Sr thermometer for porites corals: little need to measure Ca?

Abstract

Cubic sub-samples of 1 mm3 (about 0.5 mg) were obtained from a Porites coral slice collected from Xisha Island, South China Sea, and analysed for Sr and Ca concentrations by ID-TIMS after precise weighing. Ca contents in the coral are quite uniform (mean = 38.16 wt%, 2σ = 0.11 wt%). The variation corresponds to ±0.4°C, about the level of uncertainty of the Sr/Ca-SST calibration. Sr concentrations and Sr/CA ratios covary linearly, and the line can be defined by the equation Sr/Ca x 103 = 0.111 Sr - 0.5061 (correlation coefficient r = 0.9955), where Sr is in μ mol/gm. Consequently, our results demonstrate that the previously established coral Sr/Ca thermometer, Sr/Ca x 103 = A + B x T, can be simplified as a Sr thermometer, 0.111Sr - 0.5061 = A + B x T, where Sr is in μ mol/gm. Therefore, measurement of Ca in corals could be unnecessary. If different levels of impurity exist in different corals, a few Ca measurements in each coral may be needed, but definitely there is no need to measure every sub-sample.