Species specific Sr/Ca-δ18O relationships for three Tridacnidae species from the northern South China Sea

Skeletal remains of marine bivalves, Tridacna spp., can provide multi-proxy records of environmental variables. Sr/Ca ratio, which has been widely used in corals as a paleo-temperature proxy, has also been explored in Tridacna spp. shells in recent decades, but some controversies remain, especially regarding the different Sr/Ca-SST relationships across Tridacnidae species. In this study, ten specimens of three different species (Tridacna gigas, Tridacna squamosa and Tridacna derasa) were collected from the northern South China Sea and the monthly resolution Sr/Ca and δ18O ratios were investigated. Almost all high-resolution Sr/Ca profiles, determined by ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer), show pronounced annual cycles and are significantly correlated with paired δ18O value. However, Sr/Ca ratios of T. gigas are positively correlated with paired δ18O values, suggesting a negative correlation between T. gigas Sr/Ca and in-situ SST, while Sr/Ca ratios of T. derasa and T. squamosa are negatively correlated with paired δ18O, suggesting a positive correlation between SST and Sr/Ca in T. derasa and T. squamosa. These interspecies differences highlight the necessity of species identification before using Tridacnidae Sr/Ca ratios in paleoclimate reconstructions. Although the Sr/Ca-δ18O relationship has obvious interspecies differences, the clear annual cycles in all these specimens indicate that the Sr/Ca ratios of Tridacna spp. may have the potential to be used in reconstructing the past climate seasonality, ENSO variability and so on.