Marine calcification as a source of carbon dioxide: positive feedback of increasing atmospheric CO2

Calcification is a well‐recognized source of CO2 to the surrounding water and thus a potential source of atmospheric CO2 due to chemical equilibria involving the CO2 species. The released CO2: precipitated carbonate ratio (Ψ) has recently been estimated to be 0.6 in seawater (taking into account the buffering capacity of the latter). We report an analytical expression enabling the computation of this ratio. Calculations show that the amount of CO2 that must be released to equilibrate seawater increases with increasing partial pressure of CO2 in seawater (pCO2), which results from human impact on atmospheric CO2. We show that at 15°C Ψ increased from 0.55 during the time of glaciation to 0.67 at present and would increase to 0.84 for a pCO2 of 1,000 µatm. Doubling the preindustrial pCO2 value results in a total CO2 source of ∼5 Gt C (taking into account the described buffering effect).