Calcium Isotopic Constraints on the Transition From Aragonite Seas to Calcite Seas in the Cambrian
Guang‐Yi Wei, Ashleigh v.S. Hood, Noah J. Planavsky, Da Li, Hong‐Fei Ling, Lidya G. Tarhan
Abstract
Abstract 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 (δ 44 Ca) 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 δ 44 Ca 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.