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 SO4-rich 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.