An aragonite sea contains aragonite

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

 and high-magnesium calcite

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

 as the primary inorganic carbonate precipitates. Therefore, the chemical conditions of the seawater must be notably high in magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 content for an aragonite sea to form. This is in contrast to a calcite sea

Calcite sea

A calcite sea is one in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic carbonate precipitates...

 in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate.

The Early Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 and the Middle to Late Mesozoic

Mesozoic

The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and the Cenozoic

Cenozoic

The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

 (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

Sea level

Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

 and the temperature of the surrounding system also determine whether an aragonite sea will form. Aragonite is the predominant mineral in warm, shallow marine environments. Calcite 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

Marl

Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...

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.