Constraining CaCO 3 Export and Dissolution With an Ocean Alkalinity Inverse Model

Ocean alkalinity plays a fundamental role in the apportionment of CO 2 between the atmosphere and the ocean. The primary driver of the ocean's vertical alkalinity distribution is the formation of calcium carbonate (CaCO 3 ) by organisms at the ocean surface and its dissolution at depth. This so‐called “CaCO 3 counterpump” is poorly constrained, however, both in terms of how much CaCO 3 is exported from the surface ocean, and at what depth it dissolves. Here, we created a steady‐state model of global ocean alkalinity using Ocean Circulation Inverse Model transport, biogeochemical cycling, and field‐tested calcite and aragonite dissolution kinetics. We find that limiting CaCO 3 dissolution to below the aragonite and calcite saturation horizons cannot explain excess alkalinity in the upper ocean, and that models allowing dissolution above the saturation horizons best match observations. Linking dissolution to organic matter respiration, or imposing a constant dissolution rate both produce good model fits. Our best performing models require export between 1.1 and 1.8 Gt PIC y −1 (from 73 m), but all converge to 1.0 Gt PIC y −1 export at 279 m, indicating that both high‐ and low‐export scenarios can match observations, as long as high export is coupled to high dissolution in the upper ocean. These results demonstrate that dissolution is not a simple function of seawater CaCO 3 saturation (Ω) and calcite or aragonite solubility, and that other mechanisms, likely related to the biology and ecology of calcifiers, must drive significant dissolution throughout the water column. The acid neutralizing capacity, or alkalinity, in the ocean surface affects how much CO 2 the ocean can absorb from the atmosphere. Alkalinity is lost from the surface ocean when marine organisms make calcium carbonate (CaCO 3 ) shells; in contrast, alkalinity is returned to seawater when shells dissolve. Both processes are not well quantified. Here, we created a model to constrain how much CaCO 3 sinks out of the surface ocean, and at what depth it dissolves. We find that CaCO 3 must dissolve in the upper ocean where calcite and aragonite—the two most common forms of CaCO 3 —are thermodynamically oversaturated. We also find that a range of CaCO 3 exports—from 1.1 to 1.8 Gt PIC y −1 —can match observations, as long as high export is coupled to high dissolution in the upper ocean. These results suggest that CaCO 3 dissolution is not only controlled by seawater chemistry, and that other mechani