Core-Top Calibrations for Element-To-Calcium Proxies of Seawater Properties in Bamboo Corals (Keratoisididae) From the South China Sea

Bamboo corals are promising archives of paleoceanographic conditions. Existing calibrations for element-to-calcium ratio (El/Ca) proxies of bamboo corals, however, are not necessarily calibrated to contemporaneous environmental parameters, thus weakening the reliability of the proxies. Here, we aim at calibrating the proxies by comparing El/Ca in the outermost surface of the calcareous skeletons of live-collected bamboo corals from the South China Sea (SCS) with modern environmental records. Statistical analysis suggests that Mg/Ca and Ba/Ca can be expressed as a function of in situ seawater temperature and silicate concentration, respectively, that is, Mg/Ca (mmol/mol) = 2.17 ± 0.51 * T (°C) + 74.90 ± 2.66 and Ba/Ca (μmol/mol) = 0.070 ± 0.020 * Silicate (μmol/kg) + 7.27 ± 2.42. The slope of the Mg/Ca-T equation from this study is slightly different from that in a previous study on bamboo corals, likely due to taxonomic and/or geographic differences of the corals and/or differences in sampling strategy and pre-treatment method. Intra- and inter-coral variations have small effects on Mg/Ca, yielding an uncertainty of 2.04 mmol/mol in Mg/Ca (95% confidence interval), equivalent to 0.94°C in estimated temperature. The slope of the Ba/Ca-silicate equation is the same as that in a previous study, suggesting little effect of geographic difference on Ba/Ca. Intra- and inter-coral variations in Ba/Ca are larger than those in Mg/Ca, possibly reflecting incorporation of multiple Ba-rich particulate phases and/or highly variable nutrient concentrations in the micro-environment near corals. These new calibrations allow reconstructions of paleo-temperature and nutrient concentration in the SCS on decadal and longer timescales.