Two forms of calcium carbonate: calcite and aragonite

The shells and skeletons of many marine organisms are made from either calcite or aragonite – two mineral forms of calcium carbonate. Scientists are particularly interested in aragonite, which is produced by many tropical corals, cold-water corals, pteropods and some molluscs. It is more soluble than calcite.

Organisms grow shells and skeletons more easily when carbonate ions in water are abundant – it is supersaturated. Unprotected shells and skeletons dissolve when carbonate ions in water are scarce – it is undersaturated or corrosive.

The saturation state Omega (Ω) describes the level of calcium carbonate saturation in seawater. If the saturation state for aragonite is less than 1 (Ω<1), conditions are corrosive (undersaturated) for aragonite-based shells and skeletons. If the saturation state is above 1 (Ω>1), waters are supersaturated with respect to calcium carbonate and conditions are favourable for shell formation. Coral growth benefits from a saturation state of 3 (Ω≥3).

Computer model projections show that the saturation state will be less than 3 in surface waters around tropical reefs by 2100 if CO2 emissions continue on the current trajectory.