Inorganic Carbon Cycle

Iswariya, Luca

When gaseous CO2 is transferred from the atmosphere to the ocean, it immediately reacts with water to form carbonic acid (H2CO3) which dissociates, leading to the formation of bicarbonate ( HCO3-) and carbonate ions ( CO32-):
C O2ga s+H2 O⇌H2 CO3 H2CO3 ⇌H++ HCO3- HCO 3-⇌ H++CO3 2-

The sum of these three forms of carbon is often referred to as Dissolved Inorganic Carbon (DIC):

The reactions are so fast that, to a good approximation, the three components are always in equilibrium. The equilibrium relationship between the different molecules involved in reaction can then be used to define the solubility KH of CO2, which relates the concentration of carbonic acid to the partial pressure of carbon dioxide

By definition, for the same atmospheric pCO2, the amount of carbonic acid in the ocean at equilibrium will be larger for a high solubility than for a low solubility. The transfer of CO2 between the ocean and the atmosphere can then easily be expressed as a function of [H2CO3] using


In the sea, the equilibrium between the different forms of carbon occurs when nearly 90% of the dissolved inorganic carbon is in the form of bicarbonate, around 10% is carbonate, and only 0.5% is carbonic acid. This predominance of carbonate and bicarbonate ions explains why the ocean is able to store much more carbon than the atmosphere, while it is not true for other gases (such as oxygen) which have similar solubility to CO2. Furthermore, reactions show that atmospheric CO2 must balance the whole pool of DIC, not just H2CO3. As DIC is dominated by HCO3- and CO32-, the atmosphere-ocean exchanges will be strongly influenced by the concentration in these ions. For instance, it has been estimated that only 1 molecule in 20 of the CO2 entering the ocean stays as H2CO3, the large majority reacting with CO32-to form HCO3-, the dominant species in DIC: