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

Abstract

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.