The carbonate ion is a primary ingredient in calcium carbonate, or CaCO_3(s), which makes up the hard, outer shells of many marine animals such as corals, crustaceans, and molluscs. Solid calcium carbonate can exist in multiple arrangements (polymorphs), similar to the way that solid carbon can exist as coal or as diamond, depending on the arrangement of the carbon atoms in the lattice.

Left: Image shows the estimated change in annual mean sea surface pH between the pre-industrial period and the 1990s.
Right: Image shows the estimated change in annual mean sea surface carbonate ion concentration between the pre-industrial period and the 1990s.
Image Courtesy: Global Ocean Data Analysis Project(GLODAP)

The two most biologically relevant arrangements of CaCO₃ are aragonite and calcite, both of which are currently supersaturated in the surface layer of the ocean. Deeper waters are unsaturated, starting at a depth ranging anywhere between 200- 3500 m, depending on the location and polymorph. This unsaturation begins and ends at the saturation horizon, or lysocline, the depth above which CaCO₃ is supersaturated, and below which calcium carbonate is undersaturated. Solid CaCO₃ dissolves below the calcite or aragonite saturation horizons. Most oceanic calcium carbonate forms the calcareous ooze in both surface layers and deep layers of the ocean.