1. Introduction
Latest Sandbian to early Katian sequences provide a record of major environmental changes across Laurentia's epicontinental sea that are commonly interpreted as representing the initiation of climatic cooling and glaciation ~10 Ma prior to the end Ordovician (Hirnantian) glacial interval (e.g. Lavoie, Reference LAVOIE1995; Pope & Read, Reference Pope, Read, Noel and Clarke1997; Pope & Steffen, Reference Pope and Steffen2003; Saltzman & Young, Reference Saltzman and Young2005). The proposed lithologic expression of the environmental changes is a shift from deposition of muddy, matrix-supported ‘warm-water’ carbonates to skeletally supported ‘cool-water’ carbonates across the early Late Ordovician M4/M5 sequence boundary (Holland & Patzkowsky, Reference Holland, Patzkowsky, Wittke, Ludvigson and Day1996; Pope & Read, Reference Pope, Read, Noel and Clarke1997; Kolata, Huff & Bergström, Reference Kolata, Huff and Bergström2001). Additional observations that have been cited in favour of cooling across the M4/M5 include a migration of cool-water fauna into Laurentia, a ~3‰ positive carbon isotope excursion (Guttenberg Isotope Carbon Excursion: GICE), and abundant phosphate and chert deposition above the M4/M5 sequence boundary (Holland & Patzkowsky, Reference Holland, Patzkowsky, Wittke, Ludvigson and Day1996; Pope & Read, Reference Pope, Read, Noel and Clarke1997; Pope & Steffen, Reference Pope and Steffen2003 [cf. Leslie & Bergström, Reference Leslie and Bergström2003]; Saltzman & Young, Reference Saltzman and Young2005).
Interpretations favouring early Katian climatic cooling pose some theoretical problems, however, and do not fit all of the available data. In particular, if temperature decrease is the dominant early Katian change, then the large magnitude of cooling at tropical to subtropical latitudes required to explain the lithological shift to interpreted ‘cool-water’ carbonates challenges existing thoughts about the relative stability of conditions in the tropics. Additionally, warm-water carbonates continued to accumulate in some regions within North America (e.g. parts of the mid-continent and the Upper Mississippi Valley), stratigraphically above the M4/M5 boundary (Bergström et al. Reference Bergström, Schmitz, Saltzman, Huff, Finney and Berry2010; Ettensohn, Reference Ettensohn, Finney and Berry2010), and the shift to deposition of carbonates with ‘cool-water’ features is diachronous even near the M4/M5 boundary (Lavoie & Asselin, Reference Lavoie and Asselin1998; Dronov, Reference Dronov2013). Further, in contrast to a glacio-eustatic regression expected if the M4/M5 shift is associated with an initiation or expansion of continental glaciers (e.g. Bergström et al. Reference Bergström, Schmitz, Saltzman, Huff, Finney and Berry2010), the sequence boundary marks a relative rise in sea level across much of Laurentia (Holland & Patzkowsky, Reference Holland, Patzkowsky, Wittke, Ludvigson and Day1996).
There are explanations for both the lithologic changes and the GICE that do not invoke early Katian cooling as a forcing mechanism. The Taconic tectophase of the Taconic Orogeny began during the Late Ordovician (Kolata, Huff & Bergström, Reference Kolata, Huff and Bergström2001; Ettensohn et al. Reference Ettensohn, Hohman, Kulp and Rast2002; Ettensohn, Reference Ettensohn, Finney and Berry2010). Nutrient availability, light penetration, terrigenous input, and/or salinity play important roles in determining carbonate production and, thus, influence lithology (Samankassou, Reference Samankassou2002; Pedley & Carannante, Reference Pedley, Carannante, Pedley and Carannante2006). Some, if not all, of these variables likely changed in response to initial uplift of the Taconic highlands (including indirect responses related to modified circulation patterns within the basin). Increased upwelling of nutrient-rich open ocean water into the shallow epicontinental sea has also been invoked as a causal mechanism for changes in carbon cycling that resulted in the GICE (Young et al. Reference Young, Saltzman and Bergström2005). In short, a tectonic model invoking uplift with associated crustal loading, increased weathering rates, and increased input of terrigenous material provides an alternative hypothesis to various cooling scenarios as an explanation for observed lithologic changes during the early Late Ordovician.
Geochemical-based palaeotemperature records can provide an independent perspective on environmental forcing. Several studies have used δ18O measurements from conodont apatite (a phase considered to be quite resistant to diagenetic overprinting due to the strong phosphate–oxygen bond, e.g. Luz, Kolodny & Kovach, Reference Luz, Kolodny and Kovach1984; Wenzel, Lecuyer & Joachimski, Reference Wenzel, Lecuyer and Joachimski2000; Joachimski et al. Reference Joachimski, Breisig, Buggisch, Talent, Mawson, Gereke, Morrow, Day and Weddige2009) to estimate temperature trends in Laurentia's epicontinental sea during the early Katian (Buggisch et al. Reference Buggisch, Joachimski, Lehnert, Bergström, Repetski and Webers2010; Herrmann, MacLeod & Leslie, Reference Herrmann, MacLeod and Leslie2010; Rosenau, Herrmann & Leslie, Reference Rosenau, Herrmann and Leslie2012). These studies have yielded somewhat contradictory interpretations. Two relatively high δ18O values measured near the base of the Katian in Oklahoma were cited as evidence of a brief cooling interval (Rosenau, Herrmann & Leslie, Reference Rosenau, Herrmann and Leslie2012), whereas results from Minnesota, Iowa and Kentucky suggest warming during the early Katian followed by stable conditions through the remainder of the Stage (Buggisch et al. Reference Buggisch, Joachimski, Lehnert, Bergström, Repetski and Webers2010). In addition, a low-resolution δ18O record spanning the entire Ordovician period suggests stable temperatures in the Katian (Trotter et al. Reference Trotter, Williams, Barnes, Lecuyer and Nicoll2008) while variation in δ18O values purported to be correlated with sea level change in samples from Kentucky and Anticosti Island were interpreted to support the presence of glacial–interglacial cycles throughout the Katian (Elrick et al. Reference Elrick, Reardon, Labor, Martin, Desrochers and Pope2013). Attempting to integrate these δ18O records into a general palaeoclimatic scenario is complicated by the fact that these studies were conducted at different scales and generally present data from a few, closely spaced sections or composites of sections without significant time overlap. It is therefore difficult to rule out a role for local influences on observed δ18O trends that could attenuate or obscure the signature of global trends.
Change in the oxygen isotopic composition of the seawater (δ18Osw) from which the apatite precipitates is a major confounding factor in palaeotemperature estimates (e.g. Longinelli, Reference Longinelli1965; Kolodny, Luz & Navon, Reference Kolodny, Luz and Navon1983; Luz, Kolodny & Kovach, Reference Luz, Kolodny and Kovach1984). The δ18Osw value of a water mass can vary on local and regional scales due to differences in evaporation/precipitation rates and freshwater input or, globally, due to large-scale climatic forcing including the volume of glacial ice. To constrain the relative contribution of temperature to an observed shift in δ18O values, it is necessary either to independently determine changes in δ18Osw (a difficult proposition) or to estimate local control on δ18Osw by documenting δ18O trends across multiple sections separated by distances that are long relative to likely gradients in δ18Osw.
In this study we take the latter approach. We measured δ18O values from species-specific and mixed conodont assemblages from six sections across eastern North America to test for consistent δ18Osw patterns across the region. The compiled δ18O record can be compared to predicted patterns of models that have been proposed. If the ‘cool-water’ interpretation of early Katian carbonates is correct, an increase in oxygen isotopic values is expected to correlate with the change in carbonate lithology across the M4/M5. Similarly, if the GICE records a net burial of organic carbon (and thus drawdown of atmospheric CO2 levels), an increase in oxygen isotopic values is expected to begin within or shortly above that carbon isotope excursion. Either of these isochronous increases in δ18O values would be consistent with global cooling and/or glaciation, whereas a diachronous increase could indicate an important role for cooling related to oceanographic changes such as an incursion and/or upwelling of cooler sub-thermocline water with many potential ultimate causes (e.g. basin deepening and/or changes in ocean circulation). Finally, and in marked contrast to the first three, stable or decreasing δ18O values would argue against cooling as a primary driver of observed lithological and geochemical changes during the early Katian.
2. Materials and methods
2.a. Study sections
During the Late Ordovician, Laurentia was rotated clockwise ~45° relative to modern geographic position and was in a subtropical latitude in the southern hemisphere (Torsvik & Cocks, Reference Torsvik, Cocks, Harper and Servais2013). Much of E Laurentia was covered in a shallow epicontinental sea bounded by the Taconic highlands to the SE and the Transcontinental Arch to the NE. Connection to the Iapetus Ocean was established along the SE margin of the carbonate platform.
Data are presented from six sections that span the seaway along a roughly N–S transect and represent different depositional settings within the shallow tropical epicontinental sea that covered much of North America (Fig. 1). The sections studied are the Tidwell Hollow and Red Mountain Expressway sections in Alabama (33.87806°N, 86.57555°W and 33.49598°N 86.78822°W), the Dickeyville section in Wisconsin (42.640586°N 90.57556°W), the Decorah section in Iowa (43.29021°N 91.76561°W), the Rochester section in Minnesota (43.97207°N 91.38358°W) and the Hagan section in Virginia (36.28333°N 86.69250°W). Stratigraphic details for the Alabama sections can be found in Quinton et al. Reference Quinton, Herrmann, Leslie and MacLeod(2016a), for the Decorah section in Emerson et al. (Reference Emerson, Ludvigson, Witzke, Schneider, González, Carpenter, Ludvigson and Bunker2005), and the Rochester and Hagan sections in Leslie (Reference Leslie1995). All sampled sections span the Sandbian–Katian boundary and contain the Deicke and Millbrig K-bentonites (Haynes, Reference Haynes1994; Kolata, Huff & Bergström, Reference Kolata, Huff and Bergström1996, Reference Kolata, Huff and Bergström1998; Sell et al. Reference Sell, Samson, Mitchell, McLaughlin, Koenig and Leslie2015). The conodont fauna has previously been described for the Tidwell Hollow, Red Mountain Expressway, Decorah, Rochester and Hagan sections (Hall, Bergström & Schmidt, Reference Hall, Bergström, Schmidt, Benson and Stock1986; Leslie, Reference Leslie1995, Reference Leslie2000; Quinton et al. Reference Quinton, Herrmann, Leslie and MacLeod2016a). In addition, these sections have been placed into the Midcontinent biostratigraphic framework (Leslie, Reference Leslie1995, Reference Leslie2000; Quinton et al. Reference Quinton, Herrmann, Leslie and MacLeod2016a).
Figure 1. Global palaeogeographic reconstruction of the Late Ordovician (modified from Torsvik & Cocks, Reference Torsvik, Cocks, Harper and Servais2013). The position of previously documented oxygen isotopic records spanning the M4/M5 (Buggisch et al. Reference Buggisch, Joachimski, Lehnert, Bergström, Repetski and Webers2010; Rosenau, Herrmann & Leslie, Reference Rosenau, Herrmann and Leslie2012) and sections from this study are identified on a generalized reconstruction of the bathymetry of the epicontinental sea (adapted from Brett et al. Reference Brett, McLaughlin, Cornell and Baird2004). Water mass boundaries as identified by Holmden et al. (Reference Holmden, Creaser, Muehlenbachs, Leslie and Bergström1998) are indicated with black dotted lines. Regional stratigraphy and position of the Deicke and Millbrig K-bentonite is provided for each composite section (adapted from Leslie, Reference Leslie2000).
One complicating factor in attempting to understand the nature of environmental changes across M4/M5 is that the proposed stratigraphic position of the boundary among sections is inconsistent relative to other chronostratigraphic data (e.g. Sell et al. Reference Sell, Samson, Mitchell, McLaughlin, Koenig and Leslie2015). These chronostratigraphic markers include the Deicke and Millbrig K-bentonites, altered volcanic ash beds that have been recognized and correlated throughout eastern North America, and conodont biozones. For example, the base of the Plectodina tenuis zone has been identified above the Millbrig K-bentonite in some sections and below the Millbrig K-bentonite in other sections. The M4/M5 boundary has also been proposed to be above the Millbrig K-bentonite in some sections and below it in others. Further complication is added by the fact that the P. tenuis zone has been recognized both above and below the M4/M5 in different sections. These inconsistencies suggest at least three possibilities: (1) that the sequence boundary is diachronous (and thus not eustatic), (2) that the sequence boundary has been misidentified in some sections, or (3) that the sequence boundary is correctly located and isochronous but that other chronostratigraphic markers have been incorrectly located and/or identified in some sections (e.g. Kolata, Huff & Bergström, Reference Kolata, Huff and Bergström2001; Brett et al. Reference Brett, McLaughlin, Cornell and Baird2004; Sell et al. Reference Sell, Samson, Mitchell, McLaughlin, Koenig and Leslie2015). Because ash deposited from a given eruption should provide an isochronous event bed where present (an assumption that is not necessarily true for biostratigraphic events or lithologic shifts), we use the Deicke and Millbrig K-bentonites to correlate among sections in addition to the stratigraphic position of the M4/M5. Unrecognized reworking or the presence of additional bentonites in a given section can be sources of uncertainty, but the correlations used are supported by phenocryst mineralogy in the study sections (e.g. Haynes, Reference Haynes1994; Sell et al. Reference Sell, Samson, Mitchell, McLaughlin, Koenig and Leslie2015).
2.b. Oxygen isotope samples and methods
Limestone samples of ~5kg and shale samples of ~2kg were collected at selected horizons in the Tidwell Hollow, Red Mountain Expressway, Decorah, Dickeyville and Rochester sections. To maintain consistency between sections, sampling was performed at regular stratigraphic intervals (1–0.5 m intervals) all measured relative to the Millbrig or Deicke K-bentonite. To control for possible bias related to lithology, samples were also collected at significant changes in lithology.
Limestone samples were dissolved using either a ~10% acetic solution or a ~10% buffered formic solution following standard dissolution and extraction procedures (e.g. Jeppsson & Anehus, Reference Jeppsson and Anehus1995; Jeppsson et al. Reference Jeppsson, Anehus and Fredholm1999; Quinton et al. Reference Quinton, Leslie, Herrmann and MacLeod2016b). Shale samples were disaggregated by soaking in kerosene followed by a water and Calgon solution. All samples were washed on a 63 micron screen, and apatite microfossils were concentrated using heavy liquid (either tetrabromoethane or lithium heteropolytungstate: LST Heavy Liquid). Conodont elements and brachiopod shell fragments were isolated and identified using a binocular microscope. Conodonts used to document δ18O values for the Hagan section are from previous collections (Leslie, Reference Leslie1995, Reference Leslie2000).
Conodont elements are composed of highly crystalline bioapatite that is relatively resistant to alteration of the δ18O values in phosphate-bound oxygen (Kolodny & Luz, Reference Kolodny, Luz and Taylor1991; Wenzel, Lecuyer & Joachimski,. 2010; MacLeod, Reference MacLeod2012). The conodont elements analysed in this study are well preserved. The elements lack evidence of overgrowths and have low conodont colour alteration index (CAI) values. CAI values range from 1 to 1.5, with the exception of the two sections from Alabama where elements have a CAI of 3. Together these observations suggest that measured δ18O values should be close to the primary oxygen isotopic ratios of the conodont elements, but diagenetic alteration cannot be completely ruled out.
To assess the extent and direction of any diagenetic shifts in the study sections, we measured phosphate δ18O values from co-occurring linguliform brachiopod shells. Bioapatite from linguliform brachiopod shells is generally thought to be more susceptible to alteration than conodont elements (Wenzel, Lecuyer & Joachimski, Reference Wenzel, Lecuyer and Joachimski2000). This difference is due to the fact that brachiopods build their shells out of alternating layers of generally porous apatite and organic tissue and this architecture seems to have provided a relatively attractive substrate for microbial colonization. Measured oxygen isotopic values from brachiopod shells thus provide a geochemical taphonomic control on the direction and magnitude of local diagenetic overprints (Bassett et al. Reference Bassett, MacLeod, Miller and Ethington2007; Hughes et al. Reference Hughes, MacLeod, Haynes, Quinton, Martin and Ethington2015).
Previous conodont oxygen isotopic studies have documented significant variability in measured δ18O values, likely due to palaeoecological differences among conodont taxa, incomplete time averaging, and/or differences in diagenetic susceptibility among element types (Trotter et al. Reference Trotter, FitzGerald, Kokkonen and Barnes2007; Rosenau, Herrmann & Leslie, Reference Rosenau, Herrmann and Leslie2012; Sun et al. Reference Sun, Joachimski, Wignall, Yan, Chen, Jiang, Wang and Lai2012; Quinton & MacLeod, Reference Quinton and MacLeod2014; Quinton et al. Reference Quinton, Leslie, Herrmann and MacLeod2016b). To reduce potential bias related to these factors, oxygen isotopic measurements were performed both on species-specific (Belodina compressa, Curtognathus sp., Drepandoistodus suberectus, Icriodella sp., Oulodus serratus, Panderodus gracilis, Phragmodus cognitus, Phragmodus undatus, Plectodina aculeta, Polyplacognathus ramosus) and mixed conodont assemblages. When possible, multiple oxygen isotopic measurements were generated for an individual sample horizon to increase confidence that values reflect the average for that stratigraphic horizon. Finally, oxygen isotopic values were also measured on brachiopod shell fragments to constrain digenetic influences. The PO43− anion was isolated using techniques modified from LaPorte et al. (Reference Laporte, Holmden, Patterson, Prokopiuk and Eglington2009) and described by Quinton & MacLeod (Reference Quinton and MacLeod2014). All samples were analysed on a continuous flow Thermo-Finnigan Delta Plus XL gas source mass spectrometer connected to a Thermo-Finnigan TC-EA (high-temperature conversion – elemental analyzer) at 1400°C with an external precision of ±0.3‰ (1 standard deviation) based on replicate analyses of NBS-120c. Values are reported in per mil notation (‰) on the Vienna Standard Mean Ocean Water (VSMOW) scale and correspond to a NBS-120c value of 21.7‰. Oxygen isotopic values from previous studies (Buggisch et al. Reference Buggisch, Joachimski, Lehnert, Bergström, Repetski and Webers2010; Roseneau et al. Reference Rosenau, Herrmann and Leslie2012) have been renormalized to a NBS-120c value of 21.7‰. Replicate analyses were performed for samples that yielded >0.800 mg of silver phosphate crystals (106 out of 257 samples). Reproducibility for these replicates was generally better than reported external precision (Table 1). For these samples (those with replicate analyses) the value reported is the average of the replicate δ18O measurements; for samples without replicates, the value measured is reported (Figs 2–4).
Table 1. Conodont oxygen isotopic results for the Decorah section in Iowa (43.29021°N, 91.76561°W), the Rochester section in Minnesota (43.97207°N, 91.38358°W), the Dickeyville section in Wisconsin (42.640586°N, 90.57556°W), the Hagen section in Virginia (36.28333°N, 86.69250°W) and the Red Mountain Expressway and Tidwell Hollow sections in Alabama (33.49598°N, 86.78822°W and 33.87806°N, 86.57555°W).Footnote *. Averages are calculated from δ18O values of replicate analyses of silver phosphate crystals. One standard deviation (σ) is calculated for samples with replicate analyses. Conodont species analysed include Belodina compressa, Curtognathus sp., Drepandoistodus suberectus, Icriodella superba, Oulodus serratus, Panderodus gracilis, Phragmodus cognitus, Phragmodus undatus, Plectodina aculeta and Polyplacognathus ramosus
* Detailed descriptions of outcrops from Alabama are available in Quinton et al. (Reference Quinton, Leslie, Herrmann and MacLeod2016), and for Hagen, VA, detailed descriptions are available in Young, Saltzmann & Bergström (Reference Young, Saltzman and Bergström2005) and Leslie (Reference Leslie1995).
† metre above the Millbrig K-bentonite.
Figure 2. Average conodont and brachiopod δ18O results from Decorah, IO, plotted against bulk carbonate and organic carbon δ13C values. Carbon isotopic results are plotted with three-point moving averages. Averages are calculated from δ18O values of all replicate analyses of each silver phosphate crystal precipitation. Error bars on δ18O averages represent the standard deviation of replicate analyses for that sample. The ~1‰ positive carbon isotope excursion above the Millbrig K-bentonite is interpreted as the GICE.
Figure 3. Conodont and brachiopod δ18O results from Rochester, MN, plotted against bulk carbonate δ13Ccarb values. Carbon isotopic results are plotted with three-point moving averages. Averages are calculated from δ18O values of all replicate analyses of each silver phosphate crystal precipitation. Error bars on δ18O averages represent the standard deviation of replicate analyses for that sample. The low resolution of bulk carbonate δ13Ccarb record prevents identification of the GICE with any confidence in this section. The Platteville Formation is abbreviated as Plat.
Figure 4. Conodont and brachiopod δ18O results from Red Mountain Expressway, AL, plotted against bulk carbonate δ13Ccarb values from Quinton et al. Reference Quinton, Herrmann, Leslie and MacLeod(2016a). Averages are calculated from δ18O values of all replicate analyses of each silver phosphate crystal precipitation. Error bars on δ18O averages represent the standard deviation of replicate analyses for that sample.
2.c. Carbon isotope samples and methods
To place our oxygen isotopic results within the general chemostratigraphic framework for the late Sandbian – early Katian, we measured co-occurring bulk carbonate and organic carbon δ13C values for the two studied sections for which δ13C trends have not yet been documented. Carbon isotope measurements were performed on carbonate and trace organic matter for the Rochester and Decorah sections using methods described in Quinton et al. Reference Quinton, Herrmann, Leslie and MacLeod(2016a). All carbonate δ13Ccarb measurements were performed on a Thermo-Finnigan Delta Plus Dual Inlet isotope ratio mass spectrometer connected to a Kiel III Carbonate Interface. Analytical precision calculated from multiple analyses of NBS-19 run during the course of this study is ±0.04‰ (1 standard deviation) for δ13Ccarb and ±0.06‰ for δ18Ocarb (1 standard deviation). Organic carbon δ13Corg analyses were performed on decarbonated sample powders on a continuous flow Thermo-Finnigan Delta Plus XL gas source mass spectrometer connected to an NA1500 EA chemical elemental analyser. Analytical precision is ±0.1‰ (1 standard deviation) based on multiple analyses of an internal lab standard (Acetanilide). All δ13C values are reported in ‰ on the Vienna PDB scale.
3. Results
Conodont apatite δ18O values for the latest Sandbian to early Katian range from 16.1‰ to 19.9‰, with an average of 18.8‰ (Table 1; Figs 2–4). This average corresponds to an estimated temperature of 31.3°C when using the temperature equation of Pucéat et al. (Reference Pucéat, Joachimski, Bouilloux, Monna, Bonin, Motreuil, Morinière, Hénard, Mourin, Dera and Quesne2010), a NBS-120c value of 21.7‰, assuming no change in δ18Osw (isotopic composition of seawater) due to ice volume effects, an ice-free δ18Osw value of −1‰ VSMOW (Shackleton & Kennett, 1975), and preservation of original isotopic values. Conodont δ18O regional averages for the Upper Mississippi Valley, Alabama and Virginia are 18.9‰, 18.1‰ and 19‰ respectively (Fig. 4). In contrast, brachiopod apatite oxygen isotopic values average ~17.4‰ and are generally lower (by 0.5–1.6‰) and more variable than values for co-occurring conodont elements (Figs 2–4; Supplementary Material Table 1 available at https://doi.org/10.1017/S0016756817000589). Average oxygen isotopic values from brachiopods would correspond to an estimated temperature of 37.3°C using the assumptions above.
Bulk carbonate δ13Ccarb values from Rochester and Decorah range from −3.9‰ to 1.0‰, and organic carbon δ13Corg values range from −31.7‰ to −25.4‰ (Figs 1 and 2). Bulk carbonate δ13Ccarb values with an average of −0.9‰ are relatively consistent through the sampled sections. At Decorah, both bulk carbonate and organic carbon results record a ~1‰ positive excursion above the Millbrig K-bentonite.
4. Discussion
4.a. Carbon isotopes
North American carbon isotope stratigraphy is relatively well established for the Late Ordovician, with at least four globally recognized positive carbon isotope excursions. These excursions are used for chemostratigraphic correlation (e.g. Bergström et al. Reference Bergström, Chen, Gutiérrez-Marco and Dronov2009, Reference Bergström, Saltzman, Leslie, Ferretti and Young2015), and the GICE, the second most prominent of these excursions, has also been proposed to represent the initiation of early Katian climatic cooling (Saltzman & Young, Reference Saltzman and Young2005; Young et al. Reference Young, Saltzman, Bergström, Leslie and Xu2008). The stratigraphic position of the GICE has been previously identified in the Hagen (Young, Saltzmann & Bergström, Reference Young, Saltzman and Bergström2005), Tidwell Hollow and Red Mountain Expressway Quinton et al. Reference Quinton, Herrmann, Leslie and MacLeod(2016a), and Dickeyville (Wright et al. in press) sections. New bulk carbonate δ13Ccarb results for the Decorah and Rochester sections allow for identification of the GICE and placement of δ18O results within the stable carbon isotope chemostratigraphic framework. At Decorah we interpret the ~1‰ positive excursion starting at 3.7 m as the GICE. This interpretation would place the entirety of the excursion within the Guttenberg Member. Results from Rochester record a ~1‰ positive excursion below the Millbrig K-bentonite. However, given the stratigraphic position of this excursion and limited data above the Millbrig K-bentonite we cannot confidently identify the GICE at Rochester.
4.b. Diagenetic considerations
We interpret brachiopod δ18O results as having been shifted by diagenetic alteration. Linguliform brachiopods are benthic organisms and as such likely lived at a different position in the water column than most conodont taxa. Because temperatures generally decline with depth, this difference in habitat means brachiopods likely lived at temperatures lower than or similar to temperatures conodonts experienced. Thus, if primary oxygen isotopic values were preserved, brachiopods should have δ18O values the same as or higher than values in co-occurring conodont elements. In contrast to this prediction, brachiopod δ18O results are generally lower and more variable than values from conodonts in the same samples. These results are consistent with diagenetic overprints shifting brachiopod δ18O toward lower values. Note, though, that temperatures estimated from brachiopod δ18O values (~37.3°C) are significantly lower than burial temperatures estimated based on the CAI (lower than 50–80°C) of co-occurring conodont elements (Epstein, Epstein & Harris, Reference Epstein, Epstein and Harris1977). Thus, brachiopod oxygen isotopic values were not completely reset during burial.
The relatively modest overprint on brachiopod oxygen isotopic values and the relative resistance to alteration of conodont apatite suggests that conodont δ18O results likely primarily reflect palaeoenvironmental conditions. Supporting this argument, isotopic values and trends across most of the sampled sections argue against diagenesis as an explanation for observed patterns as it is unlikely that δ18O values would be consistently altered across multiple sections separated by > 1000 km and influenced by different secondary processes and with different burial histories. Further, conodont δ18O values yield a temperature estimate of 30–32°C, that falls near the range of modern tropical sea surface temperatures and is comparable to predictions from numerical climate models for Katian equatorial sea surface temperatures (e.g. Herrmann et al. Reference Herrmann, Haupt, Patzkowsky, Seidov and Slingerland2004; Pohl et al. Reference Pohl, Donnadieu, Hir, Buoncristiani and Vennin2014). Results from Red Mountain Expressway are the one exception to these general observations and are discussed below.
4.c. Local influences on δ18O values
Conodont oxygen isotopic results from this study combined with previous results (Buggisch et al. Reference Buggisch, Joachimski, Lehnert, Bergström, Repetski and Webers2010; Rosenau, Herrmann & Leslie, Reference Rosenau, Herrmann and Leslie2012) provide the first record of δ18O values across the SE region of Laurentia's epicontinental sea during the Late Ordovician. With these results, it is possible to test for systematic variations in δ18Osw values and patterns in the region using the Deicke and Millbrig K-bentonites as stratigraphic markers for purposes of regional correlation (Fig. 5). First, it should be noted that there is significant scatter in the δ18O data. Scatter appears to be a common feature of high-resolution conodont oxygen isotopic records and likely reflects some combination of intra- and inter-annual temperature/salinity differences, and taxonomic differences. These differences are made more noticeable by analytical uncertainty that is several times larger in phosphate measurements than carbonate measurements (Quinton & MacLeod, Reference Quinton and MacLeod2014; Quinton et al. Reference Quinton, Leslie, Herrmann and MacLeod2016b). The documented variability is of a similar magnitude within all the study sections and is observed in both species-specific and mixed conodont assemblage oxygen isotopic results. Further, generated δ18O records for each section include multiple measurements for many sample horizons, maximizing potential to capture the δ18O average for that time interval. For these reasons, observed variability should not bias our results, but it does highlight the importance of testing for patterns in average δ18O values over many samples rather than interpreting differences between adjacent samples.
Figure 5. Compilation of Late Ordovician species-specific and mixed conodont δ18O values from the Upper Mississippi Valley (this study), Minnesota (from Buggisch et al. Reference Buggisch, Joachimski, Lehnert, Bergström, Repetski and Webers2010), Oklahoma (Rosenau, Herrmann & Leslie, Reference Rosenau, Herrmann and Leslie2012), Alabama (this study), Virginia (this study) and Kentucky (Buggisch et al. Reference Buggisch, Joachimski, Lehnert, Bergström, Repetski and Webers2010), with Deicke and Millbrig K-bentonites (red lines) as planes of correlation. The Upper Mississippi Valley data from this study includes results from Decorah (black circles), Rochester (grey circles) and Dickeyville (open circles). Oxygen isotopic values from Alabama include results from Tidwell Hollow (grey circles) and Red Mountain Expressway (black circles). Regional averages are 18.9‰ for the Upper Mississippi Valley, 18.7‰ for Minnesota, 18.3‰ for Oklahoma, 18.1‰ for Alabama, 19‰ for Virginia and 18.4‰ for Kentucky. The M4/M5, the GICE, and K-bentonites are identified in Minnesota (Buggisch et al. Reference Buggisch, Joachimski, Lehnert, Bergström, Repetski and Webers2010), Alabama Quinton et al. Reference Quinton, Herrmann, Leslie and MacLeod(2016a), Virginia (Young, Saltzmann & Bergström, 2005) and Kentucky (Buggisch et al. Reference Buggisch, Joachimski, Lehnert, Bergström, Repetski and Webers2010). It should be noted that while the GICE has been labelled in Oklahoma (Rosenau, Herrmann & Leslie, Reference Rosenau, Herrmann and Leslie2012) others have questioned this identification (Westrop et al. Reference Westrop, Amati, Brett, Swisher, Carlucci, Goldman, Leslie and Burkhalter2015). The GICE is identified in the Upper Mississippi Valley based on δ13C results from this study (see online Supplementary Material available at https://doi.org/10.1017/S0016756817000589). The M4/M5 boundary (identified in Kentucky) is correlated with the contact between the Platteville and Decorah formations in the Upper Mississippi Valley (following Holland & Patzkowsky, Reference Holland, Patzkowsky, Wittke, Ludvigson and Day1996; Kolata, Huff & Bergström, Reference Kolata, Huff and Bergström2001); this correlation of the M4/M5 shows that the sequence boundary is regionally diachronous assuming that none of the K-bentonites has been misidentified or miscorrelated. All δ18O values are normalized to an NBS-120c value of 21.7‰.
The strata exposed in the Upper Mississippi Valley and Kentucky were deposited on the shallow Galena and Lexington carbonate platforms respectively. Exposures in Alabama and Virginia were deposited along the margin of the Taconic Foreland Basin while those from Oklahoma were deposited in the Arkoma basin. Despite differences in position and depth in the epicontinental sea, regional δ18O averages are relatively consistent (Upper Mississippi Valley: 18.9‰, 1σ: 0.5; Kentucky: 18.4‰, 1σ: 0.4; Tidwell Hollow Alabama: 18.7‰, 1σ: 0.7; Virginia: 19‰, 1σ: 0.4; and Oklahoma: 18.3‰, 1σ: 0.4) and record a similar pattern of decreasing values in the early Katian.
Similarly, δ18O values do not significantly vary across proposed temperature- and salinity-defined aquafacies (Holmden et al. Reference Holmden, Creaser, Muehlenbachs, Leslie and Bergström1998) as would be expected if local differences in freshwater input and evaporation/precipitation rates exerted a significant influence on δ18Osw values. Based on differences in Nd and C isotopic values among regions, Holmden et al. (Reference Holmden, Creaser, Muehlenbachs, Leslie and Bergström1998) suggested that Laurentia's epicontinental sea contained three distinct aquafacies: the Midcontinent, Taconic and Southern (Fig. 1). These authors suggested that basin morphology and geographic patterns in the source and amount of freshwater runoff controlled the presence of each aquafacies. Both environmental factors can have a significant influence on the oxygen isotopic composition of regional and local seawater. As such, measured δ18O values from these aquafacies are expected to be distinct if local environmental influences on δ18Osw were significant. In contrast to this prediction, measured δ18O values from the Midcontinent aquafacies (e.g. Upper Mississippi Valley), Southern aquafacies (e.g. Kentucky) and Taconic aquafacies (e.g. Virginia) are generally consistent around 18.4–19‰, arguing that δ18Osw values did not vary systematically due to local environmental conditions that impart signals observed in Nd and C trends in the region. Therefore, it seems values and trends in δ18O values primarily reflect regional patterns.
Oxygen isotopic results from Red Mountain Expressway, at an average of 17.5‰, are the one exception to basin-wide similarity among conodont δ18O values. During the Late Ordovician, Red Mountain Expressway was situated at the highest palaeolatitude (~30°S) and represents one of the greatest water depths of the studied sections, yet δ18O values are up to 1‰ lower at this site than others. The lower values could be interpreted as an indication of local water 4–5°C warmer than elsewhere, but this interpretation is difficult to reconcile with the Red Mountain Expressway's palaeogeographic and palaeoceanographic position. Diagenetic alteration provides an alternative explanation for the low values at Red Mountain Expressway. Exposures in Alabama were subjected to the highest burial temperatures, in excess of 110°C as indicated from the conodont elements with a CAI of 3, of all the sections sampled. Brachiopod δ18O values are also generally lowest and the degree of offset between conodont δ18O values and brachiopod values at Red Mountain Expressway is the greatest among the sections studied. Both observations are consistent with a more pervasive diagenetic overprint at Red Mountain Expressway than other sections.
4.d. Oxygen isotopes and ‘cool-water’ carbonates
Our compilation of δ18O records argues against ‘cool-water’ models that invoke climatic cooling to explain the history of Sandbian–Katian carbonate deposition on Laurentia. Conodont δ18O values do not increase across the interval; in fact, if anything they decrease on average by 1.5‰ following the M4/M5. As noted above, there are no basin-scale gradients suggestive of strong local influences on δ18Osw values that compromise temperature inferences, and diagenetic overprints do not seem to be a major concern except at the Red Mountain Expressway section. Further, additional considerations about global fluctuations in δ18Osw due to possible increases in ice volume are not required. Glacial growth, which can cause an increase in δ18Osw values, and climatic cooling can both contribute to recorded δ18O values, but both shift values in the same direction (i.e. increased δ18O values). Distinguishing between the relative contributions of either on observed δ18O trends only becomes necessary when attempting to estimate the amount of temperature and/or ice volume change. We suggest that the lack of oxygen isotopic increase in conodonts through the sections studied indicates that water temperature was not a dominant factor controlling the stratigraphic trends among early Katian carbonates in SE Laurentia.
The tectonic model is the only one that does not predict an increase in δ18O values and thus is favoured by these data. In a tectonic scenario, the change in carbonate lithology resulted from increased turbidity, terrigenous input, and/or changing circulation patterns in response to basin deepening across SE Laurentia and uplift of the Taconic Mountains (e.g. Ettensohn, Reference Ettensohn, Finney and Berry2010). Reduced light penetration due to increased water turbidity could explain the shift to skeletally supported fabrics and the absence of chlorozoan fauna (both characteristics of early Katian ‘cool-water’ carbonates). Detailed analyses of the carbonates themselves would be required to distinguish among the relative importance of different proximate forcing mechanisms (terrigenous input, nutrient increase, changing circulation) brought about by the Taconic uplift.
4.e. Possible palaeoenvironmental implications of early Katian δ18O decrease
Our results show no evidence of cooling in the Sandbian – early Katian, but they are consistent with diachronous warming across SE Laurentia during this interval. All regions record a ~1– 0.5‰ decrease in average δ18O values following the M4/M5, but the timing of this increase varies among sections. This observation becomes apparent when the compiled oxygen isotope data are separated into a pre-M4/M5, a post-M4/M5 and a GICE + post-GICE time-slice using the stratigraphic position of these boundaries and the Deicke K-bentonite to scale the data (Fig. 6).
Figure 6. Comparison of new and previously documented conodont oxygen isotopic values from the Upper Mississippi Valley (black circles), Oklahoma (dark grey circles), Alabama (hollow circles), Virginia (open grey circles) and Kentucky (light grey circles) at three different time-slices in the Late Ordovician. Data from the Red Mountain Expressway section are not plotted due to suspected diagenetic overprint. All sections are scaled and plotted against stratigraphic thickness in the Rochester section. Metre scale for each time-slice refers to stratigraphic thickness in the Rochester section. For the pre-M4/M5 interval, all data are scaled to the position of the M4/M5 and the Deicke K-bentonite. For the post-M4/M5 interval, all data are scaled based on the stratigraphic position of the M4/M5 and the base of the GICE. For the GICE and post-GICE interval, all data were scaled based on the thickness of the GICE in each section.
Stratigraphically below the M4/M5 sequence boundary, oxygen isotopic values from the Upper Mississippi Valley, Oklahoma, Alabama, Virginia and Kentucky overlap and record similar scatter around an average of ~19‰. Values from the Red Mountain Expressway are markedly lower but believed to be diagenetically altered as discussed above. Following the M4/M5, oxygen isotopic values become more variable and values from marginal settings (Oklahoma and Alabama) decrease and diverge from the interior settings (Upper Mississippi Valley). During the GICE, values from all sections again overlap and record similar fluctuations but at a lower average (~0.5‰) than prior to the M4/M5. The divergence in δ18O values between marginal and interior sections following the M4/M5 could reflect diachronous warming across the region, with warming first along the margin of the epicontinental sea, then in interior settings.
On the other hand, it is possible to explain the diachronous decrease in oxygen isotopic values through a change in the moisture balance (and therefore δ18Osw) within the basin or an influx of open ocean waters (with different δ18Osw than the water it replaces) into a semi-restricted shallow epicontinental sea characterized by net evaporation. Net evaporation within the epicontinental sea has been proposed (Railsback et al. Reference Railsback, Ackerly, Anderson and Cisne1990; Ludvigson & Jacobson, Reference Ludvigson, Jacobson, Wittke, Ludvigson and Day1996) and would result in a 18O enrichment in basin interior settings. In such a scenario, marginal settings would be expected to experience decreasing oxygen isotopic values first as they are closest to the open ocean, an interpretation that fits well with the observed sequence of δ18O changes.
While it is not possible to distinguish between these alternatives without additional data (e.g. Nd to constrain changes in circulation patterns), we favour an interpretation invoking warming. As argued above, there is no compelling evidence to indicate significant δ18Osw gradients. Additionally, independent geochemical evidence (Nd and C) suggests that marginal settings like Alabama were well mixed with open-ocean water (Holmden et al. Reference Holmden, Creaser, Muehlenbachs, Leslie and Bergström1998). Therefore, values from Tidwell Hollow should be lower than those from interior settings where evaporative enrichment of 18O is expected. Contrary to this prediction, δ18O values from the Tidwell Hollow section are generally higher than those observed from the interior section (i.e. Kentucky). Finally, average temperature estimates assuming an open ocean δ18Osw value (i.e. not evaporatively enriched) at ~32°C match predicted Katian sea surface temperatures for the region (e.g. Herrmann et al. Reference Herrmann, Haupt, Patzkowsky, Seidov and Slingerland2004; Pohl et al. Reference Pohl, Donnadieu, Hir, Buoncristiani and Vennin2014).
5. Conclusions
New conodont oxygen isotope data from this study provide the first record of δ18O variability across SE North America from which to test for local environmental influences and, thereby, constrain regional δ18O trends. Oxygen isotopic values are consistent (i.e. exhibit a similar range and degree of scatter) across bathymetric features as well as across proposed water mass boundaries for the sampled interval, suggesting that local environmental factors (e.g. evaporation, precipitation and/or freshwater influx) did not result in significant δ18Osw differences across the region. We argue that the diachronous ~1.5‰ decrease in oxygen isotopic values across the M4/M5 likely reflects regional warming.
Based on these arguments, new oxygen isotopic records do not support the ‘cool-water’ interpretation for Sandbian – early Katian carbonates. Our results show no evidence of cooling across the M4/M5 boundary and so we argue that cooling was not the dominant factor controlling the fabrics and fossil content of early Katian carbonates in SE Laurentia. Instead, the shift in carbonate lithology across the M4/M5 seems better explained by a tectonically driven increase in turbidity in the basin. It should be noted that discriminating among the alternative hypotheses is possible because the compiled δ18O data provide temporal and spatial control on a continental scale, a result that highlights the importance of documenting patterns with relatively high resolution from multiple sections when inferring palaeoenvironmental forcing.
Acknowledgements
We thank Katie Meierdiercks and Ian Knoerl at James Madison University and Claire Jones, Zach Wright, Bryan Mogrovejo, John Michael Callen and Cody Schulte at Louisiana State University for assistance with fieldwork and laboratory work. Additionally, we thank Cheryl Kelley and Shannon Haynes for assistance with analyses at the University of Missouri Stable Isotope and Biogeochemistry Laboratory, and two anonymous reviewers. This research was funded by NSF EAR 1323444 to K.G.M., NSF EAR 1324577 to S.A.L., and NSF EAR 1324954 to A.D.H.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/S0016756817000589
