Seawater temperature proxies based on D_Sr, D_Mg, and D_U from culture experiments using the branching coral Porites cylindrica

In order to investigate the incorporation of Sr, Mg, and U into coral skeletons and its temperature dependency, we performed a culture experiment in which specimens of the branching coral (Porites cylindrica) were grown for 1month at three seawater temperatures (22, 26, and 30°C). The results of this study showed that the linear extension rate of P. cylindrica has little effect on the skeletal Sr/Ca, Mg/Ca, and U/Ca ratios. The following temperature equations were derived: Sr/Ca (mmol/mol)=10.214(±0.229)-0.0642(±0.00897)×T (°C) (r²=0.59, p<0.05); Mg/Ca (mmol/mol)=1.973(±0.302)+0.1002(±0.0118)×T (°C) (r²=0.67, p<0.05); and U/Ca (μmol/mol)=1.488(±0.0484)-0.0212(±0.00189)×T (°C) (r²=0.78, p<0.05). We calculated the distribution coefficient (D) of Sr, Mg, and U relative to seawater temperature and compared the results with previous data from massive Porites corals. The seawater temperature proxies based on D calibrations of P. cylindrica established in this study are generally similar to those for massive Porites corals, despite a difference in the slope of D_U calibration. The calibration sensitivity of D_Sr, D_Mg, and D_U to seawater temperature change during the experiment was 0.64%/°C, 1.93%/°C, and 1.97%/°C, respectively. These results suggest that the skeletal Sr/Ca ratio (and possibly the Mg/Ca and/or U/Ca ratio) of the branching coral P. cylindrica can be used as a potential paleothermometer.