Calcite sea

Short description: 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]

<gallery> Image:CalciteSeasTable.jpg|Table showing the conditions for calcite and aragonite seas Image:MgCaRatioChanges.jpg|Tectonic mechanism for changing Mg/Ca ratios in seawater File:OoidSurface01.jpg|Grainstone with calcitic ooids and sparry calcite cement; Carmel Formation, Middle Jurassic, of southern Utah Image:EncrustedBivalveMold.jpg|An encrusted Ordovician bivalve external mold showing contemporaneous dissolution of the original aragonite shell and calcitic cementation of the mold File:Encrusted Ordovician nautiloid InternalExternal.jpg|An encrusted Ordovician nautiloid internal mold showing contemporaneous dissolution of the original aragonite shell and calcitic cementation Image:Paleosabella4CS.jpg|The boring Palaeosabella in an Ordovician bivalve shell. The borings penetrated an inner aragonitic shell layer which dissolved away. Image:Petroxestes_borings_Ordovician.jpg|Petroxestes borings in an Upper Ordovician hardground, southern Ohio

## References

1. ↑ 1.0 1.1 Wilkinson, 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.
2. ↑ 2.0 2.1 2.2 Wilkinson, 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. doi:10.1086/629032. Bibcode: 1986JG.....94..321W.
3. ↑ 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.
4. ↑ 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 (Geological Society of America) 24 (3): 279–283. doi:10.1130/0091-7613(1996)024<0279:svisca>2.3.co;2. Bibcode: 1996Geo....24..279H.
5. ↑ 5.0 5.1 5.2 Lowenstein, T.K.; Timofeeff, M.N.; Brennan, S.T.; Hardie, L.A.; Demicco, R.V. (2001). "Oscillations in Phanerozoic seawater chemistry: evidence from fluid inclusions". Science 294 (5544): 1086–1088. doi:10.1126/science.1064280. PMID 11691988. Bibcode: 2001Sci...294.1086L.
6. ↑ 6.0 6.1 6.2 Palmer, T.J.; Wilson, M.A. (2004). "Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas". Lethaia 37 (4): 417–427 [1]. doi:10.1080/00241160410002135. Bibcode: 2004Letha..37..417P.
7. ↑ 7.0 7.1 Palmer, T.J. (1982). "Cambrian to Cretaceous changes in hardground communities". Lethaia 15 (4): 309–323. doi:10.1111/j.1502-3931.1982.tb01696.x. Bibcode: 1982Letha..15..309P.
8. ↑ 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. doi:10.1038/335809a0. Bibcode: 1988Natur.335..809P.
9. ↑ Wilson, M.A.; Palmer, T.J. (1992). "Hardgrounds and hardground faunas". University of Wales, Aberystwyth, Institute of Earth Studies Publications 9: 1–131.
10. ↑ Sandberg, P.A. (1983). "An oscillating trend in Phanerozoic non-skeletal carbonate mineralogy". Nature 305 (5929): 19–22. doi:10.1038/305019a0. Bibcode: 1983Natur.305...19S.
11. ↑ 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. doi:10.1130/0091-7613(2000)28<791:MMAEOL>2.0.CO;2. Bibcode: 2000Geo....28..791C.
12. ↑ Wilkinson, B.H. (1979). "Biomineralization, paleooceanography, and the evolution of calcareous marine organisms". Geology 7 (11): 524–527. doi:10.1130/0091-7613(1979)7<524:BPATEO>2.0.CO;2. Bibcode: 1979Geo.....7..524W.
13. ↑ 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. doi:10.1016/S0031-0182(98)00109-6. Bibcode: 1998PPP...144....3S.
14. ↑ 14.0 14.1 Stanley, S.M.; Hardie, L.A. (1999). "Hypercalcification; paleontology links plate tectonics and geochemistry to sedimentology". GSA Today 9: 1–7.
15. ↑ Porter, S.M. (2007). "Seawater chemistry and early carbonate biomineralization". Science 316 (5829): 1302–1304. doi:10.1126/science.1137284. PMID 17540895. Bibcode: 2007Sci...316.1302P.
16. ↑ Pojeta, J. Jr. (1988). "Review of Ordovician pelecypods". U.S. Geological Survey Professional Paper 1044: 1–46.
17. ↑ Harper, E.M.; Palmer, T.J.; Alphey, J.R. (1997). "Evolutionary response by bivalves to changing Phanerozoic sea-water chemistry". Geological Magazine 134 (3): 403–407. doi:10.1017/S0016756897007061. Bibcode: 1997GeoM..134..403H.
18. ↑ 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". Global Biogeochemical Cycles 29 (5): 517–533. doi:10.1002/2014GB004986. ISSN 0886-6236. Bibcode: 2015GBioC..29..517H. https://earth-system-biogeochemistry.net/wp-content/uploads/2021/05/Hain_et_al_2015_GBC.pdf.

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