Balancing the Oceanic Calcium Carbonate Cycle: Consequences of Variable Water Column Ψ

The paired chemical reactions, Ca2+ + 2HCO3− ↔ CaCO3 + CO2 + H2O, overestimate the ratio of CO2 flux to CaCO3 flux during the precipitation or dissolution of CaCO3 in seawater. This ratio, which has been termed ψ, is about 0.6 in surface seawater at 25°C and at equilibrium with contemporary atmospheric CO2 and increases towards 1.0 as seawater cools and pCO2 increases. These conclusions are based on field observations, laboratory experiments, and equilibrium calculations for the seawater carbonate system. Yet global geochemical modeling indicates that small departures of Ψ from 1.0 would cause dramatic, rapid, and unrealistic change in atmospheric CO2. Ψ can be meaningfully calculated for a water sample whether or not it is in equilibrium with the atmosphere. The analysis presented here demonstrates that the atmospheric CO2 balance can be maintained constant with respect to seawater CaCO3 reactions if one considers the difference between CaCO3 precipitation and burial and differing values for ψ (both <1.0) in regions of precipitation and dissolution within the ocean.