One of the most important components of the chemical perspective of oceanography is the carbonate system, primarily because it controls the acidity of seawater and acts as a governor for the carbon cycle. Within the mix of acids and bases in the Earth-surface environment, the carbonate system is the primary buffer for the acidity of water, which determines the reactivity of most chemical compounds and solids. The carbonate system of the ocean plays a key role in controlling the pressure of carbon dioxide in the atmosphere, which helps to regulate the temperature of the planet. The formation rate of the most prevalent authigenic mineral in the environment, CaCO3, is also the major sink for dissolved carbon in the long-term global carbon balance.

Dissolved compounds that make up the carbonate system in water (CO2, HCO3− and CO32 −) are in chemical equilibrium on time scales longer than a few minutes. Although this is less certain in the heterogeneous equilibrium between carbonate solids and dissolved constituents, to a first approximation CaCO3 is found in marine sediments that are bathed by waters that are saturated or supersaturated thermodynamically and absent where waters are undersaturated. It has become feasible to test models of carbonate thermodynamic equilibrium because of the evolution of analytical techniques for the carbonate system constituents and thermodynamic equilibrium constants. During the first major global marine chemical expedition, Geochemical Sections (GEOSECS) in the 1970s, marine chemists argued about concentrations of dissolved inorganic carbon, DIC (= HCO3− + CO32 − + CO2), and alkalinity at levels of 0.5%–1%, and the fugacity of CO2, at levels of ± 20 %.