Calcium Isotopic Constraints on the Transition From Aragonite Seas to Calcite Seas in the Cambrian

The primary mineralogy of marine carbonates has varied over geological time in concert with the secular evolution of global climate and seawater chemistry. Here, we employed a multi-proxy geochemical and petrographic approach, including measuring the Ca isotope (delta Ca-44) and Sr content of Ediacaran-Cambrian carbonates, to provide new insights into the timing of the transition from a "dolomite-aragonite sea" to a "calcite sea" across the Ediacaran-Cambrian transition. We find robust evidence for the persistence of an aragonite sea well into Cambrian Age 2 (and potentially up through Age 4). Together with an updated petrographic compilation of abiotic carbonate precipitates (i.e., ooids and cements), these new delta Ca-44 and Sr data provide further evidence that there was a protracted transition from aragonite seas to calcite seas in the Cambrian. We propose that this transition was mediated, in part, by changes in seawater Mg/Ca ratios potentially regulated by the global marine redox state and extents of authigenic clay precipitation.