Calcite Seas and Aragonite Seas

The carbonate mineral that precipitates preferentially from the global ocean has not been constant through Earth history. There have been alternating intervals — called calcite seas and aragonite seas — during which either low-magnesium calcite or aragonite plus high-magnesium calcite dominated carbonate precipitation, for both skeletal organisms and inorganic precipitates. Although calcite does not precipitate freely in the modern ocean owing to abundant Mg²⁺ ions, evidence suggests that calcite was precipitated in preference to aragonite at times in the geologic past when Mg²⁺ concentrations in the ocean were low. ^[1] The key variable is the Mg/Ca ratio of seawater: when it is low, calcite precipitation is favored; when it is high, aragonite and high-magnesian calcite are favored.

The Seafloor-Spreading Driver

The alternation between calcite seas and aragonite seas is linked to rates of seafloor spreading. High rates of seafloor spreading increase removal of Mg²⁺ from seawater by absorption into hot seafloor basalts, lowering the ocean’s Mg/Ca ratio and promoting calcite precipitation; low spreading rates allow Mg²⁺ to accumulate in seawater, raising the ratio and promoting aragonite precipitation. ^[1] This means that the long-term mineralogy of carbonate rocks carries a signal not just about the depositional environment but about the tectonic state of the planet at the time of deposition.

The Phanerozoic Pattern

Skeletal and nonskeletal carbonates deposited during early Cambrian through middle Mississippian time and again during middle Jurassic through late Tertiary time were dominantly low-magnesium calcite (calcite seas), whereas those deposited during middle Mississippian through middle Jurassic time and during the Neogene–Quaternary were dominantly aragonite and high-magnesium calcite (aragonite seas). ^[1] Seas that preferentially precipitate aragonite are referred to as aragonite seas. ^[1]

Expression in Ooids

The generation of ooids illustrates this effect concretely: ooids in modern environments (an aragonite sea) consist mainly of aragonite, whereas many ancient ooids may have precipitated as calcite during calcite-sea intervals. ^[1] This is the same pattern seen for the two varieties of calcite discussed on the Low/High-Magnesian Calcite page — the ocean’s bulk chemistry controls which polymorph of CaCO₃ nucleates most easily at a given time.

Biological Response

Marine organisms that secrete carbonate skeletons appear to track the ocean’s ambient mineralogy, secreting the carbonate mineral that is easiest to precipitate under prevailing conditions. During calcite seas, groups that typically secrete aragonite in modern oceans may instead produce calcite or mixed mineralogies. This biological plasticity means that the skeletal mineralogy record in ancient limestones can be used — with care — to infer past ocean chemistry, but direct correlation requires accounting for diagenetic overprinting since aragonite converts to calcite and high-magnesian calcite loses Mg over geologic time.

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