Calcite sea

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] 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.

[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.