Air-Sea Disequilibrium Enhances Glacial Ocean Carbon Storage

The changes of atmospheric CO2 concentrations during glacial-interglacial cycles have been attributed to various mechanisms, mostly related to changes in ocean carbon storage. Whereas the direct effect of ocean temperatures has been suggested to be minor, many recent studies imply changes in ocean circulation, sea ice and the biological pump to be major factors. However, the ocean’s carbon cycle is complex and quantifying the impacts of different mechanisms on ocean carbon storage, and by extension on atmospheric CO2 changes, is not straightforward. Here we present results from a novel method using a data-constrained model and a novel decomposition of the ocean’s carbon cycle that is both precise and complete. The model has been constrained by nitrogen and carbon isotopes and is consistent with a large variety of reconstructions. It features a weak and shallow AMOC, expanded sea ice cover and a large increase in soluble iron fluxes in the Southern Ocean. However, surprisingly the large circulation and sea ice changes only have minor effects on atmospheric CO2 due to their compensating effects on different ocean carbon components. On the other hand, temperature (44-45 ppm) and iron (26-39 ppm) explain most of the simulated atmospheric CO2 changes of 67-87 ppm. Our decomposition indicates that the biological pump was less efficient during the LGM, contrary to many previous inferences. However, ocean storage was enhanced mainly due to an increase in air-sea disequilibrium, which also explains the larger effects of temperature and iron compared to previous studies.