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 ∼35% and its sequestration time by ∼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 ∼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.