Sensitivity of pelagic CaCO₃ dissolution to ocean acidification in an ocean biogeochemical model

Abstract. In ocean biogeochemical models pelagic CaCO₃ dissolution is usually calculated as R = k * Sⁿ, where k is the dissolution rate constant transforming S, the degree of (under-) saturation of seawater with respect to CaCO₃, into a time dependent rate R, and n is the reaction rate order. Generally, there are two ways to define the saturation state of seawater with respect to CaCO₃: (1) Δ[CO₃²⁻], which reflects the difference between the in-situ carbonate ion concentration and the saturation concentration, and (2) Ω, which is approximated by the ratio of in-situ carbonate ion concentration over the saturation concentration. Although describing the same phenomenon, the deviation from equilibrium, both expressions are not equally applicable for the calculation of CaCO₃ dissolution in the ocean across pressure gradients, as they differ in their sensitivity to ocean acidification (change of [CO₃²⁻]) over depth. In the present study we use a marine biogeochemical model to test the sensitivity of pelagic CaCO₃ dissolution to ocean acidification (1–4 × CO₂ + stabilization), exploring the possible parameter space for CaCO₃ dissolution kinetics as given in the literature. We find that at the millennial time scale there is a wide range of CaCO₃ particle flux attenuation into the ocean interior (e.g. a reduction of −55 to −85% at 1000 m depth), which means that there are significant differences in the impact on particle ballasting, depending on the kinetic expression applied.