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

Ellen Cliff1, Samar Khatiwala1 and Andreas Schmittner2, (1)University of Oxford, Department of Earth Sciences, Oxford, United Kingdom, (2)Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, United States
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.