Secular changes in ocean chemistry are related to the mineralogy of carbonate precipitates, and had been predicted by global ocean models. Ten years ago, Dickson described the Mg content of Cambrian to Eocene echinoderm ossicles and compared the data to other first-order geochemical cycles and proxies. The data revealed systematic changes coincident with previous interpretations of ocean Mg/Ca ratios, with high mole% MgCO3 in Early Cambrian and late Carboniferous to Triassic samples, and low values in Silurian and Jurassic to Cretaceous echinoderms. The paper suggested that the data “add another independent line of evidence that collectively can leave little doubt that major changes in the seawater Mg/Ca ratio have occurred,” but noted other short-term changes.
EchinodermSkeletal Preservation: Calcite-Aragonite Seas and the Mg/Ca Ratio ofPhanerozoic Oceans by J.A.D. (Tony) Dickson, Journal of Sedimentary Research, v. 74, p. 355-365.