Ocean Carbonate System
Seawater carbonate chemistry is governed by a series of chemical reactions. The following equations explain the reactions that take place during ocean acidification1:
CO2 + H2O ⇔ H2CO3
⇔ HCO3 -1
+ H+ ⇔ H+ + CO3-2
- Equation 1: When atmospheric carbon dioxide (CO2) combines with seawater (H2O), it reacts to form carbonic acid (H2CO3).
- Equation 2: Carbonic acid then dissociates by losing hydrogen ions to form bicarbonate (HCO3). The increase in hydrogen ion concentration lowers the pH of the water, thus increasing acidity.
- Equation 3: Available carbonate ions (CO3-2 ) in the water then combine with hydrogen, and are no longer available for organisms to use for biological processes like building calcium carbonate shells or skeletons.
Over great timescales, the ocean’s ability to absorb atmospheric CO2 depends on the extent of CaCO3 dissolution in the water column or sediments:
|CaCO3 ⇔ CO32- + Ca2+|
The mineral CaCO3 derives from shells and skeletons of marine organisms, including plankton, corals, and coralline algae, and many other invertebrates. In pelagic environments, carbonates fall through the water column and are either dissolved or deposited in shallow or deep-water sediments.