Fossil Echinoderms As Monitor of the Mg/Ca Ratio of Phanerozoic Oceans

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Abstract
Opinion has long been divided as to whether the Mg/Ca ratio of seawater remained constant during the Phanerozoic or underwent substantial secular change. Existing empirical evidence for the Mg/Ca of ancient seawater provides a poorly resolved and often controversial signal. Echinoderm fossils that have retained their bulk original chemistry, despite micrometer-scale changes, preserve a record of seawater Mg/Ca and confirm that major changes in Mg/Ca occurred during the Phanerozoic. Echinoderms from the Cambrian and from the Carboniferous to the Triassic indicate a seawater Mg/Ca of ∼3.3, whereas echinoderms from the Jurassic to the Cretaceous indicate a Mg/Ca of ∼1.4. The present seawater Mg/Ca is ∼5.