Chemical conditions of the sea favouring aragonite deposition
The alternation of calcite and aragonite seas throughgeologic time.
High-Mg Calcite and, less abundantly, Aragonite
Calcite; Mg content generally lower, increasing towards "Threshold"
Anaragonite sea containsaragonite and high-magnesiumcalcite as the primary inorganiccalcium carbonate precipitates. The reason lies in the highly hydratedMg2+divalention, the second most abundantcation inseawater afterNa+, known to be a strong inhibitor ofCaCO3crystallization at thenucleation stage.[1][2] The chemical conditions of the seawater must be notably high inmagnesium content relative to calcium (high Mg/Ca ratio) for an aragonite sea to form. This is in contrast to acalcite sea in which seawater low in magnesium content relative to calcium (low Mg/Ca ratio) favors the formation of low-magnesium calcite as the primary inorganic marine calcium carbonate precipitate.
The EarlyPaleozoic and the Middle to LateMesozoic oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and theCenozoic (including today) are characterized by aragonite seas.[3][4][5][6][7][8][9][10]
Aragonite seas occur due to several factors, the most obvious of these is a high seawater Mg/Ca ratio (Mg/Ca > 2), which occurs during intervals of slowseafloor spreading.[6] However, thesea level, temperature, and calcium carbonate saturation state of the surrounding system also determine whichpolymorph of calcium carbonate (aragonite, low-magnesium calcite, high-magnesium calcite) will form.[11][12]
Likewise, the occurrence of calcite seas is controlled by the same suite of factors controlling aragonite seas, with the most obvious being a low seawater Mg/Ca ratio (Mg/Ca < 2), which occurs during intervals of rapid seafloor spreading.[6][10]
This trend has been observed by looking at the chemistry of both biogenic and abiogenic carbonates, dating them, and analyzing the conditions under which they were formed. Various studies have examined these relationships and concluded that the mineralogy of both biogenic (major carbonate sediment and rock-forming organisms)[10] and abiogenic marine carbonates (limestones andmarls)[13] throughoutPhanerozoic time has generally been synchronized with calcium carbonate mineralogies expected from seawater magnesium/calcium ratios reconstructed from derivatives of ancient seawater trapped inhalite crystals in the geologic record (fluid inclusions).[7]
^Deleuze, Marl; Brantley, Susan L. (1997). "Inhibition of calcite crystal growth by Mg2+ at 100 °C and 100 bars: Influence of growth regime".Geochimica et Cosmochimica Acta.61 (7):1475–1485.doi:10.1016/s0016-7037(97)00024-0.ISSN0016-7037.
^Pan, Yiwen; Li, Yifan; Ma, Qianwei; He, Hangqi; Wang, Shuyuan; Sun, Zhentao; Cai, Wei-Jun; Dong, Bo; Di, Yanan; Fu, Weiqi; Chen, Chen-Tung Arthur (2021). "The role of Mg2+ in inhibiting CaCO3 precipitation from seawater".Marine Chemistry.237 104036.doi:10.1016/j.marchem.2021.104036.ISSN0304-4203.
^Ries, J. (2011). "Skeletal mineralogy in a high-CO2 world".Journal of Experimental Marine Biology and Ecology.403 (1–2):54–64.doi:10.1016/j.jembe.2011.04.006.
Adabi, Mohammad H. (2004), "A re-evaluation of aragonite versus calcite seas",Carbonates and Evaporites,19 (2):133–141,doi:10.1007/BF03178476,S2CID128955184
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
Hardie, Lawrence A. (2003), "Secular variations in Precambrian seawater chemistry and the timing of Precambrian aragonite seas and calcite seas",Geology,31 (9):785–788,Bibcode:2003Geo....31..785H,doi:10.1130/g19657.1
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.
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.
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
Stanley, S.M.; Hardie, L.A. (1999), "Hypercalcification; paleontology links plate tectonics and geochemistry to sedimentology",GSA Today,9:1–7
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.Bibcode:1986JG.....94..321W.doi:10.1086/629032.S2CID128840375.
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.
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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