Secular changes in seawater chemistry, including the Mg/Ca ratio, control the preferred abiotic carbonate precipitate from aragonite to calcite and back. In this paper, Pettijohn Medalist winner Tony Dickson (2004) empirically tested hypotheses regarding these controls using the composition of Mg-calcite rich echinoderms through the Phanerozoic. The data revealed low Mg/Ca ratios in the Jurassic and Cretaceous, and highs during the Cambrian, late Carboniferous to Triassic, and today; trends broadly follow proposed first-order Mg/Ca raios from geochemical models and fluid inclusions. These results were interpreted to represent independent confirmation of the general first-order trends derived from other means.
Echinoderm skeletal preservation: Calcite-aragonite seas and the Mg/Ca ratio of Phanerozoic oceans by J.A.D. (Tony) Dickson. Journal of Sedimentary Research, v. 74, p. 355-365.