# Aragonite sea

An aragonite sea contains aragonite and high-magnesium calcite as the primary inorganic calcium carbonate precipitates. The reason lies in the highly hydrated Mg$^{2+}$ divalent ion, the second most abundant cation in seawater after Na$^+$, known to be a strong inhibitor of CaCO$_3$ crystallization at the nucleation stage. The chemical conditions of the seawater must be notably high in magnesium content relative to calcium (high Mg/Ca ratio) for an aragonite sea to form. This is in contrast to a calcite 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 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.
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 slow seafloor spreading. However, the sea level, temperature, and calcium carbonate saturation state of the surrounding system also determine which polymorph of calcium carbonate (aragonite, low-magnesium calcite, high-magnesium calcite) will form.
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.
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) and abiogenic marine carbonates (limestones and marls) throughout Phanerozoic time has generally been synchronized with calcium carbonate mineralogies expected from seawater magnesium/calcium ratios reconstructed from derivatives of ancient seawater trapped in halite crystals in the geologic record (fluid inclusions).

## Citations

* ↑ Deleuze, Marl; Brantley, Susan L. (1997). "Inhibition of calcite crystal growth by Mg$^{2+}$ 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. ISSN 0016-7037.
* ↑ 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 Mg$^{2+}$ in inhibiting CaCO$_3$ precipitation from seawater". Marine Chemistry. 237: 104036. doi:10.1016/j.marchem.2021.104036. ISSN 0304-4203.
* ↑ Wilkinson, Owen & Carroll 1985
* ↑ Wilkinson & Given 1986
* ↑ Morse & Mackenzie 1990
* 1 2 3 Hardie 1996
* 1 2 Lowenstein et al. 2001
* ↑ Hardie 2003
* ↑ Palmer & Wilson 2004
* 1 2 3 Ries, J. (2010). "Geological and experimental evidence for secular variation in seawater Mg/Ca (calcite-aragonite seas) and its effects on marine biological calcification". Biogeosciences. 7 (9): 2795–2849. Bibcode:2010BGeo....7.2795R. doi:10.5194/bg-7-2795-2010.
* ↑ Adabi 2004
* ↑ 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.
* ↑ Westphall & Munnecke 2003