| Summary: | Deep ocean deoxygenation inferred from proxies has been used to support the hypothesis that lower atmospheric carbon dioxide during glacial times was due to an increase in the strength of the ocean’s biological pump. This relies on the 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. However, this assumption has been shown to be imperfect because of disequilibrium. Here I use an Earth System Model tuned to a suite of observations, which reproduces the pattern of glacial-to-Holocene oxygenation change seen in proxy data, to show that disequilibrium plays an important role in glacial deep ocean deoxygenation. Using a novel decomposition method to track O2, I find a whole-ocean loss of 33 Pmol O2 from the preindustrial to the Last Glacial Maximum (LGM) despite a 27 Pmol gain from increased solubility due to cooler temperatures. This loss is driven by biologically-mediated O2 disequilibrium, which increases from contributing 10% of the reduction of the O2 inventory from solubility equilibrium in the preindustrial, to 27% during the LGM. Sea ice and iron fertilization are found to be the largest contributors to LGM deoxygenation, which occurs despite overall reduced production and respiration of organic matter in the glacial ocean. These results challenge the notion that deep ocean glacial deoxygenation was caused by a stronger biological pump or more sluggish circulation, and instead highlights the importance and previously under-appreciated role of O2 disequilibrium.
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