Throughout the Phanerozoic, the primary inorganic marine calcium carbonate mineralogy oscillated between calcite and aragonite, reflecting changes in seawater chemistry. These variations in seawater composition also appear to have influenced the evolution of calcifying organisms. However, the processes controlling these mineralogical and biological changes are poorly constrained. Previous work has focused mainly on the Mg/Ca ratio in seawater as the primary driver. Here, we examine the role of dissolved SO4 in these processes by performing controlled laboratory precipitation experiments and geochemical modeling of evaporite formation. We show that an increase in dissolved SO4 decreases the Mg/Ca ratio at which calcite is destabilized and aragonite becomes the dominant CaCO3 polymorph. Our data suggest that the Mg/Ca and SO4 thresholds for formation of calcite seas are significantly lower than previous estimates and are mutually dependent. This shows that our understanding of Phanerozoic changes in seawater chemistry and the models relating this to primary CaCO3 mineralogy need reevaluation.