Echinoderm Skeletal Preservation: Calcite-Aragonite Seas and the Mg/Ca Ratio of Phanerozoic Oceans

The alternation between abiotic calcite and aragonite pre-cipitation in shallow seawater and the oscillation of seawater Mg/Ca ratio throughout the Phanerozoic are both popular hypotheses. Echi-noderms with well-preserved stereom provide new empirical evidence that supports both these hypotheses. Analyses are reported from 29 specimens, Cambrian to Eocene age, containing ossicles preserved ei-ther as Mg calcite, 3.3 to;8 mole % MgCO3, or as mixed calcite and dolomite,;5 to 12.5 mole % MgCO3. Echinoderms with high mole% MgCO3 occur in the Early Cambrian and late Carboniferous to Tri-assic; low values come from the Silurian and Jurassic to Cretaceous. The average composition of echinoderms with preserved stereom does not fall below 3.5 mol % MgCO3 during the Phanerozoic and reaches its highest mean value of 16.0 mol % MgCO3 today. An empirical partition coefficient of 0.03182 for modern tropical echinoids is used to indicate the Mg/Ca ratio of ancient seawater from the Mg/Ca ratio of fossil stereom; some assumptions are involved and a likely error is calculated from the Mg21 variation of modern echi-noids. High mean seawater Mg/Ca ratios are calculated for early Cam-brian (3.3) and the late Carboniferous to Triassic (2.3) but never reached today’s value of 5.2; low Mg/Ca ratios (1.1) are indicated from Jurassic to Cretaceous echinoderms. The 29 echinoderm samples plot close to first-order Mg/Ca seawater oscillations derived from geochem-ical models and Mg/Ca ratios determined from fluid inclusions, but considerable discrepancies exist when shorter (106 Myr) time intervals are considered. Further data and improved understanding of Mg par-titioning is required before accurate secular variation in the Mg/Ca of seawater can be determined from echinoderms. However, they are an underused resource in this context and provide an excellent seawater archive.