Possible overestimation of shallow-depth calcium carbonate dissolution in the ocean

We argue that diagnostics of excess dissolved calcium carbonate (TA*) above the saturation horizon cannot be unambiguously interpreted in terms of local in situ dissolution. We examine a three-dimensional numerical model of global ocean circulation and biogeochemistry with explicit representation of the formation and dissolution of calcium carbonate. In particular, dissolution is only allowed to occur below the saturation horizon. The model qualitatively captures the observed basin- and global-scale patterns of alkalinity and TA* as well as the relationship between TA* and chlorofluorocarbon age in the thermocline. The existence of TA* above the saturation horizon in the model can only be explained in terms of transport subsequent to dissolution. The model study does not rule out the possibility of shallow-depth calcium carbonate dissolution but suggests that dissolution rates derived from tracer observations have not adequately accounted for the influence of transport.