The dominant sink of oceanic calcium carbonate occurs in undersaturated seawater

Calcium carbonate (CaCO₃) particles formed by marine organisms dissolve in seawater and on the ocean floor, constituting a key component of the global ocean carbon cycle. However, neither the magnitude nor the distribution of CaCO₃ dissolution in the modern oceans is well constrained. Here we diagnose CaCO₃ dissolution rates in major oceanic regions and reconstruct CaCO₃ settling flux using a data-constrained ocean circulation model and observational climatologies of global seawater alkalinity and dissolved oxygen. Our approach distinguishes the effects of ocean circulation and CaCO₃ dissolution on seawater alkalinity and dissolved oxygen, and involves only one free parameter, a restoring timescale, which is constrained by compiling carbonate export rates estimated in the literature. We find that CaCO₃ dissolution in shallow waters (above the saturation horizon of aragonite) is much smaller than some recent estimates. Excluding the upper 114 m of the euphotic zone, the fraction of CaCO₃ dissolution within aragonite saturated waters is only about 16.5 ± 5.5%. About 39.8 ± 4.5% of CaCO₃ dissolution occurs below the aragonite saturation horizon but above the saturation horizon of calcite. The remaining 43.7 ± 3.8% dissolves in the deep ocean in undersaturated seawater. These results suggest that the seawater CaCO₃ saturation state is the primary control on CaCO₃ dissolution in the ocean.