Introduction
Radiocarbon dating of marine material such as bivalve shells or foraminifera tests requires an estimate of the marine reservoir effect, because the carbon reservoir of the oceans is depleted in 14C compared to the atmosphere (Stuiver et al., Reference Stuiver, Pearson and Braziunas1986; Stuiver and Braziunas, Reference Stuiver and Braziunas1993; Alves et al., Reference Alves, Macario, Ascough and Bronk Ramsey2018). This leads to discrepancies between radiocarbon ages for marine and terrestrial material of the same calendar age, the marine samples appearing on average 400 14C years older. For the mixed surface layer of the oceans, a global marine calibration curve exists (Marine20; Heaton et al., Reference Heaton, Köhler, Butzin, Bard, Reimer, Austin, Bronk Ramsey, Grootes, Hughen, Kromer, Reimer, Adkins, Burke, Cook, Olsen and Skinner2020). However, regional differences need to be corrected by including a specific offset ΔR (Stuiver et al., Reference Stuiver, Pearson and Braziunas1986; Reimer & Reimer, Reference Reimer and Reimer2017). Rapid atmospheric 14C changes are dampened in marine waters and the marine calibration curve appears ‘smoothed’ relative to the atmospheric one. Hence, the use of the marine calibration curve in combination with a local correction factor ΔR should be preferred over the use of the terrestrial calibration curve with a generalised estimated age correction for the marine reservoir effect (Reimer & Reimer, Reference Reimer and Reimer2001).
The local offset ΔR can be determined by 14C dating of pre-bomb (commonly pre-AD 1955) material either from long-lived species with an ‘internal’ age control by annual growth bands (e.g. corals or bivalves) or from specimens of museum collections with known collection date. To go deeper in time, the dating of reliable paired marine and terrestrial material obtained from the same chronostratigraphic context can be used (e.g. Facorellis et al., Reference Facorellis, Maniatis and Kromer2016).
For the North Sea area, available ΔR values show ambiguous results. In the Calib database (www.calib.org/marine), the closest ΔR values available for the German Wadden Sea area are data obtained from a living Artica islandica (Linnaeus 1767) individual collected in the German Bight (Weidman, Reference Weidman1995). Accelerator mass spectrometry (AMS) 14C dating of annual pre-bomb growth rings (AD 1948, 1950 and 1954) of this long-lived bivalve resulted in ΔR values ranging from −189 ± 33 to 92 ± 116 years. This corresponds to reservoir ages between 219 and 488 14C years. In the Oyster Ground area (southern North Sea), Witbaard et al. (Reference Witbaard, Jenness, Van der Borg and Ganssen1994) measured ΔR values between −473 ± 74 and −120 ± 60 14C years for another Arctica islandica specimen. Finally, in fjords in northwestern Denmark, ΔR values obtained from several bivalve species vary from −32 ± 57 up to 375 ± 63 14C years (Heier-Nielsen et al., Reference Heier-Nielsen, Heinemeier, Nielsen and Rud1995). Therefore, a calendar age difference in the range of several hundred years must be expected for a marine 14C date depending on the chosen reference for ΔR. This is particularly important when studying human-environment developments with possible leads and lags (Ascough et al., Reference Ascough, Cook, Dugmore, Barber, Higney and Marian Scott2016).
Although coastal research, especially the reconstruction of former sea levels, largely relies on radiocarbon dating of inundated terrestrial material (e.g. basal peats; Meijles et al., Reference Meijles, Kiden, Streurman, van der Plicht, Vos, Gehrels and Kopp2018), dating of marine material is inevitable when reconstructing the dynamics of the sedimentation processes of the overlying marine sediments (e.g. Elschner et al., Reference Elschner, Scheder, Bungenstock, Bartholomä, Becker, Capperucci, Enters, Karle, Schlütz, Wehrmann and Hoffmann2020). Here, terrestrial material is sparse or reworked, and often marine organisms that build calcium carbonate shells are the only available material for 14C dating. Recently, transfer functions for relative sea-level changes based on assemblages of benthic foraminifera and ostracods have been established (e.g. Scheder et al., Reference Scheder, Frenzel, Bungenstock, Engel, Brückner and Pint2019). Sea-level reconstructions based on this approach also rely on 14C dating of marine material for chronological control.
Within the WASA project (Bittmann et al., Reference Bittmann, Bungenstock and Wehrmann2021) more than 60 samples of marine organisms have been 14C dated in order to reconstruct coastal dynamics around the East Frisian island of Norderney. Thus, a local ΔR value for solid palaeoenvironmental reconstructions of this coastal environment is crucial. For example, suspension feeders such as Cerastoderma edule and Mytilus edulis may partially incorporate terrestrial carbon from nearby river discharge into the Wadden Sea. Poirier et al. (Reference Poirier, Baumann and Chaumillon2019) hypothesise that coastal foraminifera take up 14C-depleted carbon from terrestrial lignocellulose debris via an unexplored pathway involving bacteria. Thus, a considerable ΔR offset in carbonate shells of the Wadden Sea from the global marine calibration curve can be expected.
In this study, we present results of 14C datings of shells from two bivalve specimens collected in the 19th century and demonstrate two exemplary applications of the obtained ΔR value.
Methods
At the Landesmuseum Natur und Mensch, Oldenburg, Germany, several juvenile specimens of Cerastoderma edule and Mytilus edulis (zoological taxonomy follows WoRMS, www.marinespecies.org) sampled in AD 1889 near the island of Wangerooge (Fig. 1) are stored in a sealed glass cylinder in a mixture of ethanol, glycerine and water (Fig. 2). In 2018, the original seal was broken and one specimen of each species was withdrawn. These were cooked in distilled water to remove the soft parts and the preserving agent. The shells were submitted to the Radiocarbon Laboratory, Poznań, Poland, where the ventral margin of one shell of each species was used for AMS 14C dating. The pre-treatment of samples included the removal of organic coatings and the outer carbonate layers with H2O2 and HCl, respectively. CO2 from the remaining material was obtained in a vacuum line by leaching with concentrated H3PO4 and was then reduced with H2 to prepare an AMS target. The IAEA C1 Carrara Marble was used as background sample.
Fig. 1. Map of the southern North Sea area with the islands of Norderney (N), Langeoog (L), Spiekeroog (S) and Wangerooge (W). Additionally, locations are shown for which previous ΔR data are available: German Bight (green dot; Weidmann, Reference Weidman1995), Oyster Ground (blue dot; Witbaard et al., Reference Witbaard, Jenness, Van der Borg and Ganssen1994) and Limfjorden (red dots; Heier-Nielsen et al., Reference Heier-Nielsen, Heinemeier, Nielsen and Rud1995). Crosses mark the positions of Mya arenaria shells of Essink et al. (Reference Essink, Oost, Streurman and Van der Pflicht2017). Data sources: DTM: EMODnet Bathymetry Consortium (2018) (http://doi.org/10.12770/18ff0d48-b203-4a65-94a9-5fd8b0ec35f6); coastline: EEA (www.eea.europa.eu/data-and-maps/data/); borders: Natural Earth (https://www.naturalearthdata.com).
Fig. 2. Glas cylinder with the original, sealed bivalves sampled in AD 1889 around the island of Wangerooge (A) and close-up views of the dated specimens Cerastoderma edule (B) and Mytilus edulis (C). Scale in mm (photographs (B) and (C) by Rolf Kiepe, NIhK).
The shell of Cerastoderma edule was measured with improved precision, i.e. two cathodes were prepared, analysed and the data averaged by the Radiocarbon Laboratory in Poznań.
The resulting 14C dates were used to calculate the local reservoir effect for the German Wadden Sea using the online application at http://calib.org/deltar/. Both 14C dates were combined using equations given by Ward & Wilson (Reference Ward and Wilson1978) by calculating a weighted mean, and the ΔR value of this combined age was determined in the same way.
Results and discussion
The obtained 14C ages for Mytilus edulis (530 ± 30 14C years; Table 1) and Cerastoderma edule (544 ± 21 14C years) are statistically similar (X2-test in Oxcal 4.3; Bronk Ramsey, Reference Bronk Ramsey2009) and justify the calculation of a weighted mean. Similarly, Ascough et al. (Reference Ascough, Cook, Dugmore, Scott and Freeman2005) found no significant difference in the 14C ages of several mollusc species from a distinct archaeological layer in Scotland. The resulting ΔR value of −85 ± 17 14C years (Table 1) represents the first estimate of the local marine reservoir effect for the German Wadden Sea area and corresponds to a reservoir age of 427 14C years.
Table 1. Reported 14C ages of two bivalves collected in AD 1889 with resulting regional offset ΔR from the global marine calibration curve Marine20. Please note that an earlier reported value of ΔR based on the Marine13 calibration curve (Reimer et al., Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffmann, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and Van der Plicht2013) is 74 ± 16 14C years (Elschner et al., Reference Elschner, Scheder, Bungenstock, Bartholomä, Becker, Capperucci, Enters, Karle, Schlütz, Wehrmann and Hoffmann2020).
a This sample was dated with improved precision using two cathodes at the AMS laboratory in Poznań (573 ± 26 and 511 ± 27 14C years, respectively).
This new ΔR value differs considerably from the previously available ΔR values of Weidman (Reference Weidman1995) and Witbaard et al. (Reference Witbaard, Jenness, Van der Borg and Ganssen1994) for the German Bight and the Oyster Ground with weighted averages of −160 ± 93 14C years and −261 ± 155 14C years, respectively. These negative ΔR values imply the incorporation of carbon slightly enriched in 14C (and hence reservoir ages <400 14C years), which can be explained by the hydrographical characteristics of the southern North Sea (Scourse et al., Reference Scourse, Wanamaker, Weidman, Heinemeier, Reimer, Butler, Witbaard and Richardson2012). In these shallow waters and with relatively high primary production rates an enhanced exchange with the atmosphere takes place and the carbon reservoir of the sea water is comparably small. Located closer to the Strait of Dover, the Oyster Ground might be more affected by the inflow of Atlantic surface water into the North Sea. In contrast, the German Bight is affected by freshwater inflow from the Elbe, leading to lower salinity values (Janssen et al., Reference Janssen, Schrum and Backhaus1999) and probably influencing the 14C content by water depleted in 14C (e.g. hardwater effect; Scourse et al., Reference Scourse, Wanamaker, Weidman, Heinemeier, Reimer, Butler, Witbaard and Richardson2012). Philippsen (Reference Philippsen2013) reports reservoir ages of c.1900–3000 14C years from water dissolved inorganic carbon (DIC) of the Alster, a tributary of the Elbe. Reservoir effects of similar magnitude exist for the Waal, the main distributary branch of the Rhine (Lanting & Van der Plicht, Reference Lanting and Van der Plicht1998). A riverine influence may cause the less negative ΔR value obtained from the German Bight compared to the Oyster Ground.
In addition, the large variability within the data from the German Bight and the Oyster Ground is striking. In both studies, growth bands of Arctica islandica were analysed and showed variations of >100 14C years in ΔR within one or two decades. It is speculative if this variability can be attributed solely to actual changes in the reservoir age of the North Sea water, possibly due to changing current regimes, or if larger uncertainties in the early days of AMS-14C dating also play a certain role (e.g. Chen et al., Reference Chen, Guo and Liu2011).
Even though an intense mixing by tidal currents in the Wadden Sea should favour an atmosphere–sea CO2 exchange, the new ΔR value obtained near the island of Wangerooge is considerably higher than at the two offshore sites. This might be because of a higher contribution of old, 14C-depleted terrestrial/fluvial carbon to the marine DIC, for example by CO2 originating from the decomposition of organic sediments such as peat (Lanting & Van der Plicht, Reference Lanting and Van der Plicht1998). Although all three bivalve species discussed here are suspension feeders, species effects may likewise play a role, e.g. differences in the contribution of metabolic carbon to the shell carbonate. Gillikin et al. (Reference Gillikin, Lorrain, Bouillon, Willenz and Dehairs2006) report a variable percentage (between 0 and 10%) of metabolic carbon in shells of Mytilus edulis, whereas this percentage seems to be more constant and close to the theoretically derived 10% for Arctica islandica (Beirne et al., Reference Beirne, Wanamaker and Feindel2012).
For a specific location, ΔR values are generally assumed to be constant in time (Hua, Reference Hua, Rink and Thompson2013) due to the lack of data. However, models suggest a large temporal variability of several hundred years for the regional marine reservoir effect during the late Quaternary (Franke et al., Reference Franke, Paul and Schulz2008). Regarding the use of marine 14C dates for the Wadden Sea, even variations of a few decades can be crucial (see applications below). Tisnérat-Laborde et al. (Reference Tisnérat-Laborde, Paterne, Métivier, Arnold, Yiou, Blamart and Raynaud2010) have used 14C analyses of mollusc shells collected between AD 1823 and 1952 mainly from the French Atlantic coast to infer changes in the marine reservoir effect. They report a mean ΔR value of −7 ± 50 14C years for the pre-anthropogenic period and considerable variations of the marine reservoir age from around 90 to 170 14C years for the period between AD 1885 and 1950. These differences are correlated to changes in ocean currents and therefore in reservoir sources.
Given the large but often non-catalogued collections in natural history museums in Germany and across Europe, there seems to be ample opportunity for future research on this topic.
Applications
1 Dating of marine bivalves from sediments of the East Frisian Islands
Within the WASA project (Bittmann et al., Reference Bittmann, Bungenstock and Wehrmann2021), 38 of the more than 60 AMS 14C-dated marine samples were bivalve shells (Table 2) with both valves still attached and partly occurring in living position. They were sampled at various depths in sediment cores around the island of Norderney (Fig. 1). This dataset is complemented by seven previously unpublished 14C dates from bivalves from Langeoog (H. Freund) as well as five 14C measurements of Cerastoderma edule obtained around Spiekeroog published by Behrends et al. (Reference Behrends, Goodfriend and Liebezeit2003; Table 2). Radiocarbon dates of Cerastoderma edule published by Flessa (Reference Flessa1998) were not included, because this study focuses on reworked (transported) shells collected from surficial sediments. Articulated specimens were specifically excluded. No other published 14C dates of marine bivalves are accessible from the Wadden Sea of the East Frisian Islands. Further available radiocarbon dates of pooled individuals of the foraminifera Ammonia tepida (Cushman 1926; WASA project) as well as dates obtained from the aquatic snail Peringia ulvae (Pennant 1777; courtesy of H. Freund) are not included in this example. In contrast to articulated bivalve shells, foraminifera and gastropods can potentially be reworked.
Table 2. Compiled 14C dates of bivalves for the Wadden Sea area of the East Frisian Islands. pMC: percentage of modern carbon.
With the exception of two dates, all 14C dates are younger than 3000 cal BP. Fifteen samples of the WASA project resulted in modern ages (percentage of modern carbon (pMC) > 100%) and will not be considered here. However, their young ages reflect the highly dynamic sedimentation processes in the Wadden Sea with localised, rapid sediment accumulation. For the remaining 35 samples, a cumulative probability distribution (cpd) was calculated in R (R Core Team, 2019) based on the algorithms provided in package CLAM (version 2.3.2; Blaauw, Reference Blaauw2010) using the Marine20 calibration curve (Heaton et al., Reference Heaton, Köhler, Butzin, Bard, Reimer, Austin, Bronk Ramsey, Grootes, Hughen, Kromer, Reimer, Adkins, Burke, Cook, Olsen and Skinner2020) and our new ΔR value of −85 ± 17 14C years. Cumulative (or summed) radiocarbon probability distributions are commonly used in archaeology as a proxy record for human activity phases to be compared with palaeoenvironmental records (e.g. Feeser et al., Reference Feeser, Dörfler, Kneisel, Hinz and Dreibrodt2019). However, Williams (Reference Williams2012) notes that false peaks can be obtained during the calibration process due to ‘steep’ portions in the calibration curve, whereas plateaus in the calibration curve will cause a dampening of peaks. To assess these effects, the following randomisation test was carried out: for the time period between 0 and 3000 cal BP, n = 35 calendar ages with an assumed error of 30 14C years were randomly selected from a uniform distribution, their radiocarbon age determined based on the Marine20 calibration curve and a ΔR of −85 ± 17 14C years and the cpd calculated as described above. This step was repeated 2000 times, and from the resulting dataset the 95% percentile was calculated. Any peak in the cpd of the original dataset that exceeds the 95% percentile of 2000 cpds of 35 randomly distributed 14C dates can therefore be considered to be significant on a 95% level.
The results of these calculations are shown in Fig. 3, together with the sea-level curve obtained by Behre (Reference Behre2007). The cpd of the Wadden Sea 14C dataset exhibits two prominent peaks at c.100–350 cal BP and 800–1000 cal BP exceeding the 95% percentile level. In contrast, only a few 14C dates cover the time periods 500–650 cal BP and 1350–1600 cal BP. These intervals coincide remarkably with phases of rising sea levels (peaks in the cpd) as identified by Behre (Reference Behre2007) and phases with relatively stable sea level (few 14C dates and low cpd). Apparently, bivalves are preserved in the sediment record during transgressive phases and presumably rapid sediment deposition. During phases of relatively stable sea level, highly dynamic processes of erosion and redeposition of sediments in the Wadden Sea probably prevented the sedimentological conservation of bivalves in living position. Articulated bivalves were preferentially preserved during phases of rapidly rising sea levels when a net accumulation of sediments prevailed and the abrasion base of the sea moved upwards. These findings are in agreement with conclusions of Brett (Reference Brett1995) who states that conservation Lagerstätten are typically found during transgressive and early highstand phases.
Fig. 3. Cumulative probability distribution for 35 marine 14C dates of bivalve shells from the Wadden Sea around the East Frisian Islands. Blue line: sea-level curve (Behre, Reference Behre2007); red line: 95% percentile based on a randomisation procedure as described in the text.
Although the dataset used is still limited (e.g. by its small sample size) and an (unintentional) sampling bias cannot be excluded, it suggests a remarkable relationship between taphonomic preservation and sediment dynamics controlled by sea-level changes.
2 14C dates and the timing of Mya arenaria introduction to the eastern North Atlantic
In a recent study, Essink et al. (Reference Essink, Oost, Streurman and Van der Pflicht2017) analysed a total of eight specimens of the marine bivalve Mya arenaria (Linnaeus 1758), which were found in sediment records at several sites in the Netherlands (Fig. 1). Mya arenaria disappeared in Europe during the Pleistocene (Strasser, Reference Strasser1999) until its reintroduction from North America, either by French colonisers or earlier by Vikings (Norse) via Greenland. Originally, the Mya arenaria shells were 14C-dated and calibrated using the atmospheric calibration curve (Intcal13; Reimer et al., Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffmann, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and Van der Plicht2013) with an estimated reservoir age of 400 14C years for fully marine specimens and 840 14C years (Essink et al., Reference Essink, Oost, Streurman and Van der Pflicht2017) for a sample with riverine influence. The ranges of these calibrated dates predate the landing on the American continent of Columbus in AD 1492, and therefore an introduction of this species to Europe by Viking settlers from northeastern North America was postulated. This hypothesis seems to be supported by additional datings of Mya arenaria. Petersen et al. (Reference Petersen, Rasmussen, Heinemeier and Rud1992) reported an age of AD 1245–1295 for northern Denmark, and Behrends et al. (Reference Behrends, Hertweck, Liebezeit and Goodfriend2005) dated Mya arenaria shells in the southern Baltic Sea to AD 1310 ± 70 years using aspartic acid racemisation.
When recalibrating the dates reported by Essink et al. (Reference Essink, Oost, Streurman and Van der Pflicht2017) with Calib 8.2 (Stuiver et al. Reference Stuiver, Reimer and Reimer2020) using the Marine20 calibration curve and the new ΔR value of −85 ± 17 14C years, all seven recalibrated samples might be younger than AD 1492 (Table 3). The age of Mya arenaria published by Petersen et al. (Reference Petersen, Rasmussen, Heinemeier and Rud1992) is lacking crucial information such as the original 14C age and a possibly applied marine reservoir correction and can therefore not be recalibrated. The aspartic acid racemisation method applied on Mya arenaria samples from the Baltic Sea relies itself on cross-calibration against 14C-dated bivalves from the Wadden Sea (Behrends et al., Reference Behrends, Hertweck, Liebezeit and Goodfriend2005). For this, several samples of Cerastoderma edule shells were 14C-dated and calibrated using a reservoir age of 377 ± 16 14C years (Behrends et al., Reference Behrends, Goodfriend and Liebezeit2003). Although detailed information is lacking, it is likely that recalibration of these dates will change this cross-calibration and lead to a new assessment of these results, especially if slower racemisation rates are also taken into account (Behrends et al., Reference Behrends, Hertweck, Liebezeit and Goodfriend2005). In addition, a direct 14C dating of these samples would be preferable for comparison, using available ΔR values for the Baltic Sea for calibration (e.g. Lougheed et al., Reference Lougheed, Filipsson and Snowball2013).
Table 3. Reported 14C dates of Mya arenaria by Essink et al. (2017). All highlighted calibrated age ranges include periods younger than AD 1492 and hence do not necessarily date the introduction of Mya arenaria to Europe before Columbus landed in America.
a See text for discussion.
In contrast to the conclusion by Essink et al. (Reference Essink, Oost, Streurman and Van der Pflicht2017), the recalibrated 14C age ranges do not provide conclusive evidence to support the hypothesis of an introduction of Mya arenaria to the European North Atlantic prior to Columbus’ contact with the American continent. More 14C dates of Mya arenaria and solid estimates of ΔR values for the Dutch Wadden Sea and of this particular timespan are necessary to solve this question.
Acknowledgements
We gratefully acknowledge Christina Barilaro and Kay Fuhrmann of the Landesmuseum Natur und Mensch in Oldenburg, Germany, for providing us with shells from their collection and Tomasz Goslar from the Poznań Radiocarbon Laboratory for his support. Two anonymous reviewers made helpful comments. The reported research is part of the WASA project (The Wadden Sea as an archive of landscape evolution, climate change and settlement history: exploration – analysis – predictive modelling), funded by the ‘Niedersächsisches Vorab’ of the VolkswagenStiftung within the funding initiative ‘Küsten- und Meeresforschung in Niedersachsen’ of the Ministry for Science and Culture of Lower Saxony, Germany (project VW ZN3197).
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