Investigation of Barium as a Temperature Proxy in Corals

Trace element concentrations in the aragonite skeletons of massive coral colonies can be measured to reconstruct seasonal- to centennial-scale variability in sea surface temperature (SST). Coral strontium-calcium (Sr/Ca) ratios are commonly used as a proxy for SST. However, barium-calcium (Ba/Ca) ratios, while especially sensitive to temperature in laboratory precipitation experiments, have not been explored as a coral SST proxy because seasonal Ba input from rivers overwhelms the SST signal in the coral skeleton. In this study, we investigated the Ba/Ca-SST relationship in corals from the Red Sea where Ba from river runoff is absent. Ba/Ca and Sr/Ca were measured in three Porites lutea coral cores, showing distinct seasonal cycles primarily influenced by SST, with infrequent spikes in Ba/Ca due to Ba deposition during dust storms. After removing the Ba/Ca peaks from dust storms, the resulting Ba/Ca time series show strong and significant correlations to SST, similar to Sr/Ca. Comparing Ba/Ca directly to Sr/Ca for each coral results in strong correlations that agree closely with experimental data. Our findings reveal the quantitative Ba/Ca-SST relationships in corals, and suggest that coral Ba/Ca can be used as a reliable temperature proxy in areas without river runoff.