The influence of seawater chemistry on carbonate-associated sulfate derived from coral skeletons

Abstract It has long been established that seawater Mg/Ca ratios fluctuate over geological time scales (10 6  years), synchronous with seawater sulfate (SO 4 2− ) concentrations. As carbonate minerals closely track the Mg/Ca ratio of seawater and contain a substantial amount of carbonate-associated sulfate (CAS), they may therefore be excellent proxies for coeval oscillations in seawater Mg/Ca ratios and SO 4 2− concentrations. Here, we report on a series of experiments designed to understand CAS incorporation into biogenic aragonite under seawater conditions characteristic of the Mesozoic and Cenozoic. In order to understand the combined effect of fluctuations in seawater Mg/Ca ratios and SO 4 2− concentrations on CAS incorporation in coral skeletons, fragments of Pocillopora damicornis were grown in treatments in which the SO 4 2− , Ca 2+ and Mg 2+ concentrations were varied independently and Ca 2+ and SO 4 2− concentrations were varied in combination. These experiments demonstrate that CAS incorporation in coral skeletons linearly and proportionally correlates with seawater SO 4 2− concentrations, but is complicated by concomitant changes in seawater Ca 2+ concentrations. These trends do not appear to be rate dependent and are likely related to the formation of ion complexes in seawater and/or biological changes induced by increased/decreased seawater Ca 2+ concentrations. This work has important implications for understanding fluctuations in Mesozoic and Cenozoic SO 4 2− concentrations and does not necessarily exclude coral skeletons as archives for understanding fluctuations in the SO 4 2− isotopic (δ 34 S) composition of seawater.