Assessment of Mg/Ca in Saccostrea glomerata (the Sydney rock oyster) shell as a potential temperature record

Bivalve shell Mg/Ca records can be important potential tracers of paleo-temperature, but evidence has shown that the temperature dependence of trace metal incorporation rates can vary significantly between different species and even within the same species. This study assesses one such bivalve specimen as a potential temperature proxy for the east coast of Australia. Specimens of the Sydney rock oyster (Saccostrea glomerata) were cultured for approximately one year at two locations on the east coast of Australia: Moreton Bay, Queensland, a semi-enclosed bay with consistently marine conditions, and Pambula Lake, New South Wales, a tidal lake which can undergo significant variations in salinity as a result of rainfall and river discharge. Transects of the oysters’ growth axis were analysed via LA-ICP-MS. Mg/Ca-temperature relationships for the oysters show a robust temperature dependence but there is also a clear discrepancy between the two sites. However, this discrepancy was not evident when the temperature dependence of Mg partition coefficients (D_Mg) was determined, indicating that the difference seen in the direct Mg/Ca_shell-temperature relationships can be accounted for by variation in salinity at the estuarine site of Pambula Lake. This has implications for the use of Mg/Ca as a paleo-temperature tracer within estuarine fauna as knowledge of the water chemistry is required to determine D_Mg values. However, S. glomerata Mg/Ca remains a promising paleo-temperature tracer as the direct Mg/Ca-temperature relationships can still be applied to samples from environments that experienced little variation in salinity. Combined analysis of Mg/Ca coupled with δ¹⁸O_shell records has the potential to enable the deconvolution of the combined salinity/temperature signal in the δ¹⁸O_shell and Mg/Ca_shell records.