Sea chemistry favouring low-magnesium calcite as the inorganic calcium carbonate precipitate
The alternation of calcite and aragonite seas through geologic time
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]
Jurassic hardground with encrusting oysters and borings
The most significant geological and biological effects of calcite sea conditions include rapid and widespread formation of carbonate hardgrounds,[7][8][9] calcitic ooids,[1][10] calcite cements,[2] and the contemporaneous dissolution of aragonite shells in shallow warm seas.[6][11] Hardgrounds were very common, for example, in the calcite seas of the Ordovician and Jurassic, but virtually absent from the aragonite seas of the Permian.[7]
Fossils of invertebrate organisms found in calcite sea deposits are usually dominated by either thick calcite shells and skeletons,[12][13][14][15] were infaunal and/or had thick periostraca,[16] or had an inner shell of aragonite and an outer shell of calcite.[17] This was apparently because aragonite dissolved quickly on the seafloor and had to be either avoided or protected as a biomineral.[6]
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]
Table showing the conditions for calcite and aragonite seas
Tectonic mechanism for changing Mg/Ca ratios in seawater
Grainstone with calcitic ooids and sparry calcite cement; Carmel Formation, Middle Jurassic, of southern Utah
An encrusted Ordovician bivalve external mold showing contemporaneous dissolution of the original aragonite shell and calcitic cementation of the mold
An encrusted Ordovician nautiloid internal mold showing contemporaneous dissolution of the original aragonite shell and calcitic cementation
The boring Palaeosabella in an Ordovician bivalve shell. The borings penetrated an inner aragonitic shell layer which dissolved away.
Petroxestes borings in an Upper Ordovician hardground, southern Ohio
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^ abWilkinson, B.H.; Owen, R.M.; Carroll, A.R. (1985). "Submarine hydrothermal weathering, global eustacy, and carbonate polymorphism in Phanerozoic marine oolites". Journal of Sedimentary Petrology. 55: 171–183. doi:10.1306/212f8657-2b24-11d7-8648000102c1865d.
^ abcWilkinson, B.H.; Given, K.R. (1986). "Secular variation in abiotic marine carbonates: constraints on Phanerozoic atmospheric carbon dioxide contents and oceanic Mg/Ca ratios". Journal of Geology. 94 (3): 321–333. Bibcode:1986JG.....94..321W. doi:10.1086/629032. S2CID 128840375.
^Morse, J.W.; Mackenzie, F.T. (1990). "Geochemistry of sedimentary carbonates". Developments in Sedimentology. 48: 1–707. doi:10.1016/S0070-4571(08)70330-3.
^ Hardie , Lawrence A (1996). "Secular variation in seawater chemistry: An explanation for the coupled secular variation in the mineralogies of marine limestones and potash evaporites over the past 600 my". Geology. 24 (3). Geological Society of America: 279–283. Bibcode:1996Geo....24..279H. doi:10.1130/0091-7613(1996)024<0279:svisca>2.3.co;2.
^ abcPalmer, T.J.; Wilson, M.A. (2004). "Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas". Lethaia. 37 (4): 417–427 [1]. Bibcode:2004Letha..37..417P. doi:10.1080/00241160410002135.
^ abPalmer, T.J. (1982). "Cambrian to Cretaceous changes in hardground communities". Lethaia. 15 (4): 309–323. Bibcode:1982Letha..15..309P. doi:10.1111/j.1502-3931.1982.tb01696.x.
^Palmer, T.J.; Hudson, J.D.; Wilson, M.A. (1988). "Palaeoecological evidence for early aragonite dissolution in ancient calcite seas". Nature. 335 (6193): 809–810. Bibcode:1988Natur.335..809P. doi:10.1038/335809a0. S2CID 4280692.
^Wilson, M.A.; Palmer, T.J. (1992). "Hardgrounds and hardground faunas". University of Wales, Aberystwyth, Institute of Earth Studies Publications. 9: 1–131.
^Cherns, L.; Wright, V.P. (2000). "Missing molluscs as evidence of large-scale, early skeletal aragonite dissolution in a Silurian Sea". Geology. 28 (9): 791–794. Bibcode:2000Geo....28..791C. doi:10.1130/0091-7613(2000)28<791:MMAEOL>2.0.CO;2.
^Wilkinson, B.H. (1979). "Biomineralization, paleooceanography, and the evolution of calcareous marine organisms". Geology. 7 (11): 524–527. Bibcode:1979Geo.....7..524W. doi:10.1130/0091-7613(1979)7<524:BPATEO>2.0.CO;2.
^Stanley, S.M.; Hardie, L.A. (1998). "Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in seawater chemistry". Palaeogeography, Palaeoclimatology, Palaeoecology. 144 (1–2): 3–19. Bibcode:1998PPP...144....3S. doi:10.1016/S0031-0182(98)00109-6.
^ abStanley, S.M.; Hardie, L.A. (1999). "Hypercalcification; paleontology links plate tectonics and geochemistry to sedimentology". GSA Today. 9: 1–7.
^Porter, S.M. (2007). "Seawater chemistry and early carbonate biomineralization". Science. 316 (5829): 1302–1304. Bibcode:2007Sci...316.1302P. doi:10.1126/science.1137284. PMID 17540895. S2CID 27418253.
^Pojeta, J. Jr. (1988). "Review of Ordovician pelecypods". U.S. Geological Survey Professional Paper. 1044: 1–46.
^Hain, Mathis P.; Sigman, Daniel M.; Higgins, John A.; Haug, Gerald H. (2015). "The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for Eocene and Cretaceous ocean carbon chemistry and buffering" (PDF). Global Biogeochemical Cycles. 29 (5): 517–533. Bibcode:2015GBioC..29..517H. doi:10.1002/2014GB004986. ISSN 0886-6236. S2CID 53459924.
A calcitesea is a sea in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate...
An aragonite sea contains aragonite and high-magnesium calcite as the primary inorganic calcium carbonate precipitates. The chemical conditions of the...
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them from dissolution in sea water. When the diatom dies, the frustule is stripped of its organic layer and exposed to sea water. As a result, only 1%...
period, exoskeletons made of various materials – silica, calcium phosphate, calcite, aragonite, and even glued-together mineral flakes – sprang up in a range...
thickening, armouring and biomineralization occurs. Biomineralization with calcite is particularly common in Crustacea, whereas sclerotization particularly...
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three polymorphic forms, which in the order of their usual stabilities are calcite, aragonite and vaterite. The main groups of microorganisms that can induce...
extinction event. The Ordovician was a time of calcitesea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of...
transitioned from an aragonite sea to a calcitesea chemistry, favouring the dissolution of aragonite and precipitation of calcite. The rise of calcareous plankton...
in living organisms. However, some minerals are often biogenic (such as calcite) or organic compounds in the sense of chemistry (such as mellite). Moreover...
upper photic zones. The Great Calcite Belt of the Southern Ocean is a region of elevated summertime upper ocean calcite concentration derived from coccolithophores...
aragonite and high-magnesium calcite, known as aragonite seas, and low ratios result in calciteseas where low-magnesium calcite is the primary calcium carbonate...
produce skeletons of amorphous silica and one (Calcarea) of magnesium-calcite. It is these skeletons that are composed of the elements called spicules...
involved in mineralization in diatoms – even though diatoms use silica, not calcite, to form their tests! The shell-secreting area is differentiated very early...
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