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

Abstract

Abstract 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.

Keywords

• bamboo corals
• element‐to‐calcium ratio proxies
• South China Sea