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 GSA Today, v. 9, no. 2, February 1999

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Science Article: (View Abstract)
Hypercalcification: Paleontology Links Plate Tectonics and Geochemistry to Sedimentology
by Steven M. Stanley, Lawrence A. Hardie

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In Memoriam 2
History of Geology Award 3
Executive Director Position 3
Award Nominations 7
Memorial Preprints 7
SAGE Remarks 8
Biggs Award 9
GeologyCo-Editor 9
Student Research Grants 10
Washington Report 12
GSA On the Web 12
GSAF Update 14
1999 Section Meeting -- Rocky Mountain 17
1999 Section Meeting -- North-Central 23
Shlemon Mentors Workshops 23
Penrose Conference Report: Ophiolites and Oceanic Crust 30
GSA Meetings 33
Bulletin and Geology Contents 34
1999 GeoVentures 35
Classifieds 38

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Hypercalcification: Paleontology Links Plate Tectonics and Geochemistry to Sedimentology
by Steven M. Stanley, Lawrence A. Hardie

ABSTRACT
During the Phanerozoic Eon, the mineralogies of nonskeletal marine cements and oolites have oscillated on a 100-200 m.y. scale between aragonite high-Mg calcite (aragonite seas) and low-Mg calcite (calcite seas). Oscillations in the carbonate mineralogy of dominant reef-building and sediment-producing organisms are in harmony with the oscillations for nonskeletal carbonates. These oscillations, together with synchronous oscillations in the mineralogy of marine potash evaporites, can be explained by secular variation in the Mg/Ca ratio of seawater driven by changes in the spreading rates along midocean ridges. The temporal patterns for biocalcification have come to light through a focus on (1) simple taxa that exert relatively weak control over the milieu in which they secrete their skeletons, and (2) taxa that hypercalcify-i.e., secrete massive skeletons or are exceptionally productive, for example, in forming voluminous chalk deposits. Most major reef-building and sediment-producing taxa belong to both of these categories. It appears that the Mg/Ca ratio of seawater has not only controlled Phanerozoic oscillations in hypercalcification by simple taxa, such as calcareous nannoplankton, sponges, and bryozoans, but has strongly influenced their skeletal evolution.


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