Aragonite sea

The alternation of calcite and aragonite seas through geologic time.

An aragonite sea contains aragonite and high-magnesium calcite as the primary inorganic carbonate precipitates. Therefore, the chemical conditions of the seawater must be notably high in magnesium content for an aragonite sea to form. This is in contrast to a calcite sea in which low-magnesium calcite is 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 form due to several factors, the most obvious of these is a high magnesium content. However, the sea level and the temperature of the surrounding system also determine whether an aragonite sea will form.

Calcite seas occurred at times of rapid seafloor spreading and global greenhouse climate conditions. Calcite is the predominant mineral in warm, shallow marine environments. Aragonite on the other hand, is the dominant mineral in cool marine water environments.

This trend has been observed by looking at the chemistry of carbonates, dating them and analyzing the conditions under which they were formed. One study has examined the temporal and spatial distribution of limestone-marl alternations in Ordovician, Jurassic and Cretaceous (times of calcite seas). This study concluded that the most abundant of the limestone-marl alternations occurred in settings similar to today's seas which favor aragonite production.