Deconvolving the biogeochemical controls on coral Sr ∕ Ca and Ba ∕ Ca proxies: new perspectives from paired stable Ca, Sr and Ba isotope compositions

This study introduces a novel approach to disentangle the biogeochemical controls on Sr/Ca and Ba/Ca signatures in coral skeletons using paired stable Ca, Sr and Ba isotopes to assess their specific uptake dynamics during coral biomineralization. The observed seasonal variations in stable Ca (δ44/42Ca) and Sr isotopes (δ88/86Sr) underscore the capability of corals to actively mediate the transport of Ca2+ and Sr2+ ions to the calcifying fluid prior to aragonite precipitation. We suggest that while the individual concentrations of Ca and Sr in the calcifying fluid vary seasonally, the Sr/Ca ratio of the fluid is likely comparable to that of seawater due to similar ion uptake dynamics. In contrast, the observed coral stable Ba isotope compositions (δ138/134Ba) remain essentially constant, suggesting a passive transport mechanism of Ba2+ ions, possibly through direct seawater leakage. The contrasting ion transport behaviours of Ba and Ca elucidate the underlying cause of the temperature-dependent variations in coral Ba/Ca records. By evaluating the uptake dynamics of Ca, Sr and Ba via their respective isotope systems, this study provides useful implications for the accurate application of coral Sr/Ca and Ba/Ca as proxies for paleoclimate reconstructions.