Glacial Deep Ocean Deoxygenation Driven By Biologically Mediated Air-sea Disequilibrium

Deep ocean deoxygenation is used to support the hypothesis that lower atmospheric carbon dioxide during glacial times was due to increased strength of the ocean’s biological pump. This relies on the convenient but flawed assumption that surface ocean oxygen (O₂) is equilibrated with the atmosphere such that any O₂ deficiency observed in deep waters is a result of organic matter respiration consuming O₂ and producing dissolved inorganic carbon. The importance of O₂ disequilibrium has not been investigated previously in glacial ocean conditions. We show for the first time that O₂ disequilibrium plays a significant role in glacial deep ocean deoxygenation with an observationally-constrained earth system model. Using a novel decomposition method to accurately track O₂, we find a net loss of 33 Pmol O₂ from the preindustrial (PI) to the Last Glacial Maximum (LGM) despite a 27 Pmol increase of O₂ due to changes in physical conditions. This loss is driven by biologically mediated O₂ disequilibrium which increases from contributing removal of 10% of O₂ in the PI to removing 27% of the O₂ inventory in the LGM. Perturbations show sea ice and iron fertilization are the largest contributors to this O₂ depletion in the LGM which occurs despite reduced respiration of organic matter in the glacial deep ocean. The model results are consistent with an updated qualitative set of O₂ proxy data showing deep ocean deoxygenation in the LGM relative to the PI. Our results challenge the widely held notion that deep ocean glacial deoxygenation is caused by increased strength of the biological pump, instead highlighting the importance and previously unappreciated role of O₂ disequilibrium in distribution of O₂ throughout the glacial ocean.