The Influence of Air-Sea CO₂ Disequilibrium on Carbon Sequestration by the Ocean's Biological Pump

The ocean's biological carbon pump (BCP) affects the Earth's climate by sequestering CO2 away from the atmosphere for decades to millennia. One primary control on the amount of carbon sequestered by the biological pump is air-sea CO2 disequilibrium, which is controlled by the rate of air-sea CO2 exchange and the residence time of CO2 in surface waters. Here, we use a data-assimilated model of the soft tissue BCP to quantify carbon sequestration inventories and time scales from remineralization in the water column to equilibration with the atmosphere. We find that air-sea CO2 disequilibrium enhances the global biogenic carbon inventory by similar to 35% and its sequestration time by similar to 70 years compared to identical calculations made assuming instantaneous air-sea CO2 exchange. Locally, the greatest enhancement occurs in the subpolar Southern Ocean, where air-sea disequilibrium increases sequestration times by up to 600 years and the biogenic dissolved inorganic carbon inventory by >100% in the upper ocean. Contrastingly, in deep-water formation regions of the North Atlantic and Antarctic margins, where biological production creates undersaturated surface waters which are subducted before fully equilibrating with the atmosphere, air-sea CO2 disequilibrium decreases the depth-integrated sequestration inventory by up to similar to 150%. The global enhancement of carbon sequestration by air-sea disequilibrium is particularly important for carbon respired in deep waters that upwell in the Southern Ocean. These results highlight the importance of accounting for air-sea CO2 disequilibrium when evaluating carbon sequestration by the biological pump and for assessing the efficacy of ocean-based CO2 removal methods.