Ocean chemistry for shell and skeleton formation

Under the microscope: the pteropod Limacina helicina lives in the Pacific Ocean. Photo: Ben Moore-Maley, Univeristy of British Columbia, and Dr. Nina Bednarsek, NOAA PMEL

In the ocean, CO₂ reacts with water and carbonate ions to create carbonic acid. Elevated CO₂ levels reduce the concentration of carbonate ions. Shells of many marine organisms are made of calcium carbonate, which has two main forms: calcite and aragonite.

Both minerals dissolve at low carbonate ion concentrations – known as “undersaturated conditions” – unless the calcifying organisms have evolved mechanisms to prevent dissolution, such as protective layers or other means to isolate their carbonate structures from exposure to corrosive water⁴³. Aragonite, which is formed by corals, by the first larval stages of many molluscs, and by some adult molluscs (including pteropods), is more soluble than calcite, which is produced by coccolithophores, foraminifera, echinoderms and crustaceans. The scale for describing the level of saturation of calcium carbonate in seawater is the “saturation state”, Omega (Ω), where Ω < 1 indicates undersaturated (corrosive) and Ω > 1, supersaturated waters.

In an acidifying ocean, skeletal and shell growth generally slows down significantly as the saturation state decreases. For example, coral growth benefits from high aragonite saturation states, larger than 3 (Ω ≥ 3)⁴⁴.

Conditions in the deep

Deep waters naturally contain more CO₂ and have a lower pH compared with the surface ocean. Furthermore, calcite and aragonite saturation decreases with increasing pressure. The boundary between these deeper undersaturated waters and the shallower saturated waters is known as the “saturation horizon”.

Increasingly, the aragonite saturation horizon is becoming shallower and thus corrosive conditions for shell-forming organisms are moving
closer to the surface.

This process is well understood and well documented at a global scale. The saturation horizon will reach the ocean surface by the end of this century in the North Pacific, the Arctic seas and Southern Ocean, under RCP 8.5 (see Future CO₂ Emissions).

In upwelling areas such as in the northeast Pacific Ocean and along the western coasts of South America and Africa, naturally more acidic deep waters upwell onto continental shelves. The ocean uptake of anthropogenic CO₂ has increased the extent of the areas affected by more acidic waters⁴⁵, which include important fisheries grounds.

[This text is from the Ocean Acidification Summary for Policy Makers, 2013, and is available online as a PDF with full references.]