The in situ Mg/Ca ratios of planktonic foraminifera shells in the northeastern South China Sea: an attempt to get efficient and reliable proxies

Planktonic foraminifera are one of the important carriers of the physico-chemical environments. Recent advances in in situ microanalysis techniques provide new insights into the spatial distribution and variations of trace element concentrations of individual foraminifera shells. In this study, we focus on the variations of the in situ Mg/Ca ratios of four planktonic foraminifera species, which live in the different depths, from the surface sediments of the northeastern slope of the South China Sea (SCS). Both electron microprobe mapping and LA-ICP-MS results indicate that Globigerinoides ruber exhibits periodic bands with high Mg contents and Mg/Ca ratios, which may be attributed to symbionts. In contrast, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia inflata exhibit thick calcite layers with low Mg/Ca ratios, potentially associated with calcite crusts. Meanwhile, high Mg/Ca ratios may be attributed to the contaminants, especially on the shell surface. These findings demonstrate that the variations of Mg/Ca ratios in foraminifera shells are controlled not only by the surrounding seawater temperature but also by other factors.

Applying this method, we reconstructed Mg/Ca-SST in the northeastern SCS over the past ∼3000 years. The results indicate that the reconstructed SST trend aligns with the previous regional records, which validates the robustness of the method. We hope that this method can be widely applied in the future due to the efficiency, rapid processing, and high spatial resolution with minimal sample requirements.