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Fossil Echinoderms As Monitor of the Mg/Ca Ratio of Phanerozoic Oceans
J. A. D. Dickson
Opinion has long been divided as to whether the Mg/Ca ratio of
seawater remained constant during the Phanerozoic or underwentsubstantial secular change. Existing empirical evidence for theMg/Ca
of ancient seawater provides a poorly resolved and oftencontroversial
signal. Echinoderm fossils that have retained theirbulk original
chemistry, despite micrometer-scale changes, preservea record of
seawater Mg/Ca and confirm that major changes in Mg/Caoccurred during
the Phanerozoic. Echinoderms from the Cambrianand from the
Carboniferous to the Triassic indicate a seawaterMg/Ca of ~3.3,
whereas echinoderms from the Jurassic to the Cretaceousindicate a
Mg/Ca of ~1.4. The present seawater Mg/Ca is ~5.
Department of Earth Sciences, University of Cambridge, Downing
Street, Cambridge, CB2 3EQ, UK. E-mail: jadd1{at}esc.cam.ac.uk
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