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Earliest Holocene occurrence of the soft-shell clam, Mya arenaria, in the Greifswalder Bodden, Southern Baltic
Marine Geology 216 (2005) 79 – 82 www.elsevier.com/locate/margeo
Earliest Holocene occurrence of the soft-shell clam, Mya arenaria, in the Greifswalder Bodden, Southern Baltic Brigitte Behrendsa,b,*,1, Gqnther Hertweckc, Gerd Liebezeit a,b, Glenn Goodfriendd a Forschungszentrum Terramare, Schleusenstrage 1, 26382 Wilhelmshaven, Germany Carl von Ossietzky-Universita¨t, Institute for Pure and Applied Chemistry, P.O. Box 2503, 26111 Oldenburg, Germany c Forschungsinstitut Senckenberg, Su¨dstrand 40, 26382 Wilhelmshaven, Germany d Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Ranch Road, NW, Washington, DC 20015, USA b
Received 3 October 2003; received in revised form 4 January 2005; accepted 7 January 2005
Abstract Shells of Mya arenaria from the Greifswalder Bodden, southern Baltic Sea, were dated using aspartic acid racemisation. The soft shell clam occurs in the sediments of the bay to a depth of 20 cm and exceptionally to 36.5 cm. Consistent ages of 682F70 and 687F70 years before 1997 were obtained for three articulated shells recovered in life position. This places the first occurrence of M. arenaria clearly before the time of Columbus. These new data support earlier findings from northern Jutland, Denmark. D 2005 Elsevier B.V. All rights reserved. Keywords: Baltic; Holocene; amino acid dating; aspartic acid; Mya arenaria
1. Introduction The postglacial history of the Baltic Sea has been divided into different stages named after the dcharacteristic fossilsT: the Yoldia Sea, the Ancylus Lake, the Litorina Sea and the Limnea Sea (Dietrich and Ko¨ster, 1974). According to these authors, from T Corresponding author. Fax: +44 1912227891. E-mail addresses: [email protected] (B. Behrends)8 [email protected] (G. Liebezeit). 1 Present address: University of Newcastle, School of Marine Sciences and Technology, Newcastle upon Tyne NE1 7RU, UK. 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.01.002
about 500 years before present (BP) the soft-shell clam Mya arenaria has dominated the fossil fauna prompting Munthe already in 1894 to name this period accordingly the Mya Sea (Munthe, 1894). There is, however, debate on the earliest occurrence of the soft clam M. arenaria in European waters. This species, present in the Pleistocene, became extinct during the last glaciation and was introduced by man to Europe only after deglaciation (Strauch, 1972; Strasser, 1999). Hessland (1946) argues that Mya could not have been re-introduced before the time of Columbus, i.e. before 1492, as larvae could not have been transferred from America to Europe by natural
80
B. Behrends et al. / Marine Geology 216 (2005) 79–82
transport processes. Petersen et al. (1992), however, provide evidence from radiocarbon dating that Mya was present in Danish waters, i.e. at the Kattegatt coast in northern Jutland, already 720F80 years BP thus placing its earliest occurrence clearly before the voyage of Columbus. These authors attributed this early occurrence to a transfer from North America to Europe by the vikings. On the other hand, Duphorn et al. (1995) place the earliest occurrence of Mya shells in the Baltic to about 500 years BP. In detailed sediment sampling of the Greifswalder Bodden, southern Baltic Sea, we found that Mya shells were restricted to the upper decimetres of the sediment column exclusively. However, the sedimentary environment did not provide clues as to the exact age of the shells. To obtain more information on this we used amino acid racemisation dating of Mya shells. Especially aspartic acid has the potential due to its fast racemisation rate to date Holocene samples younger than approximately 600 years (Goodfriend et al., 1992; 1996; Goodfriend and Stanley, 1996). Due to the reservoir effect these are difficult to date using the radiocarbon technique. For the southern Baltic reservoir ages ranging from 162F65 to 332F80 years have been reported (Engstrand, 1965; Ha˚kansson, 1987; Olsson, 1980; see also http://radiocarbon. pa.qub.ac.uk/marine/).
2. Geological setting The Greifswalder Bodden, one of the numerous bays bordering the southern coast of the Baltic Sea, is situated between the island of Rqgen and the western Pomeranian mainland. It covers an area of 514 km2 and reaches a maximum water depth of 13.8 m. The characteristics of both the shorelines and the sea floor topography originate from the configuration of the end moraines built up during the last glaciation, i.e. the Weichselian: Rqgen Stage (e.g. Kolp, 1976). The depressions of this Pleistocene relief were filled with water during the Holocene transgression (Kolp, 1976). At 4000 years BP the present-day sea level was reached during phase 2 of the Litorina transgression. After a temporary slight regression at about 3300 BP the present sea level was reached again at 2000 years BP during the Litorina transgression phase 3 (Kliewe and Janke, 1982).
The coarse sediments of the marginal zone and the shallow areas in the eastern part of the Greifswalder Bodden originate from moraine material which, however, has undergone a complete marine reworking during the transgression. The fine-grained surface sediments of the western basin area are Holocene marine deposits (Niedermeyer et al., 1995). The most conspicuous biogenic feature of the Greifswalder Bodden sediments is the occurrence of M. arenaria in the upper few decimetres of the sediment column. In the basin area in the western part of the bay the living M. arenaria population is less abundant and represented only in some of the box cores. In the marginal belt and in the ridge area of the eastern part most of the sites are populated by living M. arenaria down to a sediment depth of about 10 cm. Dead M. arenaria shells in life position extend to sediment depths generally not exceeding 20 cm. Only in two samples shells were found in deeper layers, i.e., down to 25 cm in core GB 115 and 36.5 cm in core GB 124. Thus, only the upper 20 cm of the profiles exhibited in the box cores regularly represent the Mya Period. However, this representation is not valid for the sediment surrounding the M. arenaria shells in life position found in deepest position in the cores. For endobenthic bivalves contemporaneous sediment is situated on the level of the siphon passage openings, i.e. considerably above burrowing depth. Accordingly, the bdeepest Mya shellQ from core GB 115 was sticking in older limnic deposits belonging to a preLitorina stage of the Baltic Sea. Reworked Mya shells have been found only in box cores taken in water depths of less than 5.8 m. Three reworking horizons in 2, 8 and 15 cm sediment depth suggest an origin by three century-scale high magnitude storm events during the whole Mya Period (G. Hertweck, in preparation). Along with M. arenaria, living specimens and dead shells of Cerastoderma edule and Macoma balthica were found in the upper portions of the box cores from the Greifswalder Bodden. Shell material of these two species also occurs in the deeper portion of the cores, together with shells of Scrobicularia plana. This species regularly populates the western Baltic Sea which has a higher salinity than the average value of 7.3x measured in the Greifswalder Bodden. Its absence in the contemporary southern Baltic and in the sediments of the Mya Period points to a decrease
B. Behrends et al. / Marine Geology 216 (2005) 79–82
in salinity at the end of the Litorina Period in this region.
3. Dating To determine the date of the first occurrence of a postglacial M. arenaria population in the Greifswalder Bodden, shell samples from three individual specimen found in life position from the deepest stratigraphic levels of two box cores were analysed. (1) core GB 115; 54808.20VN, 013829.01VE; z=7.3 m; 22–25 cm sediment depth (two shells) and (2) core GB 124; 54811.18VN, 013843.56VE; z=5.2 m; 6–10.5 cm sediment depth (one shell). All three shells analysed were found sub-articulated, i.e. right and left valves close together, and in life position thus indicating that no transport has occurred after death. D/L ratios of aspartic acid were determined following the protocol of Goodfriend (1992). Briefly, amino acids were liberated by acid hydrolysis from mechanically cleaned shell material with 12 M HCl using stoichiometric amounts of acid. The amino acids were then derivatised to the corresponding N-trifluoroacetyl isopropyl esters with SO2Cl and trifluoroacetic acid and analysed by gas chromatography using a chiral capillary column (Chirasil/Val, 0.25 mm i.d., 50 m; Alltech) and a N/P sensitive detector. The results were calibrated against artificially heated recent Mya shells from the Lower Saxonian Wadden Sea which were cross-calibrated against 14C-dated Cerastroderma edule shells (Behrends, 1997; Behrends et al., 2003). From the individual errors in these calibrations (2.7%, 0.9%, 3.5%) a total maximum error in age of 10% is estimated. The individual samples gave ages of 682F70 and twice 687F70 years before 1997 placing the age at around 1310 ADF70 years. This makes the Mya specimens from the Greifswalder Bodden about 30 years younger than the samples of Petersen et al. (1992) which were recovered in northern Jutland. Our findings thus add further evidence that M. arenaria has been transferred to Europe earlier than the time of Columbus, most probably by the Vikings. Wehmiller and Miller (2000) discussed the advantages and problems of amino acid dating in detail.
81
Besides effects of a.o. water in the environment, acidity/alkalinity, type of protein and its protection by the inorganic matrix one major effect influencing amino acid D/L ratios is temperature in the sedimentary environment. For the Baltic it can be ruled out that water temperatures higher than present-day summer ones occurred since the formation of the Mya shells investigated from the Greifswalder Bodden. Although Andre´n et al. (2000) report tropical and subtropical diatom species to have been present in the Baltic during the Medieval warm period from 1000 to 1200 AD and hence higher sea surface temperatures this does not affect our data. Lower temperatures during the Little Ice Age may, on the other hand, have had an influence on the aspartic acid racemisation rate. From about 1300 AD to 1700 AD annual mean temperatures in England and presumably in the whole of northern Europe decreased by about 1 K (Lamb, 1977). Estimates of a slowdown in racemisation rate by a decrease in temperature by 1 K range from 19% (mammalian protein amino acids; Bada, 1982), 16% (isoleucine; Brown, 1985) and 19% (aspartic acid in biogenic carbonate; Goodfriend et al., 1995). Thus, lower ambient temperatures during the Little Ice Age may have had an effect on the aspartic acid racemisation in the Mya specimen analysed although this is difficult to substantiate without a detailed knowledge of the actual temperature history (Bada, 1982).
4. Conclusions The data presented above clearly provide evidence that M. arenaria was present in the southern Baltic Sea around 1300F70 AD. From this the start of the Mya Period can now be firmly placed at 700 rather than 500 years BP.
Acknowledgements Gqnther Hertweck gratefully acknowledges support by the captain and crew of RV Senckenberg during sampling. Part of the work was carried out while Brigitte Behrends held a grant by the Deutscher Akademischer Austauschdienst in Washington, DC. We are indebted to R.O. Niedermeyer for numerous useful discussions. We are indebted to
82
B. Behrends et al. / Marine Geology 216 (2005) 79–82
Kay Strand Petersen and an anonymous reviewer for helpful comments on an earlier version of the manuscript.
References Andre´n, E., Andre´n, T., Sohlenius, G., 2000. The Holocene history of the southwestern Baltic Sea as reflected in a sediment core from the Bornholm Basin. Boreas 29, 233 – 250. Bada, J.L., 1982. Racemization of amino acids in nature. Interdiscip. Sci. Rev. 7, 30 – 46. Behrends, B., 1997. Aminos7uren in Sedimenten und Partikeln des Wattenmeeres. PhD thesis, Oldenburg, 1–168. Behrends, B., Goodfriend, G.A., Liebezeit, G., 2003. Amino acid dating of recent intertidal sediments in the Wadden Sea, Germany. Senckenbergiana Marit. 32, 155 – 164. Brown, R.H, 1985. Amino acid dating. Origins 12, 8 – 25. Dietrich, G., Kfster, R., 1974. Geschichte der Ostsee. In: Magaard, L., Rheinheimer, G. (Eds.), Meereskunde der Ostsee. Springer, Berlin, pp. 5 – 10. Duphorn, K., Kliewe, H., Niedermeyer, R.-O., Janke, W., Werner, F., 1995. Die deutsche Ostseekqste. Sammlung geologischer Fqhrer 88. Gebr. Borntr7ger, Stuttgart. 282 pp. Engstrand, L.G., 1965. Stockhom natural radiocarbon measurements VI. Radiocarbon 7, 257 – 290. Goodfriend, G.A., 1992. Rapid racemization of aspartic acid in mollusc shells and potential for dating over recent centuries. Nature 357, 399 – 401. Goodfriend, G.A., Hare, P.E., Druffel, E.R.M., 1992. Aspartic acid racemization and protein diagenesis in corals over the last 350 years. Geochim. Cosmochim. Acta 56, 3847 – 3850. Goodfriend, G.A., Kashgarian, M., Harasewych, M.G., 1995. Use of aspartic acid racemization and post-bomb 14C to reconstruct growth rate and longevity of the deep-water slit shell Entemnotrochus adansonianus. Geochim. Cosmochim. Acta 59 (6), 1125 – 1129. Goodfriend, G.A., Brigham-Grette, J., Miller, G.H., 1996. Enhanced age resolution of the marine Quaternary record in the Arctic using aspartic acid racemization dating of bivalve shells. Quat. Res. 45, 176 – 187.
Goodfriend, G.A., D.J., Stanley, 1996. Reworking and discontinuities in Holocene sedimentation in the Nile Delta: documentation from amino acid racemization and stable isotopes in mollusk shells. Mar. Geol. 129, 271 – 283. H3kansson, S., 1987. University of Lund Radiocarbon Dates XX. Radicoarbon 29, 353 – 379. Hessland, Y., 1946. On the Quaternary Mya period in Europe. Ark. Zool. 37A, 1 – 51. Kliewe, H., Janke, K., 1982. Der holoz7ne Wasserspiegelanstieg der Ostsee im nordfstlichen Kqstengebiet der DDR. Petermanns Geogr. Mitt. 126, 65 – 74. Kolp, O., 1976. Submarine Uferterrassen der sqdlichen Ost- und Nordsee als Marken des holoz7nen Meeresanstiegs und der ¨ berflutungsphasen der Ostsee. Petermanns Geogr. Mitt. 120, U 1 – 22. Lamb, H.H., 1977. Climate—Present, Past, and Future, vol. 2. Climatic History and the Future Methuen, London. 837 pp. Munthe, H., 1894. Preliminary report on the physical geography of the Litorina Sea. Bullettin, 1 – 38. Niedermeyer, R.-O., Scholle, T., Flemming, B.W., Hertweck, G.B., 1995. Holocene sediments and basin development of a Pomeranian coastal lagoon (Greifswalder Bodden, southern Baltic). In: Mojski, J.E. (Ed.), Proceedings of the 3rd Marine Geological Conference bThe BalticQ. Prace Panstwowgo Instytutu Geologicznego, Warzawa, pp. 117 – 121. Olsson, I.U., 1980. Content of 14C in marine mammals from northern Europe. Radiocarbon 22, 662 – 675. Petersen, K.S., Rasmussen, K.L., Heinemeier, J., Rud, N., 1992. Clams before Columbus? Nature 359, 679. Strasser, M., 1999. Mya arenaria—an ancient invader of the North Sea coast. Helgol. Meeresunters. 52, 309 – 324. Strauch, F., 1972. Phylogenese, Adaptation und Migration einiger nordischer mariner Molluskengenera (Neptunea, Cyrtodaria und Mya). E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart. 210 pp. Wehmiller, J.F., Miller, G.H., 2000. Aminostratigraphic dating methods in Quaternary geology. In: Noller, J.S., Sowers, J.M., Lettis, W.R. (Eds.), Quaternary geochronology, methods and applications. American Geophysical Union Reference Shelf, pp. 187 – 222.
Earliest Holocene occurrence of the soft-shell clam, Mya arenaria, in the Greifswalder Bodden, Southern Baltic Brigitte Behrendsa,b,*,1, Gqnther Hertweckc, Gerd Liebezeit a,b, Glenn Goodfriendd a Forschungszentrum Terramare, Schleusenstrage 1, 26382 Wilhelmshaven, Germany Carl von Ossietzky-Universita¨t, Institute for Pure and Applied Chemistry, P.O. Box 2503, 26111 Oldenburg, Germany c Forschungsinstitut Senckenberg, Su¨dstrand 40, 26382 Wilhelmshaven, Germany d Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Ranch Road, NW, Washington, DC 20015, USA b
Received 3 October 2003; received in revised form 4 January 2005; accepted 7 January 2005
Abstract Shells of Mya arenaria from the Greifswalder Bodden, southern Baltic Sea, were dated using aspartic acid racemisation. The soft shell clam occurs in the sediments of the bay to a depth of 20 cm and exceptionally to 36.5 cm. Consistent ages of 682F70 and 687F70 years before 1997 were obtained for three articulated shells recovered in life position. This places the first occurrence of M. arenaria clearly before the time of Columbus. These new data support earlier findings from northern Jutland, Denmark. D 2005 Elsevier B.V. All rights reserved. Keywords: Baltic; Holocene; amino acid dating; aspartic acid; Mya arenaria
1. Introduction The postglacial history of the Baltic Sea has been divided into different stages named after the dcharacteristic fossilsT: the Yoldia Sea, the Ancylus Lake, the Litorina Sea and the Limnea Sea (Dietrich and Ko¨ster, 1974). According to these authors, from T Corresponding author. Fax: +44 1912227891. E-mail addresses: [email protected] (B. Behrends)8 [email protected] (G. Liebezeit). 1 Present address: University of Newcastle, School of Marine Sciences and Technology, Newcastle upon Tyne NE1 7RU, UK. 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.01.002
about 500 years before present (BP) the soft-shell clam Mya arenaria has dominated the fossil fauna prompting Munthe already in 1894 to name this period accordingly the Mya Sea (Munthe, 1894). There is, however, debate on the earliest occurrence of the soft clam M. arenaria in European waters. This species, present in the Pleistocene, became extinct during the last glaciation and was introduced by man to Europe only after deglaciation (Strauch, 1972; Strasser, 1999). Hessland (1946) argues that Mya could not have been re-introduced before the time of Columbus, i.e. before 1492, as larvae could not have been transferred from America to Europe by natural
80
B. Behrends et al. / Marine Geology 216 (2005) 79–82
transport processes. Petersen et al. (1992), however, provide evidence from radiocarbon dating that Mya was present in Danish waters, i.e. at the Kattegatt coast in northern Jutland, already 720F80 years BP thus placing its earliest occurrence clearly before the voyage of Columbus. These authors attributed this early occurrence to a transfer from North America to Europe by the vikings. On the other hand, Duphorn et al. (1995) place the earliest occurrence of Mya shells in the Baltic to about 500 years BP. In detailed sediment sampling of the Greifswalder Bodden, southern Baltic Sea, we found that Mya shells were restricted to the upper decimetres of the sediment column exclusively. However, the sedimentary environment did not provide clues as to the exact age of the shells. To obtain more information on this we used amino acid racemisation dating of Mya shells. Especially aspartic acid has the potential due to its fast racemisation rate to date Holocene samples younger than approximately 600 years (Goodfriend et al., 1992; 1996; Goodfriend and Stanley, 1996). Due to the reservoir effect these are difficult to date using the radiocarbon technique. For the southern Baltic reservoir ages ranging from 162F65 to 332F80 years have been reported (Engstrand, 1965; Ha˚kansson, 1987; Olsson, 1980; see also http://radiocarbon. pa.qub.ac.uk/marine/).
2. Geological setting The Greifswalder Bodden, one of the numerous bays bordering the southern coast of the Baltic Sea, is situated between the island of Rqgen and the western Pomeranian mainland. It covers an area of 514 km2 and reaches a maximum water depth of 13.8 m. The characteristics of both the shorelines and the sea floor topography originate from the configuration of the end moraines built up during the last glaciation, i.e. the Weichselian: Rqgen Stage (e.g. Kolp, 1976). The depressions of this Pleistocene relief were filled with water during the Holocene transgression (Kolp, 1976). At 4000 years BP the present-day sea level was reached during phase 2 of the Litorina transgression. After a temporary slight regression at about 3300 BP the present sea level was reached again at 2000 years BP during the Litorina transgression phase 3 (Kliewe and Janke, 1982).
The coarse sediments of the marginal zone and the shallow areas in the eastern part of the Greifswalder Bodden originate from moraine material which, however, has undergone a complete marine reworking during the transgression. The fine-grained surface sediments of the western basin area are Holocene marine deposits (Niedermeyer et al., 1995). The most conspicuous biogenic feature of the Greifswalder Bodden sediments is the occurrence of M. arenaria in the upper few decimetres of the sediment column. In the basin area in the western part of the bay the living M. arenaria population is less abundant and represented only in some of the box cores. In the marginal belt and in the ridge area of the eastern part most of the sites are populated by living M. arenaria down to a sediment depth of about 10 cm. Dead M. arenaria shells in life position extend to sediment depths generally not exceeding 20 cm. Only in two samples shells were found in deeper layers, i.e., down to 25 cm in core GB 115 and 36.5 cm in core GB 124. Thus, only the upper 20 cm of the profiles exhibited in the box cores regularly represent the Mya Period. However, this representation is not valid for the sediment surrounding the M. arenaria shells in life position found in deepest position in the cores. For endobenthic bivalves contemporaneous sediment is situated on the level of the siphon passage openings, i.e. considerably above burrowing depth. Accordingly, the bdeepest Mya shellQ from core GB 115 was sticking in older limnic deposits belonging to a preLitorina stage of the Baltic Sea. Reworked Mya shells have been found only in box cores taken in water depths of less than 5.8 m. Three reworking horizons in 2, 8 and 15 cm sediment depth suggest an origin by three century-scale high magnitude storm events during the whole Mya Period (G. Hertweck, in preparation). Along with M. arenaria, living specimens and dead shells of Cerastoderma edule and Macoma balthica were found in the upper portions of the box cores from the Greifswalder Bodden. Shell material of these two species also occurs in the deeper portion of the cores, together with shells of Scrobicularia plana. This species regularly populates the western Baltic Sea which has a higher salinity than the average value of 7.3x measured in the Greifswalder Bodden. Its absence in the contemporary southern Baltic and in the sediments of the Mya Period points to a decrease
B. Behrends et al. / Marine Geology 216 (2005) 79–82
in salinity at the end of the Litorina Period in this region.
3. Dating To determine the date of the first occurrence of a postglacial M. arenaria population in the Greifswalder Bodden, shell samples from three individual specimen found in life position from the deepest stratigraphic levels of two box cores were analysed. (1) core GB 115; 54808.20VN, 013829.01VE; z=7.3 m; 22–25 cm sediment depth (two shells) and (2) core GB 124; 54811.18VN, 013843.56VE; z=5.2 m; 6–10.5 cm sediment depth (one shell). All three shells analysed were found sub-articulated, i.e. right and left valves close together, and in life position thus indicating that no transport has occurred after death. D/L ratios of aspartic acid were determined following the protocol of Goodfriend (1992). Briefly, amino acids were liberated by acid hydrolysis from mechanically cleaned shell material with 12 M HCl using stoichiometric amounts of acid. The amino acids were then derivatised to the corresponding N-trifluoroacetyl isopropyl esters with SO2Cl and trifluoroacetic acid and analysed by gas chromatography using a chiral capillary column (Chirasil/Val, 0.25 mm i.d., 50 m; Alltech) and a N/P sensitive detector. The results were calibrated against artificially heated recent Mya shells from the Lower Saxonian Wadden Sea which were cross-calibrated against 14C-dated Cerastroderma edule shells (Behrends, 1997; Behrends et al., 2003). From the individual errors in these calibrations (2.7%, 0.9%, 3.5%) a total maximum error in age of 10% is estimated. The individual samples gave ages of 682F70 and twice 687F70 years before 1997 placing the age at around 1310 ADF70 years. This makes the Mya specimens from the Greifswalder Bodden about 30 years younger than the samples of Petersen et al. (1992) which were recovered in northern Jutland. Our findings thus add further evidence that M. arenaria has been transferred to Europe earlier than the time of Columbus, most probably by the Vikings. Wehmiller and Miller (2000) discussed the advantages and problems of amino acid dating in detail.
81
Besides effects of a.o. water in the environment, acidity/alkalinity, type of protein and its protection by the inorganic matrix one major effect influencing amino acid D/L ratios is temperature in the sedimentary environment. For the Baltic it can be ruled out that water temperatures higher than present-day summer ones occurred since the formation of the Mya shells investigated from the Greifswalder Bodden. Although Andre´n et al. (2000) report tropical and subtropical diatom species to have been present in the Baltic during the Medieval warm period from 1000 to 1200 AD and hence higher sea surface temperatures this does not affect our data. Lower temperatures during the Little Ice Age may, on the other hand, have had an influence on the aspartic acid racemisation rate. From about 1300 AD to 1700 AD annual mean temperatures in England and presumably in the whole of northern Europe decreased by about 1 K (Lamb, 1977). Estimates of a slowdown in racemisation rate by a decrease in temperature by 1 K range from 19% (mammalian protein amino acids; Bada, 1982), 16% (isoleucine; Brown, 1985) and 19% (aspartic acid in biogenic carbonate; Goodfriend et al., 1995). Thus, lower ambient temperatures during the Little Ice Age may have had an effect on the aspartic acid racemisation in the Mya specimen analysed although this is difficult to substantiate without a detailed knowledge of the actual temperature history (Bada, 1982).
4. Conclusions The data presented above clearly provide evidence that M. arenaria was present in the southern Baltic Sea around 1300F70 AD. From this the start of the Mya Period can now be firmly placed at 700 rather than 500 years BP.
Acknowledgements Gqnther Hertweck gratefully acknowledges support by the captain and crew of RV Senckenberg during sampling. Part of the work was carried out while Brigitte Behrends held a grant by the Deutscher Akademischer Austauschdienst in Washington, DC. We are indebted to R.O. Niedermeyer for numerous useful discussions. We are indebted to
82
B. Behrends et al. / Marine Geology 216 (2005) 79–82
Kay Strand Petersen and an anonymous reviewer for helpful comments on an earlier version of the manuscript.
References Andre´n, E., Andre´n, T., Sohlenius, G., 2000. The Holocene history of the southwestern Baltic Sea as reflected in a sediment core from the Bornholm Basin. Boreas 29, 233 – 250. Bada, J.L., 1982. Racemization of amino acids in nature. Interdiscip. Sci. Rev. 7, 30 – 46. Behrends, B., 1997. Aminos7uren in Sedimenten und Partikeln des Wattenmeeres. PhD thesis, Oldenburg, 1–168. Behrends, B., Goodfriend, G.A., Liebezeit, G., 2003. Amino acid dating of recent intertidal sediments in the Wadden Sea, Germany. Senckenbergiana Marit. 32, 155 – 164. Brown, R.H, 1985. Amino acid dating. Origins 12, 8 – 25. Dietrich, G., Kfster, R., 1974. Geschichte der Ostsee. In: Magaard, L., Rheinheimer, G. (Eds.), Meereskunde der Ostsee. Springer, Berlin, pp. 5 – 10. Duphorn, K., Kliewe, H., Niedermeyer, R.-O., Janke, W., Werner, F., 1995. Die deutsche Ostseekqste. Sammlung geologischer Fqhrer 88. Gebr. Borntr7ger, Stuttgart. 282 pp. Engstrand, L.G., 1965. Stockhom natural radiocarbon measurements VI. Radiocarbon 7, 257 – 290. Goodfriend, G.A., 1992. Rapid racemization of aspartic acid in mollusc shells and potential for dating over recent centuries. Nature 357, 399 – 401. Goodfriend, G.A., Hare, P.E., Druffel, E.R.M., 1992. Aspartic acid racemization and protein diagenesis in corals over the last 350 years. Geochim. Cosmochim. Acta 56, 3847 – 3850. Goodfriend, G.A., Kashgarian, M., Harasewych, M.G., 1995. Use of aspartic acid racemization and post-bomb 14C to reconstruct growth rate and longevity of the deep-water slit shell Entemnotrochus adansonianus. Geochim. Cosmochim. Acta 59 (6), 1125 – 1129. Goodfriend, G.A., Brigham-Grette, J., Miller, G.H., 1996. Enhanced age resolution of the marine Quaternary record in the Arctic using aspartic acid racemization dating of bivalve shells. Quat. Res. 45, 176 – 187.
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