Aragonite sea

The reason lies in the highly hydrated Mg2+ divalent ion, the second most abundant cation in seawater after Na+, known to be a strong inhibitor of CaCO3 crystallization at the nucleation stage.

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.

[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 and marls)[13] 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).