Anion elements incorporation into corals skeletons: Experimental approach for biomineralization and paleo-proxies

The elemental composition of coral skeletons provides important information for palaeoceanographic reconstructions and coral biomineralization. Partition of anions and their stable isotopes in coral skeleton enables the reconstruction of past seawater carbonate chemistry, paleo-CO₂, and past climates. Here, we investigated the partition of B, S, As, Br, I, and Mo into the skeletons of two corals, Acropora cervicornis and Pocillopora damicornis, as a function of calcium and carbonate concentrations.^* Anion-to-calcium ratio in the corals (An/Ca_Coral) were correlated with the equivalent ratios in the culturing seawater (An/CO₃^2-_SW). Negative intercepts of these relationships suggest a higher CO₃^2- concentration in the coral extracellular calcifying fluid (ECF) relative to seawater, from which the skeleton precipitates. The enrichment factor of CO₃^2- at the ECF was 2.5 for A. cervicornis and 1.9 for P. damicornis, consistent with their relative calcification rates. The CO₃^2-_ECF concentrations thus calculated are similar to those proposed by previous studies based on B/Ca coupled with δ¹¹B, as well as by direct measurements using microsensors and fluorescent dyes. Rayleigh fractionation modeling demonstrates a uniform Ca utilization at various Ca_SW concentrations, providing further evidence that coral calcification occurs directly from a semiclosed seawater reservoir as reported previously. The partition coefficients reported in this study for B, S, As, Br, I, and Mo open up wide possibilities for past ocean chemistry reconstructions based on Br having long residence time (~160 Ma) in the ocean. Other elements like S, Mo, B, as well as pCO₂ may also be calculated based on these elements in fossil coral.