Calcite sea
A calcite sea is a sea in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic carbonate precipitates. The Early Paleozoic and the Middle to Late Mesozoic oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and the Cenozoic (including today) are characterized by aragonite seas.[1][2][3][4][5][6]
The most significant geological and biological effects of calcite sea conditions include rapid and widespread formation of
Calcite seas were coincident with times of rapid seafloor spreading and global greenhouse climate conditions.[14] Seafloor spreading centers cycle seawater through hydrothermal vents, reducing the ratio of magnesium to calcium in the seawater through metamorphism of calcium-rich minerals in basalt to magnesium-rich clays.[2][5] This reduction in the Mg/Ca ratio favors the precipitation of calcite over aragonite. Increased seafloor spreading also means increased volcanism and elevated levels of carbon dioxide in the atmosphere and oceans. This may also have an effect on which polymorph of calcium carbonate is precipitated.[5] Further, high calcium concentrations of seawater favor the burial of CaCO3, thereby removing alkalinity from the ocean, lowering seawater pH and reducing its acid/base buffering.[18]
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Table showing the conditions for calcite and aragonite seas
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Tectonicmechanism for changing Mg/Ca ratios in seawater
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Grainstone with calciticooids and sparry calcite cement; Carmel Formation, Middle Jurassic, of southern Utah
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An encrustedbivalveexternal mold showing contemporaneous dissolution of the original aragonite shell and calcitic cementation of the mold
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An encrusted Ordovician nautiloid internal mold showing contemporaneous dissolution of the original aragonite shell and calcitic cementation
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The boring Palaeosabella in anbivalveshell. The borings penetrated an inner aragonitic shell layer which dissolved away.
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hardground, southern Ohio
References
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- ^ Wilson, M.A.; Palmer, T.J. (1992). "Hardgrounds and hardground faunas". University of Wales, Aberystwyth, Institute of Earth Studies Publications. 9: 1–131.
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- ^ a b Stanley, S.M.; Hardie, L.A. (1999). "Hypercalcification; paleontology links plate tectonics and geochemistry to sedimentology". GSA Today. 9: 1–7.
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- ^ Pojeta, J. Jr. (1988). "Review of Ordovician pelecypods". U.S. Geological Survey Professional Paper. 1044: 1–46.
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