Interpreting environmental change using bivalve shell geochemistry
To further our understanding of Earth’s climate beyond historical records, a number of paleoclimate proxies are used to infer how climate has changed throughout Earth’s history. Examined herein are two such geochemical proxies—the δ¹⁸O of the shell of the Australian mud or flat oyster, Ostrea angasi, and the Mg/Ca ratio of the shells of the Sydney Rock oyster, Saccostrea glomerata, and the common mussel, Mytilus galloprovincialis. The proxies were calibrated using in situ culturing experiments with modern specimens. Oysters and mussels were cultured for approximately one year in three locations representative of the broad distribution of the species along the east coast of Australia: Little Swanport, Tasmania (O. angasi, M. galloprovincialis); Pambula Lake, New South Wales (O. angasi, S. glomerata, M. galloprovincialis) and Moreton Bay, Queensland (S. glomerata). Temperature loggers were deployed with the specimens and water samples taken fortnightly. The O. angasi shells were sampled using a Merchantek microsampling system at the Vrije Universiteit Amsterdam and the University of Michigan, and δ¹⁸O was analysed via ICP-MS. The trace element ratios of S. glomerata shells were analysed via Laser Ablation ICP-MS at the Australian National University. The data from the Pambula Lake and Little Swanport O. angasi were combined to produce a regression relationship between temperature and δ¹⁸Oshell and δ¹⁸Owater. For S. glomerata, the Mg/Ca data from Pambula Lake and Moreton Bay were compiled to produce a mean Mg/Ca signal for the shells from each location. Linear regressions with temperature showed a significant difference in the S. glomerata Mg/Ca-temperature relationships for the two locations. This difference was not evident when the DMg-temperature relationship was calculated, indicating that the underlying salinity or water chemistry can impact upon the S. glomerata shell Mg/Ca. The Mg/Ca-temperature relationship within M. galloprovincialis shells from Pambula Lake and Little Swanport was also examined. The Little Swanport shells did not show a consistent relationship between Mg/Ca and temperature, while the Pambula Lake M. galloprovincialis Mg/Ca-temperature relationship compared well with published Mg/Ca-temperature relationships for other Mytilus species. The derived proxy-temperature relationships for the oyster species were then applied to archaeological specimens of O. angasi from the Severs Beach midden site, Pambula Lake, and S. glomerata from the Sandstone Point midden site, Moreton Bay, to test the potential of these species for use as paleoclimate archives. Results show that while interpretation of paleoclimate information from these proxies within these species is not straightforward, both δ¹⁸O in O. angasi and Mg/Ca in S. glomerata show good potential for use as paleoclimate proxies. Future work should focus on sampling a larger number of shells from a broader geographical area. This is the first study to investigate these Australian species, and given the abundance of oyster and mussel shells within Aboriginal midden sites along the eastern coastline of Australia there is clearly potential for this approach to make a valuable contribution to understandings of paleoclimate in the Australian region, particularly for the mid to late Holocene, for which current records are scarce.
sclerochronology, oysters, trace elements, stable isotopes, bivalve, geochemistry
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