Corals feel the water chemistry: trace elements in coral skeletons reflect accurately their seawater chemistry, biological and geochemical implications

Abstract

The incorporation of trace and minor elements into coral skeletons and the underlying chemical and biological processes that govern them, are highly relevant for understanding coral biomineralization and for accurate reconstructions of past ocean conditions. In the present experimental study, that follows our previous publications (Ram and Erez, 2021, 2023), we determined the partition of six cations (Li, Na, Mg, K, Sr and Ba) into corals skeletons by culturing nine hermatypic coral species in seawater with four different calcium concentrations (∼ 10,15, 20 and 25 mM). The cation to Ca ratios in the skeletons correlated linearly with their ratios in seawater, revealing consistent and species-specific partition coefficients (DCoral). Consistent with our previous work, we find that the partition coefficients for Li, Mg, Na, and K were significantly lower than one but higher than the inorganic values (DInorg), while for Sr and Ba, DCoral were higher than one but lower than DInorg values. In addition, DCoral for the elements with DEl < 1, showed significant inter-species systematic order, with the highest DCoral for A. lamarcki, corresponding to its highest calcification rates, whereas P. damicornis showed the lowest DCoral values, consistent with its lowest calcification rates. The opposite systematic trend was observed for the elements with DEl > 1. We attribute these systematic relationships between elemental partitioning and calcification rates to species-specific physiological control, indicating precipitation of the skeleton from a semi-closed seawater reservoir. The modified seawater that comprises this extracellular calcifying fluid follows Rayleigh distillation with respect to all the measured trace and minor elements. The level of Ca utilization (1-f) and the degree of isolation of the calcifying fluid from external seawater, control the efficiency and rate of the calcification. Given the consistent DCoral values observed across all Ca treatments (for all elements and all coral species) we conclude that kinetic effects on the partition coefficients were unlikely. The deviations from DInorg values (the so-called “vital effect”) are governed by the physiology of the calcification process, involving mainly pH and DIC elevation in the ECF that is well known for corals. In addition, the present study provides a strong basis for utilizing multi-elemental proxies in fossil corals for reconstructing past ocean chemistry and climate changes during the Cenozoic era and possibly beyond, well into the Mesozoic.