title: Fossil echinoderms as monitor of the Mg/Ca ratio of Phanerozoic oceans.
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 approximately 3.3, whereas echinoderms from the Jurassic to the Cretaceous indicate a Mg/Ca of approximately 1.4. The present seawater Mg/Ca is approximately 5.
authors.affiliation: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK. jadd1@esc.cam.ac.uk
authors.forename: J A D
authors.initials: JA
authors.lastname: Dickson
mesh_terms: D000818:Animals; D002118:Calcium / Q000032:analysis*; D002119:Calcium Carbonate / Q000032:analysis; D004447:Echinodermata / Q000737:chemistry*; D005580:Fossils; D055593:Geological Phenomena; D005844:Geology; D008274:Magnesium / Q000032:analysis*; D012617:Sea Urchins / Q000737:chemistry*; D012623:Seawater / Q000737:chemistry*; D013696:Temperature; D013995:Time
chemical_list: C005479:magnesium carbonate; D002119:Calcium Carbonate; D008274:Magnesium; D002118:Calcium