carbonatesystem

The carbonate system refers to the interconnected set of dissolved inorganic carbon species in water, including carbon dioxide, carbonic acid, bicarbonate, and carbonate. In natural waters, the dominant forms depend on pH: CO2 and H2CO3 at low pH, HCO3− at moderate pH, and CO3^2− at higher pH. The system is governed by acid–base equilibria: CO2(g) ↔ CO2(aq) + H2O; CO2(aq) + H2O ↔ H2CO3 ↔ H+ + HCO3−; HCO3− ↔ H+ + CO3^2−. The distribution among species is often described by total inorganic carbon (DIC) and total alkalinity (A_T). pH, temperature, salinity, and pressure determine the relative proportions and the dissolved inorganic carbon reservoir.

The sea surface pCO2 and the exchange of CO2 with the atmosphere regulate DIC and pH in the surface waters. The carbonate system also defines the saturation states of calcium carbonate minerals: Omega = [Ca2+][CO3^2−]/Ksp, which influences precipitation and dissolution of calcite and aragonite.

Significance: The carbonate system buffers pH changes and acts as a reservoir for atmospheric CO2, influencing climate and ocean chemistry. In oceans, rising atmospheric CO2 lowers pH and reduces carbonate ion availability, decreasing saturation states of calcite and aragonite and challenging calcifying organisms. Freshwater systems and soils also rely on the carbonate system for buffering acidity and supporting aquatic life.

Measurement and modeling: scientists monitor pH, DIC, and alkalinity to characterize the carbonate system and use carbonate chemistry equations to compute speciation, pH, and saturation states.