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.
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