Present-day global patterns of the ocean carbonate pump and the key drivers of CaCO3 dissolution

Calcifying organisms produce calcium carbonate (CaCO₃) shells and skeletons. When they die, biogenic CaCO₃ is vertically exported from the euphotic zone and dissolves throughout the water column and in sediments. The alkalinity generated from this process can influence the ocean’s buffer capacity for absorbing atmospheric CO₂. However, the magnitude and driver of surface CaCO₃ export and subsequent dissolution in the ocean’s interior – a process called the carbonate pump – are highly uncertain. We present key drivers of pelagic CaCO₃ dissolution constrained by an inverse ocean biogeochemistry model combined with multiple observation databases. Within the upper twilight zone (shallower than 300 m), we found a tight association between particulate organic carbon remineralization rates and the CaCO₃ dissolution efficiency (the fraction by which the surface exported CaCO₃ dissolves), which is further supported by the observed particle flux and concentration data. In the deep ocean (deeper than 300 m), dissolution of CaCO₃ is primarily driven by conventional thermodynamics of CaCO₃ solubility with reduced fluxes of CaCO₃ burial to marine sediments beneath more corrosive North Pacific deep waters. Shallow CaCO₃ dissolution, shown to be sensitive to ocean export production, can increase the neutralizing capacity for respired CO₂ by up to 6% in low-latitude thermocline waters. Without shallow dissolution, the ocean might lose 20% more CO₂ to the atmosphere through the low-latitude upwelling regions – the world’s largest area of CO₂ outgassing in the contemporary climate. Our work identifies a previously overlooked sensitivity of oceanic CO₂ uptake to the biological pump. We suggest that Earth system models need to include the respiration driven CaCO₃ dissolution processes for a better projection of future oceanic carbon sink.