Calcium isotopes in evaporites record variations in Phanerozoic seawater SO4 and Ca

Reconstructing variations in the major element chemistry of seawater provides constraints on how the global carbon and sulfur cycles have changed over geological time, but archives for this information are rare. This work generates a new independent record of ancient seawater composition, in particular the relative abundance of Ca and SO4, through Ca isotope measurements in marine evaporites. Evaporite sequences that reach halite saturation record large Ca isotope variability if SO4 > Ca, whereas a small range in Ca isotope ratios is observed if Ca > SO4. Analyses of geological evaporites indicate SO4rich seawater in the Neogene and Permian and Ca-rich seawater in the Cretaceous and Silurian. These results agree with previous reconstructions using fluid inclusions in halite, and demonstrate a new approach for extending our understanding of oceanic and geochemical evolution. INTRODUCTION The major element chemistry of seawater is linked to the global carbon cycle and climate through weathering of silicate rocks, formation of clays, and burial of calcium carbonate, and to the oxidation of Earth’s surface environment through cycling of seawater SO4. Changes in seawater chemistry over the Phanerozoic have been proposed to explain a range of observations, including the occurrence of inorganic marine carbonates as either aragonite or calcite (Sandberg, 1983), the presence of MgSO4 or KCl in late-stage evaporite minerals (Hardie, 1996), and the chemistry of fluid inclusions within evaporitic halite (Brennan and Lowenstein, 2002; Horita et al., 2002; Lowenstein et al., 2001, 2005). Many aspects of these records, however, remain untested. Several assumptions are required to transform fluid inclusion analyses into presumed seawater composition, due to the absence of certain ionic species, the evolved nature of the brine, and its extended storage in halite (Brennan et al., 2013). Other data sets, such as the composition of mid-ocean ridge flank veins (Coggon et al., 2010) and Mg/Ca ratios in well-preserved fossils (Dickson, 2002), generally confirm the direction of the fluid inclusion trends, but differences in the timing and magnitude of key changes suggest that the true seawater signatures contained in these records are yet to be resolved