/M at the Pliensbachian – Toarcian boundary interval at Monte Sorgenza based on its position in relation to the δ¹³C_carb curve (cf. Woodfine et al., Reference Woodfine, Jenkyns, Sarti, Baroncini and Violante2008; Xu et al., Reference Xu, Ruhl, Jenkyns, Leng, Huggett, Minisini, Ullmann, Riding, Weijers, Storm, Percival, Tosca, Idiz, Tegelaar and Hesselbo2018). A sharp step fall from >2 μM/M to a minimum ∼0.5 μM/M occurs at the base of, and continues throughout, the T-OAE interval. Interpretation of bulk carbonate I/Ca values in shallow-marine sediments requires caution due to the high sensitivity of I/Ca ratios to diagenesis (Lau & Hardisty, Reference Lau and Hardisty2022), but it is significant that comparable low ratios were recorded from Yorkshire Jet Rock belemnites (Lu et al., Reference Lu, Jenkyns and Rickaby2010, fig. 3).

I/Ca ratio values remain low until some distance above the T-OAE interval at Monte Sorgenza and only return to pre-excursion levels in the middle Toarcian. This is consistent with the persistence of global anoxia indicated by other geochemical (e.g. δ⁹⁸Mo, Section 16.b.1; ε²⁰⁵Tl, Section 16.b.2) and fossil proxies (Section 19).

15.a. Molecular carbon-isotope records

Compound-specific δ¹³C analyses were performed by French et al. (Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014) across the T-OAE interval at Hawsker Bottoms (‘beds’ 29 – 41) that provide information on changes in the isotopic composition of the marine vs atmospheric carbon reservoirs. All biomarkers record the negative CIE of the T-OAE through Unit III but have different amplitudes (Fig. 21), consistent with an earlier low-resolution study of Toarcian shales in SW Germany (Schouten et al., Reference Schouten, Van Kaam-Peters, Rijpstra, Schoell and Sinninghe Damsté2000). The n-C₁₇, n-C₁₈ and n-C₁₉ alkanes display a negative excursion of ∼2 – 3‰, with a marginally smaller excursion (∼2‰) recorded by pristane and phytane; all of these compounds are considered to represent marine sources (French et al., Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014). Long-chain n-alkanes (n-C₂₇, n-C₂₈, n-C₂₉), derived primarily from land plant wax lipids, provide a terrestrial record. These exhibit the largest compound-specific negative CIE of ∼5‰, which is lower than that derived from bulk organic matter (δ¹³C_org ∼ 6 – 7‰; Hesselbo et al., Reference Hesselbo, Gröcke, Jenkyns, Bjerrum, Farrimond, Bell and Green2000; DB Kemp et al., Reference Kemp, Coe, Cohen and Schwark2005; Trabucho-Alexandre et al., Reference Trabucho-Alexandre, Gröcke, Atar, Herringshaw and Jarvis2022).

Figure 21. Stratigraphic profiles of δ¹³C_org, δ¹³C _n-alkanes, isorenieratane, AOM, TOC and δ¹⁵N_tot within Unit III, the T-OAE interval of the Yorkshire coast. Bulk rock δ¹³C_org and TOC profiles of the composite section from Hesselbo et al. (Reference Hesselbo, Gröcke, Jenkyns, Bjerrum, Farrimond, Bell and Green2000, pale coloured lines) and DB Kemp et al. (Reference Kemp, Coe, Cohen and Schwark2005, dark lines), Kemp et al. (Reference Kemp, Coe, Cohen and Weedon2011, dark lines) – see Figure 7; ‘anoxic threshold’ of TOC_WR = 2.5 wt% follows Algeo & Maynard (Reference Algeo and Maynard2004). δ¹³C data for terrestrial wood (Hesselbo et al., Reference Hesselbo, Gröcke, Jenkyns, Bjerrum, Farrimond, Bell and Green2000, brown squares) are offset but track the bulk sediment δ¹³C_org curve. Carbon isotope values from Hawsker Bottoms of representative long-chain n-alkane biomarkers (δ¹³C_n-alkane) derived from terrestrial plants (n-C₂₇, n-C₂₉) also display the negative excursion of the T-OAE (French et al., Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014). Short-chain n-alkanes (n-C₁₇ – n-C₁₉) attributed to marine plants follow an identical δ¹³C trend (French et al., Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014, fig. 7). The isorenieratane profile, a biomarker for anaerobic phototrophic green sulfur bacteria, provides evidence of reducing conditions developing during the initial phase of the T-OAE, peaking at the time of deposition of ‘bed’ 40 (Millstones); chlorobactane and okenane (not shown) display identical patterns (French et al., Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014, fig. 5). Amorphous organic matter (AOM) and algal cysts dominate (50 – >90%) the palynological assemblages (after Slater et al., Reference Slater, Twitchett, Danise and Vajda2019) of the high-TOC anoxic – euxinic facies. Peak species richness of calcareous nannofossils, preserved as carbonate and external moulds in organic matter throughout the section (Slater et al., Reference Slater, Bown, Twitchett, Danise and Vajda2022), occurs in Whale Stones ‘bed’ 35, together with a pulse of prasinophyte algae and dense granular organic matter (Houben et al., Reference Houben, Goldberg and Slomp2021). Nitrogen isotopes (δ¹⁵N_tot) display low values attributable to enhanced N₂ fixation by cyanobacteria in a strongly redox-stratified marine environment (Wang et al., Reference Wang, Ossa, Spangenberg and Schoenberg2021). Toarcian open ocean seawater field derived from Tethyan sections (Section 16.a.1). The δ¹⁵N_tot profile (Jenkyns et al., Reference Jenkyns, Gröcke and Hesselbo2001) broadly follows TOC. A – D mark coincident sharp falls in δ¹³C_org (after DB Kemp et al., Reference Kemp, Coe, Cohen and Schwark2005; Cohen et al., Reference Cohen, Coe and Kemp2007) and δ⁹⁸Mo, as noted by Kemp et al. (Reference Kemp, Coe, Cohen and Weedon2011) – see Figure 20. Biotic extinction levels ii and iii after Caswell et al. (Reference Caswell, Coe and Cohen2009); base of trace fossil absent interval follows Caswell & Herringshaw (Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023).

The molecular C-isotope records show a more marked step down to lower values within the T-OAE than the bulk organic records, and within the CIE compound-specific δ¹³C values are relatively stable compared to the δ¹³C_org curve, which has a marked concave profile (Fig. 21). By comparison, matched δ¹³C datasets from other Toarcian sections in Spain and China show good agreement in trends between n-alkane, bulk organic and carbonate carbon isotope records (e.g. Ruebsam et al., Reference Ruebsam, Reolid and Schwark2020d, fig. 2). Offsets in the Yorkshire profiles may perhaps be, in part, an artefact of the low sampling resolution of the organic geochemical data (French et al., Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014). Alternatively, short-distance migration of the bitumen fraction, which is mobile and can move in the pore space, may have modified the trend in the compound-specific δ¹³C data. Changing organic matter composition between successive bulk samples may also play a role.

Organic matter source mixing will affect the trend and amplitude of the bulk organic δ¹³C excursion. The Yorkshire Pliensbachian – middle Toarcian succession displays large variation in the proportions of marine to terrestrial organic matter (Bucefalo Palliani et al., Reference Bucefalo Palliani, Mattioli and Riding2002; French et al., Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014; Slater et al., Reference Slater, Twitchett, Danise and Vajda2019). Calculated percentages of terrigenous organic matter based on linear regression of petrographic measurements of terrigenous organic matter and Rock-Eval Hydrogen Index (HI; French et al., Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014) indicate a fall from ∼80% at the top of Subunit IIb, immediately below T-OAE Unit III, to <20% in the TOC maximum in Whale Stones ‘bed’ 35 at the top of T-OAE Subunit IIIb, rising to ∼40% at the base of Subunit IVa.

A similar trend is shown by the palynological count data of Slater et al. (Reference Slater, Twitchett, Danise and Vajda2019) with corresponding terrestrial palynomorph values of 80 – 50% in Subunit IIb, falling to 8% in IIIb, then rising to 10 – 20% in Unit IV. It is notable that the ‘marine’ fraction in Units III and IV is overwhelmingly dominated by amorphous organic matter, peaking at 90% towards the top of Subunit IIIb. The amorphous organic matter is assumed here to be of predominantly marine origin based on its close association with prasinophyte algae and high HI values, but may include a terrestrial component (cf. Tyson, Reference Tyson and Tyson1995)

The very large increase in the proportion of marine organic matter in the lower Toarcian and particularly within the T-OAE interval will increase the amplitude of the T-OAE CIE in the Yorkshire curve. Where matching data are available for samples (e.g. Suan et al., Reference Suan, van de Schootbrugge, Adatte, Fiebig and Oschmann2015; Ruebsam et al., Reference Ruebsam, Muller, Kovacs, Palfy and Schwark2018), the δ¹³C_org curve may be normalized using HI or carbon preference index (CPI; the odd over even predominance for long-chain n alkanes, n-C₂₄ to n-C₃₄) values. Suan et al. (Reference Suan, van de Schootbrugge, Adatte, Fiebig and Oschmann2015, fig. 5d) applied this method to the Yorkshire coast paired δ¹³C_org and HI data of Sælen et al. (Reference Sælen, Tyson, Telnaes and Talbot2000) and calculated an amplitude for the corrected negative excursion of 3 – 4‰ δ¹³C_org. This value is consistent with corrected δ¹³C_org, δ¹³C_phytane and δ¹³C_carb curves from two sections in SW Germany that are dominated by marine organic matter throughout the lower Toarcian (Suan et al., Reference Suan, van de Schootbrugge, Adatte, Fiebig and Oschmann2015).

Insufficient data are available to fully assess the impact of changes in organic matter composition on the fine structure of the Yorkshire δ¹³C_org profile. Palynological data do not show large sample-to-sample variation within the T-OAE (Slater et al., Reference Slater, Twitchett, Danise and Vajda2019) interval, and steps within the Yorkshire δ¹³C_org curve have been correlated to those in records from Bornholm (Denmark), Sancerre (Paris Basin), sections in the Polish Basin, and at Peniche (Lusitanian Basin) (Fig. 3; Hesselbo et al., Reference Hesselbo, Gröcke, Jenkyns, Bjerrum, Farrimond, Bell and Green2000; Hesselbo & Pieńkowski, Reference Hesselbo and Pieńkowski2011; Hermoso et al., Reference Hermoso, Minoletti, Rickaby, Hesselbo, Baudin and Jenkyns2012; Thibault et al., Reference Thibault, Ruhl, Ullmann, Korte, Kemp, Gröcke and Hesselbo2018, fig. 9; Fantasia et al., Reference Fantasia, Adatte, Spangenberg, Font, Duarte and Föllmi2019, fig. 12).

Most significantly, the presence of a large negative CIE in long-chain n-alkanes that parallels the terrestrial wood and bulk organic δ¹³C trends in the T-OAE interval of the Cleveland Basin (Figs 3, 21) and δ¹³C_carb trends at Peniche (Littler et al., Reference Littler, Hesselbo and Jenkyns2010, fig. 3) and elsewhere, confirms the synchronous ¹³C-depletion of the entire exchangeable carbon reservoir, in both the oceans and the atmosphere.

15.b. Biomarkers as palaeoredox and salinity proxies

Specific organic molecules in the geological record provide biomarkers for green sulfur-reducing bacteria that are confined to environments with available sunlight and euxinic conditions (e.g. Koopmans et al., Reference Koopmans, Koster, vanKaamPeters, Kenig, Schouten, Hartgers, deLeeuw and Sissingh Damsté1996; Grice & Eiserbeck, Reference Grice, Eiserbeck, Holland and Turekian2014). Their presence provides a signature for photic zone euxinia (PZE). Isorenieratane is the only uniquely defining molecule of green sulfur-reducing bacteria Chlorobiaceae, but its derivatives and certain other molecular compounds such as chlorobactane can also be used as proxies for the presence of these bacteria in the water column.

There is organic geochemical evidence for PZE during the T-OAE from multiple locations throughout Europe, including the UK, France, Germany, Hungary and Italy (Schouten et al., Reference Schouten, Van Kaam-Peters, Rijpstra, Schoell and Sinninghe Damsté2000; Pancost et al., Reference Pancost, Crawford, Magness, Turner, Jenkyns and Maxwell2004; Schwark & Frimmel, Reference Schwark and Frimmel2004; Van Breugel et al., Reference Van Breugel, Baas, Schouten, Mattioli and Sinninghe Damsté2006; Ruebsam et al., Reference Ruebsam, Muller, Kovacs, Palfy and Schwark2018; Xu et al., Reference Xu, Ruhl, Jenkyns, Leng, Huggett, Minisini, Ullmann, Riding, Weijers, Storm, Percival, Tosca, Idiz, Tegelaar and Hesselbo2018; Ajuaba et al., Reference Ajuaba, Sachsenhofer, Bechtel, Galasso, Gross, Misch and Schneebeli-Hermann2022; Burnaz et al., Reference Burnaz, Littke, Grohmann, Erbacher, Strauss and Amann2024). Bowden et al. (Reference Bowden, Farrimond, Snape and Love2006) reported isorenieratane derivatives in four samples from the Whitby Mudstone Formation at Port Mulgrave (‘beds’ 35, 37, 38 and 41) with additional records from ‘beds’ 31, 32 and 34 provided by Salem (Reference Salem2013), indicating that the photic zone was intermittently euxinic. Isorenieratane is also present in the high TOC intervals in each of the three Sulphur Bands (Salem, Reference Salem2013). Samples from Hawsker Bottoms confirm the presence of aromatic carotenoid biomarkers including isorenieratane from the Whitby Mudstone (Fig. 21), but samples also yield okenane, a biomarker derived from the red-coloured aromatic carotenoid okenone of purple sulfur bacteria (French et al., Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014). The highest recorded isorenieratane/TOC values occur at the top of Unit III at Hawsker Bottoms (Fig. 21).

Iron-rich, anoxic and potentially non-sulfidic (ferruginous) bottom-water conditions have been interpreted for lower Toarcian shales from the Cleveland Basin (Runswick Bay; Song et al., Reference Song, Littke and Weniger2017), based on biomarkers [low dibenzothiophene/phenanthrene ratios (< 0.2) and gammacerane indices (< 0.1)] and elemental composition (high total S and high S/TOC ratio). The abundance of small framboidal pyrite in these rocks (> 95% of < 7 μm diameter) suggests formation in the water column which depleted the free H₂S, leading to a low degree of sulfurization of organic matter. The reactive iron was supplied by a high terrigenous influx, which is also indicated by the biomarker data.

The aryl isoprenoid ratio (AIR) relates the proportions of low to high molecular weight aromatic compounds (C_13–17/C_18–22; Schwark & Frimmel, Reference Schwark and Frimmel2004). A high abundance of short-chain forms indicate intense aerobic degradation. AIR values of ≤0.5 are associated with persistent PZE (near year-round) and high values of 0.5 – 3 reflect short-term episodic PZE (possibly seasonal). Runswick Bay samples from the Jet Rock and Bituminous Shales have yielded AIR ratios of 2 – 3 indicative of episodic PZE (Song et al., Reference Song, Littke and Weniger2017), supporting the evidence provided by aromatic carotenoid biomarkers.

Caswell & Coe (Reference Caswell and Coe2014) plotted the better-characterized Toarcian AIR record from Dotternhausen (Schwark & Frimmel, Reference Schwark and Frimmel2004) against the Yorkshire succession as a palaeoredox proxy, but it is unwise to assume that redox changes were synchronous and of the same amplitude in the two basins. For example, levels with significant bioturbation, indicating temporary oxygenation of bottom waters, characterize the beds immediately below and immediately above the Oberer Stein at Dotternhausen (Röhl et al., Reference Röhl, Schmid-Röhl, Oschmann, Frimmel and Schwark2001). The equivalent interval in the Yorkshire coastal and core successions (Top Jet Dogger – Millstone interval) shows no evidence of oxygenation and biomarkers suggest peak PZE (Fig. 21; French et al., Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014).

Modern observations suggest that the presence of okenone would require an extremely shallow PZE and a highly restricted depositional environment. Based on the abundance of okenane and evidence of microbial wavy lamination in Yorkshire Toarcian shales (O’Brien, Reference O’Brien1990), French et al. (Reference French, Sepulveda, Trabucho-Alexandre, Gröcke and Summons2014) suggested that the okenane, and potentially chlorobactane and isorenieratane, may have formed in benthic microbial mats rather than a euxinic photic-zone water column. The biomarkers must, nonetheless, reflect the presence of photic zone anoxic – euxinic bottom waters.

Multiple salinity proxies derived from the organic geochemical data [e.g. gammacerane/(gammacerane + C₃₀-hopane) index] were interpreted as indicating normal marine salinity throughout the T-OAE in the Cleveland Basin by Song et al. (Reference Song, Littke and Weniger2017). These contradict the interpretation of strong brackish to freshwater conditions promoted by Remírez & Algeo (Reference Remírez and Algeo2020) based on element-ratio data (Section 13.d). However, gammacerane is considered to originate from tetrahymanole produced by ciliates, whose habitat is in the anoxic part of the water column beneath the pycnocline, so gammacerane may be better regarded as a stratification parameter (cf. Sinninghe Damsté et al., Reference Sinninghe Damsté, Kenig, Koopmans, Köster, Schouten, Hayes and de Leeuw1995) rather than a salinity indicator. Evidence of bacterial sources of tetrahymanole (Banta et al., Reference Banta, Wei and Welander2015) further complicate the use of the gammacerane proxy.

Other organic geochemical proxies (e.g. low ratio of steranes/hopanes, < 1.0; abundant long-chain n-alkanes) were interpreted by Song et al. (Reference Song, Littke and Weniger2017) to indicate a significantly higher input of terrigenous organic matter with more bacteria and/or cyanobacteria in Yorkshire compared to Toarcian black shale sections in the Netherlands, Germany and Luxemburg. These likely constitute components of the amorphous organic matter that dominates the palynofacies within the T-OAE interval (Section 19.b.2).

A comprehensive study of biomarkers in a core from southern Luxemburg (NE Paris Basin) by Ruebsam et al. (Reference Ruebsam, Mattioli and Schwark2022) indicates that the marine phytoplankton community structure during the T-OAE was dominated by eukaryotic algae, with a decrease in the proportion of prokaryotic bacterial biomass compared to the pre- and post-excursion intervals. A dominance of algae in the phytoplankton community during the T-OAE in the Cleveland Basin is similarly evidenced by palynological data (Section 19.b.2.).

Five distinct palaeoecological events within the interval of the δ¹³C_org minimum of the Paris Basin core each show a proliferation of opportunistic green algae coincident with a decline in red algae groups and shallow- and deep-dwelling calcareous nannoplankton (Ruebsam et al., Reference Ruebsam, Mattioli and Schwark2022). These events correlate to biomarker evidence [increasing ratio of tri- /di-MTTCs (methylated 2-methyl-trimethyltridecylchromans)] for episodes of surface-water freshening. These were interpreted to have been caused by higher precipitation and surface run-off accompanying climate warming, paced by changes in the short eccentricity (100 kyr) orbital cycle (Ruebsam et al., Reference Ruebsam, Mattioli and Schwark2022). To our knowledge, comparable data are not currently available from the Yorkshire succession.

16.a. Nitrogen and sulfur isotopes

16.b. Trace-metal isotopes – Mo, Tl, U, Os

The relatively long modern ocean residence times of Mo (440 – 800 ka), U (∼450 ka), Tl (∼20 ka) and Os (∼10 – 40 ka) compared to the ocean mixing time of ∼1 – 2 ka, combined with distinctive stable-isotope fractionation association with different marine sinks, means that the isotopic composition of these metals has potential to serve as global oceanic tracers (Kendall et al., Reference Kendall, Andersen, Owens, Ernst, Dickson and Bekker2021). Uranium and Mo isotope systems offer established tracers of global oceanic redox conditions, particularly for the extent of anoxic and euxinic seafloor, and Tl isotopes provide a proxy for the extent of well-oxygenated seafloor characterized by Mn oxyhydroxide burial (Kendall et al., Reference Kendall, Andersen, Owens, Ernst, Dickson and Bekker2021). Marine carbonaceous mudstones provide the principal archive for reconstructing palaeoredox histories from these isotope records. Osmium isotopes enable the identification of changes in weathering versus hydrothermal fluxes (Peucker-Ehrenbrink & Ravizza, Reference Peucker-Ehrenbrink, Ravizza, Gradstein, Ogg and Ogg2020) that might impact global productivity.

Variations in trace-metal isotope records from the Toarcian of Yorkshire have previously been attributed principally to local changes in water chemistry in the Cleveland Basin (Cohen et al., Reference Cohen, Coe, Harding and Schwark2004; McArthur et al., Reference McArthur, Algeo, van de Schootbrugge, Li and Howarth2008; Pearce et al., Reference Pearce, Cohen, Coe and Burton2008; Nielsen et al., Reference Nielsen, Goff, Hesselbo, Jenkyns, LaRowe and Lee2011; Dickson et al., Reference Dickson, Gill, Ruhl, Jenkyns, Porcelli, Idiz, Lyons and van den Boorn2017), but there is now increasing evidence that they incorporate a significant component of global ocean change (e.g. Them et al., Reference Them, Owens, Marroquín, Caruthers, Trabucho-Alexandre and Gill2022).

16.b.2. Thallium isotopes – assessing the extent of oxic seafloor area

Thallium isotopes offer further potential to constrain the geological evolution of seawater oxygenation on a basin to a global scale (Nielsen et al., Reference Nielsen, Rehkämper and Prytulak2017; Ostrander et al., Reference Ostrander, Owens and Nielsen2017; Owens et al., Reference Owens, Nielsen, Horner, Ostrander and Peterson2017; Owens, Reference Owens2019). Thallium isotopes are reported relative to Tl metal standard SRM997 using the epsilon notation where:

$${{\rm{\varepsilon }}^{{\rm{205}}}}{\rm{Tl }} = {\rm{ [}}{\left( {^{{\rm{205}}}{\rm{Tl}}{{\rm{/}}^{{\rm{203}}}}{\rm{Tl}}} \right)_{{\rm{sample}}}}{\rm{/}}{\left( {^{{\rm{205}}}{\rm{Tl}}{{\rm{/}}^{{\rm{203}}}}{\rm{Tl}}} \right)_{{\rm{SRM9971}}}} - {\rm{1]}} \times {10^4}$$

Thallium is supplied to the oceans by rivers, mineral aerosols (dust), volcanic gases and particles and high-temperature hydrothermal fluids, all of which have similar ε²⁰⁵Tl values of around −2 and pore-water fluxes from continental-margin sediments ε²⁰⁵Tl with a value of around 0. The only two important marine Tl sinks are adsorption by authigenic phases, principally Mn oxyhydroxides in pelagic clays, with ε²⁰⁵Tl values of ∼+8 to +12 and uptake of Tl during low-temperature alteration of oceanic crust with an ε²⁰⁵Tl value of −7.2. For short-term ≤1 Ma redox events it is considered that changes in the Tl isotopic composition of seawater are driven principally by changing rates of Mn-oxyhydroxide burial (Owens et al., Reference Owens, Nielsen, Horner, Ostrander and Peterson2017), since changes in the rate of oceanic crust alteration will operate on longer time scales.

The relatively short modern ocean residence time for Tl of ∼20 ka, which is nonetheless one order of magnitude longer than the ocean mixing time, ensures a uniform ε²⁰⁵Tl value of −6.0 ± 0.6 in modern open-ocean seawater (Nielsen et al., Reference Nielsen, Rehkämper and Prytulak2017; Owens et al., Reference Owens, Nielsen, Horner, Ostrander and Peterson2017; Owens, Reference Owens2019). Mn-oxyhydroxides that are ubiquitous in sediments deposited from oxygenated bottom waters, combined with the detrital mineral fraction, generate ε²⁰⁵Tl signatures in bulk sediments and partial extracts that are generally isotopically heavier than ambient seawater (e.g. Wang et al., Reference Wang, Lu, Costa and Nielsen2022, but see discussion therein). Oxyhydroxides are absent under anoxic or euxinic conditions because the reduction of insoluble Mn (IV) to more soluble Mn (II) and Mn (III) occurs when oxygen is removed from the water in contact with the sediment. In this case, the ε²⁰⁵Tl composition from the oxic part of the water column has been shown to transfer quantitatively to the authigenic fraction (principally Fe sulfides) in sediments underlying euxinic (Owens et al., Reference Owens, Nielsen, Horner, Ostrander and Peterson2017) or highly reducing anoxic bottom waters (Fan et al., Reference Fan, Nielsen, Owens, Auro, Shu, Hardisty, Horner, Bowman, Young and Wen2020), which may then provide a record of the isotopic evolution of the surface ocean. In partially and predominantly restricted basins ε²⁰⁵Tl values vary between open-ocean (−6) and continental material (−2), scaling with the degree of restriction, as exemplified by the Cariaco Basin (−5.4 – weak restriction) and the Black Sea (−2.3 – strong restriction) (Owens et al., Reference Owens, Nielsen, Horner, Ostrander and Peterson2017).

Thallium isotope data in early diagenetic pyrite from the upper beds of the T-OAE (top ‘bed’ 32 – 39) at Port Mulgrave were published by Nielsen et al. (Reference Nielsen, Goff, Hesselbo, Jenkyns, LaRowe and Lee2011). ε²⁰⁵Tl values show a large range from −8 to −2 (Fig. 20). Thallium isotope data for authigenic Tl, based on bulk rock extracts from the upper Pliensbachian – middle Toarcian of 3 sections from the Western Canada Sedimentary Basin, Alberta, Canada and SW German Basin (Dotternhausen) were presented by Them et al. (Reference Them, Gill, Caruthers, Gerhardt, Gröcke, Lyons, Marroquin, Nielsen, Alexandre and Owens2018). The Canada profiles, obtained from sections that were subject to consistent anoxic – euxinic conditions throughout the study interval (based on Fe_HR/Fe_T and Fe_Py/Fe_HR Fe-speciation proxies) yielded ε²⁰⁵Tl values close to modern seawater of −6 through the upper Pliensbachian. They then rise upward from around the bottom of the base Toarcian D. tenuicostatum Zone equivalent, to −2 at the onset of the T-OAE. ε²⁰⁵Tl falls back to a broad trough of −4 mid-T-OAE, with a second peak of −2 at the top of the T-OAE and then falls upward towards −4 in the H. bifrons equivalent Zone. A similar trend and values were observed in the SW German Basin Toarcian profile (Them et al., Reference Them, Gill, Caruthers, Gerhardt, Gröcke, Lyons, Marroquin, Nielsen, Alexandre and Owens2018, fig. 3).

The increase in ε²⁰⁵Tl at the Pliensbachian – Toarcian boundary in the Western Canada Sedimentary Basin was interpreted by Them et al. (Reference Them, Gill, Caruthers, Gerhardt, Gröcke, Lyons, Marroquin, Nielsen, Alexandre and Owens2018) to mark the onset of increasing global bottom-water anoxia that preceded the T-OAE by 600 ka and was sustained into the middle Toarcian H. bifrons Zone, representing an interval of ∼2 Ma. Rising values were attributed to a global reduction in Fe–Mn oxyhydroxide precipitation accompanying oxygen depletion, with an accompanying decrease in ²⁰⁵Tl output flux leading to rising ε²⁰⁵Tl seawater values. Authigenic Tl concentrations show no systematic variation throughout the section (Them et al., Reference Them, Gill, Caruthers, Gerhardt, Gröcke, Lyons, Marroquin, Nielsen, Alexandre and Owens2018), which suggests the marine inventory was not depleted due to increased anoxic conditions.

Thallium isotopes from the T-OAE interval in Yorkshire show a wider range and extend to lower values (ε²⁰⁵Tl −8 to −2; Nielsen et al., Reference Nielsen, Goff, Hesselbo, Jenkyns, LaRowe and Lee2011) compared to Toarcian seawater values interpreted from the successions in western Canada and Germany (ε²⁰⁵Tl −4 to −2; Them et al., Reference Them, Gill, Caruthers, Gerhardt, Gröcke, Lyons, Marroquin, Nielsen, Alexandre and Owens2018). Shifts to values below those of open ocean water (Fig. 20) might be attributed to short-term basin-scale Mn-oxyhydroxide burial events in oxic areas of the Cleveland Basin margin driving local episodes of ²⁰⁵Tl drawdown from the local water mass.

Interestingly, the increase of ε²⁰⁵Tl in the lower Toarcian profile in Canada and to some extent in Yorkshire (Fig. 20), is matched by an increasing δ³⁴S_CAS trend in the Cleveland Basin (Fig. 19) but also in Italy (Monte Sorgena) and southern Tibet (Han et al., Reference Han, Hu, He, Newton, Jenkyns, Jamieson and Franceschi2022, Reference Han, Hu, Newton, He, Mills, Jenkyns, Ruhl and Jamieson2023). Broadly parallel trends for thallium and sulfur isotopes are a characteristic feature of the long-term Cenozoic record (Nielsen et al., Reference Nielsen, Rehkämper and Prytulak2017).

Finally, it should be noted that the comparison figure of ε²⁰⁵Tl vs δ⁹⁸Mo at Port Mulgrave by Nielsen et al. (Reference Nielsen, Goff, Hesselbo, Jenkyns, LaRowe and Lee2011, fig. 5) incorrectly plots the position of the Mo dataset relative to the stratigraphy, as presented by Pearce et al. (Reference Pearce, Cohen, Coe and Burton2008, fig. 2). The correctly aligned stratigraphic profiles are shown in Figure 20. These do not support an anti-correlation between these two isotope systems, as proposed by Nielsen et al. (Reference Nielsen, Goff, Hesselbo, Jenkyns, LaRowe and Lee2011) and modelled by Owens et al. (Reference Owens, Nielsen, Horner, Ostrander and Peterson2017).

16.c. Strontium isotopes

The strontium (Sr) isotope composition of seawater essentially represents a mixture of unradiogenic Sr from the hydrothermal alteration of mid-ocean ridge basalt (⁸⁷Sr/⁸⁶Sr ≅ 0.703) and radiogenic Sr in rivers (≅ 0.711) from the weathering of ancient continental crust (McArthur et al., Reference McArthur, Howarth, Shields, Zhou, Gradstein, Ogg, Schmitz and Ogg2020). Modern seawater has an ⁸⁷Sr/⁸⁶Sr value of 0.709174 ± 0.000002. A record of ancient seawater ⁸⁷Sr/⁸⁶Sr ratios may be obtained from the analysis of biogenic calcite in marine fossils including belemnites, brachiopods and foraminifera.

⁸⁷Sr/⁸⁶Sr ratio data obtained from Yorkshire coast belemnites (Jones et al., Reference Jones, Jenkyns and Hesselbo1994; McArthur et al., Reference McArthur, Donovan, Thirlwall, Fouke and Mattey2000) underpin the construction of the upper Pliensbachian – lower Toarcian global Sr-isotope reference curve of McArthur et al. (Reference McArthur, Howarth, Shields, Zhou, Gradstein, Ogg, Schmitz and Ogg2020). The stratigraphic profile (Fig. 19) shows falling values through the A. margaritatus Zone (Staithes Sandstone and Cleveland Ironstone Penny Nab Member; Subunits Ia – b), the continuation of a long-term trend beginning in the early Sinemurian (∼200 Ma). A step fall in ⁸⁷Sr/⁸⁶Sr ratios at the base of the P. spinatum Zone (Kettleness Member; Subunit Ic), together with possible steps in δ¹⁸O_bel and Mg/Ca_bel ratio (Fig. 19), support the presence of a large stratigraphic gap at the disconformity. The Sr-isotope trend through the P. spinatum Zone is less well defined but shows generally falling-upward values, with a reversal to gently rising values at the base of the PlToBE, towards the top of the zone. This pattern represents one of the most rapid reversals in ⁸⁷Sr/⁸⁶Sr of the Phanerozoic record (McArthur et al., Reference McArthur, Howarth, Shields, Zhou, Gradstein, Ogg, Schmitz and Ogg2020, fig. 7.2).

A marked change to steeply rising ⁸⁷Sr/⁸⁶Sr ratios is observed through the C. exaratum Subzone (Jet Rock; Subunits IIIb – d) of the T-OAE interval (Fig. 19), prior to a return to more gently increasing values through the remainder of the Toarcian. The change at the base of the C. exaratum Subzone has been ascribed to an increase in the flux of radiogenic Sr from the chemical weathering of continental crust because it coincides with a marked excursion in the seawater osmium (Cohen & Coe, Reference Cohen and Coe2007; Section 16.b.4) and other weathering proxies. The increase in chemical weathering has been linked to an increase in average temperatures across the T-OAE (Fig. 19, Section 21.d.7) and an accelerated hydrological cycle (Cohen et al., Reference Cohen, Coe, Harding and Schwark2004; Them et al., Reference Them, Gill, Selby, Gröcke, Friedman and Owens2017b; Izumi et al., Reference Izumi, Kemp, Itamiya and Inui2018).

McArthur et al. (Reference McArthur, Donovan, Thirlwall, Fouke and Mattey2000, Reference McArthur, Steuber, Page and Landman2016) have suggested that the increase in the Yorkshire profile (Fig. 19) is attributable to a decrease in the sedimentation rate driven by condensation and hiatuses in the C. exaratum Subzone (Subunits IIIb – d). Condensation at this level based on changes in the ⁸⁷Sr/⁸⁶Sr profile was also indicated by Jenkyns et al. (Reference Jenkyns, Jones, Gröcke, Hesselbo and Parkinson2002, fig. 13). Indeed, the sharp break in slope of the ⁸⁷Sr/⁸⁶Sr at the base of the P. spinatum Zone (Fig. 19) coincides with the disconformity at the base Pecten Seam in coastal sections (Section 8.a.2). However, as noted by Cohen et al. (Reference Cohen, Coe, Harding and Schwark2004), an a priori assumption that ⁸⁷Sr/⁸⁶Sr ratios ratio must always change linearly with time is hard to justify in view of the reversal of the seawater Sr-isotope curve in the latest Pliensbachian ∼1 Ma earlier.

Nonetheless, the presence of prominent carbonate concretion horizons throughout the laminated black carbonaceous mudstones of the C. exaratum Subzone supports the presence of significant hiatuses (cf. Raiswell, Reference Raiswell and Marshall1987, Reference Raiswell1988; Marshall & Pirrie, Reference Marshall and Pirrie2013) and an overall reduction in bulk sedimentation rates. The interval showing an increased slope in the Sr-isotope profile is bounded by the Cannon Ball Doggers at the bottom and the Millstones at the top of the subzone (Fig. 19).

19.a. Macrofauna and trace fossils

The Yorkshire coastal succession provides one of the most detailed published palaeontological records of the upper Pliensbachian – middle Toarcian. Howarth (Reference Howarth1955, Reference Howarth1962, Reference Howarth1973, Reference Howarth1992) published comprehensive ammonite and associated faunal records based on bed-by-bed collecting of the coastal sections. Further macrofossil records and a species-level range chart for the Cleveland Basin were produced by Little (Reference Little1995) and presented by Little (Reference Little, Ryder, Fastovsky and Gartner1996), Little & Benton (Reference Little and Benton1995), Harries & Little (Reference Harries and Little1999) and, in part, by Wignall et al. (Reference Wignall, Newton and Little2005). Additional records and a revised range chart were published by Caswell et al. (Reference Caswell, Coe and Cohen2009), with further collection reported by Danise et al. (Reference Danise, Twitchett, Little and Clémence2013) and Atkinson et al. (Reference Atkinson, Little and Dunhill2023). The trace-fossil record has been described by Morris (Reference Morris1979), Martin (Reference Martin and McIlroy2004), Caswell & Frid (Reference Caswell and Frid2017), Caswell & Dawn (Reference Caswell and Dawn2019) and Caswell & Herringshaw (Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023). The last of these studies included data from the Dove’s Nest core.

The upper Pliensbachian of the Cleveland Basin (Unit I - Staithes Sandstone and Cleveland Ironstone formations) yields rich trace-fossil and high-diversity benthic body-fossil assemblages, with most species being evenly represented and including a wide range of ecological groups that occupy all levels of benthic tiering (Little, Reference Little1995; Caswell et al., Reference Caswell, Coe and Cohen2009; Danise et al., Reference Danise, Twitchett, Little and Clémence2013; Caswell & Frid, Reference Caswell and Frid2017; Atkinson et al., Reference Atkinson, Little and Dunhill2023; Caswell & Herringshaw, Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023), indicative of well-oxygenated conditions.

The extinction of Amaltheidae family ammonites marking the base of the Toarcian occurs at Sulphur Band 1 (Littler et al., Reference Littler, Hesselbo and Jenkyns2010), a short distance above the base of the Whitby Mudstone Formation (Sections 7.b, 8.b). This is followed by a sharp drop in the taxonomic richness of trace fossils (Fig. 22; Caswell & Frid, Reference Caswell and Frid2017; Caswell & Herringshaw, Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023) but with no clear decline in benthic macrofossil diversity (Danise et al., Reference Danise, Twitchett, Little and Clémence2013; Caswell & Frid, Reference Caswell and Frid2017; Atkinson et al., Reference Atkinson, Little and Dunhill2023). A first extinction horizon (level i, Fig. 22; extinction step 2 of Harries & Little, Reference Harries and Little1999; ii of Caswell & Herringshaw, Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023), which is followed by an influx of the low-oxygen specialist bivalve Pseudomytiloides dubius (Sowerby), was placed at the base of Sulphur Band 2 (Hawsker Bottoms ‘bed’ 2) by Caswell et al. (Reference Caswell, Coe and Cohen2009).

Figure 22. TOC, biotic trends, extinction levels and palaeoredox change through the upper Pliensbachian – middle Toarcian of the Cleveland Basin. Yorkshire coast stratigraphy as in Figure 2. Rescaled whole-rock TOC profile for Dove’s Nest (Trabucho-Alexandre et al., Reference Trabucho-Alexandre, Gröcke, Atar, Herringshaw and Jarvis2022, dark green) with coast composite data of Kemp et al. (Reference Kemp, Coe, Cohen and Weedon2011; thin yellow-green high-resolution curve) and trend of the low-resolution coast datasets of Ruvalcaba Baroni et al. (Reference Ruvalcaba Baroni, Pohl, van Helmond, Papadomanolaki, Coe, Cohen, van de Schootbrugge, Donnadieu and Slomp2018) and McArthur (Reference McArthur2019) (thin pale green low-resolution curve; see Fig. 2). Ranges of low-oxygen specialist bivalve taxa compiled from Little (Reference Little1995) and Caswell et al. (Reference Caswell, Coe and Cohen2009). Trace-fossil taxonomic richness from Caswell & Frid (Reference Caswell and Frid2017), Caswell & Dawn (Reference Caswell and Dawn2019) and Caswell & Herringshaw (Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023). Macrofaunal diversity after Danise et al. (Reference Danise, Twitchett, Little and Clémence2013, Reference Danise, Twitchett and Little2015). Profiles rescaled to reference stratigraphy (Fig. 2) using subzone and marker bed datum levels. Extinction levels (i) – (iii) from Caswell et al. (Reference Caswell, Coe and Cohen2009). The boundary between pre-extinction and post-extinction survival intervals of Atkinson et al. (Reference Atkinson, Little and Dunhill2023) lies at the base of the C. exaratum Subzone (level iii). The top of the ‘survival interval’, the base of recovery phase 1, occurs in the lower H. bifrons Zone above the top of our study interval. Geochemical palaeoredox interpretations from this study (see text). Climate interpretation incorporates palynological interpretation of Slater et al. (Reference Slater, Twitchett, Danise and Vajda2019) with oxygen isotope and Mg/Ca trends from belemnites (Fig. 21).

The lower Toarcian Grey Shale (Subunit IIa) initially yields a sparse normal marine benthic and nektonic fauna reflecting continuing sedimentation in a relatively well-oxygenated environment but with increasingly dysoxic, intermittently anoxic (Sulphur Bands 1 – 3) seafloor conditions indicated by reduced species richness and the changing characteristics of trace-fossil assemblages. An interval of greater oxygen depletion is evidenced in the lower part of Subunit IIb, following step increases in TOC and P_EF, by an interval barren of trace fossils and characterized by low body-fossil diversity (Fig. 22; see also Caswell & Herringshaw, Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023). Geochemical redox proxies (e.g. Figs 15, 16) generally fail to capture evidence of this event. Trace-fossil and benthic diversity temporarily recover towards the top of Subunit IIb. Disappearances of typical upper Pliensbachian benthic macrofossil taxa then occur progressively, beginning towards the top of the Grey Shale within T-OAE Subunit IIIa through ‘beds’ 31 – 32 (Caswell et al., Reference Caswell, Coe and Cohen2009, fig. 2).

The onset of a major phase of extinction is placed at the base of the consistently laminated black carbonaceous mudstones (black shale facies) in mid-‘bed’ 31 (level ii of Caswell et al., Reference Caswell, Coe and Cohen2009), with the main extinction horizon (level iii) occurring around 2 m higher, coincident with the first large step-fall in the δ¹³C_org profile (upper ‘bed’ 32, A in Fig. 18; DB Kemp et al., Reference Kemp, Coe, Cohen and Schwark2005). Trace fossils disappear at level ii and, with the sole exception of a record of pyritized Trichichnus in ‘bed’ 48 at Saltwick Bay, remain unrecorded from all Yorkshire sections until the reappearance of an impoverished assemblage in the middle Toarcian Main Alum Shale Beds (Subunit Vc), towards the base of ‘bed’ 51 (Caswell & Herringshaw, Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023). This stratigraphically extended absence of macroscopic trace fossils indicates a longer period of continuous deoxygenation than elsewhere in NW Tethys and the adjacent European shelf basins.

A steady decline in macrofaunal diversity is observed between datum levels ii and iii (Fig. 22; Danise et al., Reference Danise, Twitchett, Little and Clémence2013). The bivalve Bositra radiata (Goldfuss) dominates the benthic fauna of the extinction interval, with shell size doubling as δ¹³C_org decreases within Subunit IIIa. Pseudomytiloides dubius is also common at some levels. Pseudomytiloides dubius and B. radiata occur as parautochthonous – autochthonous assemblages, respectively, forming 1 – 2-shell-thick monospecific shell pavements (Little, Reference Little1995; Caswell et al., Reference Caswell, Coe and Cohen2009; Caswell & Coe, Reference Caswell and Coe2013). The two low-oxygen specialist taxa rarely co-occur.

Pseudomytiloides dubius is the only abundant and consistently occurring benthic macrofossil in the laminated black shales of Subunits IIIb – Va inclusive (C. exaratum – H. falciferum subzones), evidencing the development of strongly oxygen-depleted bottom waters and anoxic – euxinic sediments within the lower part of the ‘survival interval’, following the extinctions (Fig. 22). Sporadic occurrences of the scallop Meleagrinella substriata (Munster) is the only other taxon to occur consistently through the Jet Rock (Caswell et al., Reference Caswell, Coe and Cohen2009).

Bositra radiata temporarily reappears in T-OAE Subunit IIId immediately following the reappearance of Oxytoma inequivalve (J. Sowerby) along with the lowest occurrences and temporary appearances of new bivalve species and inarticulate brachiopods towards the top of Subunit IIIc (Caswell et al., Reference Caswell, Coe and Cohen2009). These define a short interval of increased benthic diversity (Fig. 22) in the upper C. exaratum Subzone at the termination of the T-OAE. The temporary influx of a more diverse faunal assemblage coincides with a shift to intermittent dysoxic bottom waters based on the TOC/P_T and DOP_T proxies (Fig. 15) and decreasing basin restriction based on Re and Mo isotopes (Fig. 20).

Remarkably, records of P. dubius almost perfectly track the TOC curve, only occurring where values exceed the 2.5% anoxic threshold (Fig. 22). The species is largely absent from the upper Bituminous Shales Subunit Va but reappears in association with the higher-TOC beds that characterize the Hard Shales Subunit Vb, before disappearing again above. Benthic diversity increases significantly in the lower part of Subunit Va and generally remains high in Subunits Vb – c, above. Increases in benthic diversity and the re-establishment of a Toarcian infauna after the T-OAE were gradual. Diversity does not return to the level observed in the pre-T-OAE section until the upper Toarcian Phlyseogrammoceras dispansum Zone (Atkinson et al., Reference Atkinson, Little and Dunhill2023). The absence and then low abundance and taxonomic richness of trace fossils in the D. commune Subzone suggest that dissolved oxygen remained below 0.5 mL L⁻¹ into the middle Toarcian (Caswell & Dawn, Reference Caswell and Dawn2019; Caswell & Herringshaw, Reference Caswell, Herringshaw, Nawrot, Dominici, Tomašových and Zuschin2023), consistent with evidence of persisting dysoxia provided by the TOC/P_T and DOP_T redox proxies (Fig. 14).

Ammonites and belemnites occur throughout the upper Pliensbachian – middle Toarcian succession in Yorkshire, including the T-OAE interval. A ‘belemnite gap’ was proposed in T-OAE Subunits IIIa – IIIb from mid-‘bed’ 32 to lower ‘bed’ 34 in Yorkshire coastal sections by Xu et al. (Reference Xu, Ruhl, Jenkyns, Leng, Huggett, Minisini, Ullmann, Riding, Weijers, Storm, Percival, Tosca, Idiz, Tegelaar and Hesselbo2018) based on literature data. However, McArthur et al. (Reference McArthur, Donovan, Thirlwall, Fouke and Mattey2000) recorded belemnites from every ‘bed’ in T-OAE Unit III except ‘beds’ 33 (Cannon Ball Doggers) and 40 (Millstones). The ‘belemnite gap’ in the Cleveland Basin coincides with a turnover in belemnite taxa characterized by the disappearance of Passaloteuthis and the appearance of Acrocoelites in the Cleveland Basin (Ullmann et al., Reference Ullmann, Thibault, Ruhl, Hesselbo and Korte2014), attributed as a response to the progressive expansion and shoaling of euxinic bottom waters during the initiation of the T-OAE. Remarkably, Acrocoelites are relatively common in the upper beds of the T-OAE interval (mid- to upper C. exaratum Subzone, e.g. Ullman et al., Reference Ullmann, Thibault, Ruhl, Hesselbo and Korte2014, fig. 2) despite evidence of increasing episodes of PZE (Section 15b). It has been inferred that this turnover represents belemnites adapting to environmental change by shifting their habitat from cold bottom waters to warm surface waters in response to expanded seafloor anoxia.

Stratigraphically more extended ‘belemnite gaps’ but continuous ammonite records have been documented in the lower Toarcian at Mochras and Peniche, which were attributed to unfavourable redox conditions in the deeper water column (Hesselbo et al., Reference Hesselbo, Jenkyns, Duarte and Oliveira2007; Xu et al., Reference Xu, Ruhl, Jenkyns, Leng, Huggett, Minisini, Ullmann, Riding, Weijers, Storm, Percival, Tosca, Idiz, Tegelaar and Hesselbo2018, fig. 4), reflecting the nektobenthic lifestyle of the former taxon (Hoffmann & Stevens, Reference Hoffmann and Stevens2020). However, neither of these sections show palaeontological or geochemical evidence of extended anoxic – euxinic bottom-water conditions during the T-OAE. Bioturbation and low-diversity benthonic macrofauna are present throughout the sections, sediment TOC contents generally remain significantly below the 2.5% anoxic threshold and redox-sensitive trace metals show no enrichment (e.g. Fantasia et al., Reference Fantasia, Adatte, Spangenberg, Font, Duarte and Föllmi2019). Rare thin laminated beds with biomarkers that suggest transient PZE and episodic anoxia at the seafloor occurred (Xu et al., Reference Xu, Ruhl, Jenkyns, Leng, Huggett, Minisini, Ullmann, Riding, Weijers, Storm, Percival, Tosca, Idiz, Tegelaar and Hesselbo2018), like those associated with the lower Toarcian Sulphur Bands of Yorkshire, but the evidence supports predominantly oxic – dysoxic bottom-water conditions throughout the T-OAE. It is unlikely, therefore, that water column oxygenation provided the main control on belemnite occurrences at Mochras and Peniche, given that belemnites remain present in association with bottom-water euxinia in the Cleveland Basin.

Water temperature and salinity may have played a role. Belemnites are considered to favour temperatures of 10 – 30° C and salinities of 27 – 37 psu (Hoffmann & Stevens, Reference Hoffmann and Stevens2020), and have been postulated to have migrated into warm surface waters during the T-OAE in response to deep water anoxia (Ullmann et al., Reference Ullmann, Thibault, Ruhl, Hesselbo and Korte2014). Ruebsam et al. (Reference Ruebsam, Pienkowski and Schwark2020) reported tropical SSTs of 32° C on the NW Tethyan shelf during peak hyperthermal conditions at the height of the T-OAE, with southern areas dominated by a warm equatorial Tethyan current. More northerly areas were influenced by cool lower salinity Boreal arctic surface waters (Section 18).

If water temperatures, and perhaps salinity, in southern areas, exceeded the tolerance of belemnites, they would have been temporarily excluded until temperatures declined following the climate optimum. It is notable that brachiopods are also temporarily absent from the T-OAE interval of many Tethyan sites, despite evidence for the continual presence of oxygenated bottom waters (García Joral et al., Reference García Joral, Gómez and Goy2011; Danise et al., Reference Danise, Clemence, Price, Murphy, Gomez and Twitchett2019; Ullmann et al., Reference Ullmann, Boyle, Duarte, Hesselbo, Kasemann, Klein, Lenton, Piazza and Aberhan2020). The northerly location of the Cleveland Basin would have seen a greater influence of cool currents flowing through the Viking Corridor (Fig. 1), enabling belemnites to maintain their presence in the area’s surface waters throughout the early Toarcian hyperthermal, while being excluded from the anoxic – euxinic bottom waters that developed during the T-OAE.

It is significant that a marked increase in nekton diversity occurs immediately above the T-OAE interval in Yorkshire (Fig. 22, Unit IVa) with an influx of ammonites indicative of a better oxygenated upper water column and/or improved connection with open ocean water masses.

19.b. Plankton records

21.a. Late Pliensbachian – a well-oxygenated shallow-marine basin

The Staithes Sandstone and Cleveland Ironstone formations (A. margaritatus – P. spinatum zones) display the greatest lithological and geochemical variation in the study interval with short-term mineralogical and grain-size changes reflecting a shallow-water and high-energy epeiric setting. Deposition of the Staithes Sandstone (Subunit Ia) occurred above the storm wave-base, deepening to largely sub-storm wave-base conditions for the Cleveland Ironstone (Subunits Ib – c). The high silt – fine sand fraction of the Staithes Sandstone is reflected in high and variable Si/Al, Na/Al, Ti/Al and Zr/Al ratios (quartz, feldspar and heavy mineral proxies) with an overall decreasing upward (fining) trend. Low CIA values point to a dominance of physical weathering processes supplying sediment to the basin and cool climate conditions.

The Cleveland Ironstone Formation presents more stable geochemical profiles interspersed with large Fe, Mn and Mg peaks associated with sideritic ironstone horizons. Five stacked CU cycles are evidenced within the Cleveland Ironstone Penny Nab and Kettleness members (cf. Rawson et al., Reference Rawson, Greensmith and Shalaby1983; Powell, Reference Powell2010) from detrital proxy element ratios and CIA values. Cycle 2 terminates with the Avicula Seam, a prominent siderite ironstone. Subunit Ib in coarsening-upward cycle 3, spanning the Amaltheus gibbosus Subzone, incorporates high δ¹³C_org values corresponding to the late Pliensbachian CIE and terminates at a regional disconformity below the Pecten Seam, a series of beds with 3 ironstones in the core. This cryptic disconformity, at the base of the Kettleness Member (P. spinatum Zone), is represented by an unremarkable bedding plane in the core but is a prominent feature of its chemostratigraphic profiles. The coarsening-upward cycles are interpreted as shallowing-upwards parasequences (’sequences’ of Macquaker & Taylor, Reference Macquaker and Taylor1996); medium-term cycle stacking patterns correspond well to the sequences and inferred relative sea-level curve for the Cleveland Basin proposed by Hesselbo (Reference Hesselbo2008). The base P. spinatum Zone disconformity is interpreted to represent a sequence boundary and superimposed transgressive surface.

Persistent oxic bottom-water conditions during the late Pliensbachian throughout the deposition of Unit I are evidenced by low TOC, TOC/P_T and DOP_T values and low redox-sensitive trace metal (e.g. U, Mo) contents. This is supported by a high trace-fossil taxonomic richness and a high benthic macrofossil diversity that includes bivalves, gastropods, brachiopods and benthic crinoids indicating a well-oxygenated open-marine environment. A proximal basin setting is indicated by palynofacies comprising 70 – 90 % terrestrial palynomorphs. Assemblages are dominated by fern spores and bisaccate pollen indicative of a cool moist climate, with subordinate dinocysts (Slater et al., Reference Slater, Twitchett, Danise and Vajda2019), and low-abundance, low-diversity nannofossil assemblages representing marine plankton (Slater et al., Reference Slater, Bown, Twitchett, Danise and Vajda2022).

General cooling through the latest Pliensbachian is evidenced by belemnite calcite geochemistry: rising δ¹⁸O_bel and falling Mg/Ca_bel ratios. The presence of glendonites in the P. spinatum Zone of southern Germany (Ruebsam et al., Reference Ruebsam, Mayer and Schwark2019; Merkel & Munnecke, Reference Merkel and Munnecke2023) points to the episodic presence of cold bottom water masses in Europe. These have been associated with the establishment of a late Pliensbachian icehouse climate (Ruebsam et al., Reference Ruebsam, Mayer and Schwark2019; Ruebsam & Schwark, Reference Ruebsam, Schwark, Reolid, Duarte, Mattioli and Ruebsam2021; Nordt et al., Reference Nordt, Breecker and White2022), including a persistent northern-hemisphere cryosphere with the likely presence of a NE Siberia icecap and extensive permafrost. Evidence provided in support of this includes regionally distributed glaciogenic sediments (pebbly argillites, tillites and matrix-supported conglomerates), potential periglacial sediments, dropstones and glendonites in Arctic Russia (Ruebsam & Schwark, Reference Ruebsam, Schwark, Reolid, Duarte, Mattioli and Ruebsam2021, fig. 8).

21.b. Pliensbachian – Toarcian boundary event – precursor to the T-OAE

The interval of the Pliensbachian – Toarcian Boundary CIE (PlToBE) marks a major shift in the chemostratigraphic profiles, reflecting the transition from the Cleveland Ironstone Formation to finer-grained mudstones of the Grey Shale Member (D. tenuicostatum Zone) at the base of the Whitby Mudstone Formation. This represents a significant long-term change in depositional conditions in the Cleveland Basin accompanying sea-level rise and coincides with the onset of a cascade of global environmental changes during the early Toarcian.

Hallam (Reference Hallam1997) estimated a relative sea-level rise of ∼35 – 85 m during the early Toarcian based on the facies change from the Staithes Sandstone to the Mulgrave Shale. A second-order eustatic sea-level rise of up to 100 m beginning in the earliest Toarcian D. tenuicostatum Zone and reaching a maximum in the middle Toarcian upper H. bifrons Zone has been proposed (Hardenbol et al., Reference Hardenbol, Thierry, Farley, Jacquin, Graciansky, Vail, Graciansky, Hardenbol, Jacquin and Vail1998; Haq, Reference Haq2018) that flooded large parts of the European shelf and other epicontinental areas globally. A transition from shallow-water limestones and/or sandstones to deeper-water mudstones is observed in many regions including the Paris (Hermoso et al., Reference Hermoso, Minoletti and Pellenard2013) and SE France basins (Bodin et al., Reference Bodin, Fantasia, Krencker, Nebsbjerg, Christiansen and Andrieu2023), Middle and High Atlas Morocco (Ait-Itto et al., Reference Ait-Itto, Price, Ait Addi, Chafiki and Mannani2017), NE Siberia Russia (Zakharov et al., Reference Zakharov, Shurygin, Il’ina and Nikitenko2006) and Neuquén Basin Argentina (Al-Suwaidi et al., Reference Al-Suwaidi, Ruhl, Jenkyns, Damborenea, Manceñido, Condon, Angelozzi, Kamo, Storm, Riccardi and Hesselbo2022). Combined with the rising eustatic sea level, the collapse of the neritic carbonate factory in the earliest Toarcian halted carbonate mud export and caused sediment starvation in many basins previously dominated by carbonate-rich facies, as seen in the Central High Atlas Basin and elsewhere (Krencker et al., Reference Krencker, Fantasia, Danisch, Martindale, Kabiri, El Ouali and Bodin2020, Reference Krencker, Fantasia, El Ouali, Kabiri and Bodin2022; Bodin et al., Reference Bodin, Fantasia, Krencker, Nebsbjerg, Christiansen and Andrieu2023).

Step falls in detrital silicate and heavy mineral proxy element ratios at the facies change in the Cleveland Basin are matched by step increases in phyllosilicate proxies (K₂O, K/Al, Cs/Al and Rb/Al) and CIA values indicative of decreased sand and silt fractions and the change to fine mudstones with an illite-rich clay-mineral assemblage. A coincident step rise in TOC may in-part reflect enhanced organic matter preservation via adsorption to the increased number of clay mineral surfaces (cf. Kennedy et al., Reference Kennedy, Pevear and Hill2002).

21.c. Earliest Toarcian – sea-level rise, oxygen depletion and climate warming

Following the PlToBE, the Grey Shale of the P. paltum to lower D. semicelatum subzones (Unit II) shows more uniform mineralogy and geochemistry compared to the underlying Pliensbachian reflecting a deeper water environment, likely close to the limit of storm wave-base (typically 15 – 40 m depth), established following sea-level rise. Detrital proxies (Ti/Al ratio, quartz and silt content) display variation but no long-term trend following their marked fall within the interval of the PlToBE. A third thin interval of laminated black shales in the lower D. clevelandicum Subzone, Sulphur Band 3 (SB3), evidences a further short-lived episode of temporary bottom-water anoxia. This occurs above a minor CU cycle with rising K/Al ratios pointing to an increase in the proportion of illitic clays. SB3 marks a further significant environment shift, reflected by the Subunit IIa – b boundary, with a step change to higher TOC and amorphous organic matter contents, geochemical evidence of increasingly dysoxic bottom waters (e.g. rising DOP_T values), and the temporary disappearance of trace fossils.

A climate shift to a warming trend through the earliest Toarcian is indicated by falling δ¹⁸O_bel and rising Mg/Ca_bel ratios from the Yorkshire coast (Section 18), consistent with brachiopod oxygen-isotope data from Portugal (Suan et al., Reference Suan, Mattioli, Pittet, Mailliot and Lecuyer2008a; Müller et al., Reference Müller, Jurikova, Gutjahr, Tomasovych, Schlogl, Liebetrau, Duarte, Milovsky, Suan, Mattioli, Pittet and Eisenhauer2020) and TEX₈₆ results from Spain and Italy (Ruebsam et al., Reference Ruebsam, Reolid, Sabatino, Masetti and Schwark2020c). Palynological trends include a rising upward proportion of Chasmatosporites spp. cycad pollen, a warm/dry climate indicator, and amorphous organic matter, but the proportion of terrestrial palynomorphs remains generally ≥80% (Slater et al., Reference Slater, Twitchett, Danise and Vajda2019). The impact of early Toarcian climate warming on terrestrial vegetation was not limited to the Cleveland Basin, with palynological evidence of significant warming recorded from many regions, including continental settings in Central Asia (Schnyder et al., Reference Schnyder, Pons, Yans, Tramoy, Abdulanova, Brunet, McCann and Sobel2017).

21.d. T-OAE (Unit III) – expansion of anoxia and a hyperthermal event

The interval of the negative δ¹³C excursion defining the T-OAE (Unit III), spanning the upper D. semicelatum and C. exaratum subzones, represents the period of most extensive palaeoenvironmental change in the Cleveland Basin and globally. The base of T-OAE Unit III is defined by the onset of falling δ¹³C values recorded in bulk sediment organic matter, terrestrial wood, individual marine and terrestrial biomarkers, and rising TOC content, followed, a short distance above, by the onset of laminated black shale deposition and the disappearance of trace fossils and most benthic fauna. This reflects the establishment of more persistent anoxia and then, from the base of the Jet Rock (C. exaratum Subzone), euxinic bottom waters that dominated through the interval of the δ¹³C_org minimum that characterizes Subunit IIIb. Anoxia persisted after the T-OAE, throughout Unit IV, into the later early Toarcian, upper H. falciferum Subzone.