Different time scales in the transient response of the ocean carbon and oxygen cycles to deglacial climate change

The ocean carbon sink and deoxygenation are two key research focuses under the current anthropogenically warming climate, as the former is essential in regulating atmospheric CO₂, and the latter is a vital factor for the marine ecosystem. The oceanic carbon and oxygen cycles are closely linked as they are commonly influenced by several processes, such as temperature-dependent gas solubility, organic matter remineralisation in the interior ocean, and ventilation. Future predictions of ocean carbon sink and deoxygenation are still subject to considerable uncertainties as the observational data in the present-day ocean is too sparse to constrain the relevant natural processes. To deepen our understanding of the natural carbon and oxygen cycles, we use the state-of-the-art Max Planck Institute Earth System Model (MPI-ESM) to conduct transient simulations for the last deglaciation (21 ka to the present day).

The deglacial evolution of oceanic CO₂ outgassing is mainly controlled by gradual global warming and the Atlantic Meridional Overturning Circulation (AMOC) variability driven by the meltwater from the prescribed ice sheet reconstruction. The global ocean oxygen content generally captures the features of the qualitative oxygen proxies, with lower oxygen content in the glacial ocean compared to the Holocene and a decrease in global oxygen content as the AMOC declines. The low oxygen content in the glacial ocean results from lower oxygen content in the deep ocean (below 2000 m), which is partially counteracted by higher oxygen content in the upper ocean, owing to solubility increase under colder temperatures. The glacial deep-ocean deoxygenation is governed by the air-sea disequilibrium under a more extensive, longer-lasting sea ice cover in the Southern Ocean and a more sluggish transport between the upper and interior ocean. Unlike the ocean carbon content, which closely follows the temporal variation of the North Atlantic Deep Water (NADW) strength, the evolution of the oxygen content is slow and decoupled from the NADW during its recovery phase, suggesting the Southern Ocean ventilation has a more significant impact on the oxygen dynamics. For the mid and late Holocene, when the ocean circulation is quasi-stable, the global air-sea CO₂ flux is near zero, whereas the replenishment of deep-sea oxygen continues. Such different response time scales between the ocean carbon and oxygen cycles are also seen in additional sensitivity simulations where an AMOC decline and recovery are simulated by freshwater hosing. Our preliminary findings suggest that the past changes in the climate and ocean circulation are likely to have a long-lasting impact on oxygen dynamics and drive oxygen concentrations away from equilibrium states, which should be accounted for when conducting model-data comparisons.