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. Nowadays, the development of in situ microanalysis technology provides a new opportunity for further understanding the distribution and variation of trace element concentrations in foraminifera shells. In this study, we focus on the in situ Mg/Ca ratios in four planktonic foraminifera shells from the surface sediments of the northeastern slope of the South China Sea (SCS). The results of electron microprobe mapping indicate that G. ruber had periodic bands of high Mg contents and Mg/Ca ratios, consistent with the results of LA-ICP-MS. In contrast, N. dutertrei, P. obliquiloculata, and G.inflata had thick calcite layer with low Mg contents and Mg/Ca ratios. The Mg/Ca ratios of shells may be attributed to symbionts, but the physiological regulation may also have some contributions. Meanwhile, contaminants may lead to the relatively higher Mg/Ca ratios. Therefore, the large Mg/Ca variations in foraminifera shells are not only affected by the surrounding seawater temperature but also constrained by other factors. At last, we employed this method to reconstruct the Mg/Ca-SST in the northeastern SCS over the past ~3000 years. The similar trend to the previous SST records proves that this method is reliable. We hope that this method can be widely applied in the future due to the efficient, fast, and high spatial resolution with small sample amounts.