Glacial deep ocean deoxygenation driven by biologically mediated air–sea disequilibrium

Abstract

Deep ocean deoxygenation inferred from proxies has been used to support the hypothesis that a 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, which consumes O2 and produces dissolved inorganic carbon. However, this assumption has been shown to be imperfect because of disequilibrium. Here we used 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, we found a whole-ocean loss of 33 Pmol O2 from the preindustrial to the Last Glacial Maximum despite a 27 Pmol gain from the increased solubility due to cooler temperatures. This loss was driven by a biologically mediated O2 disequilibrium, which contributed 10% of the reduction of the O2 inventory from the solubility equilibrium in the preindustrial compared with 27% during the Last Glacial Maximum. Sea ice and iron fertilization were found to be the largest contributors to the Last Glacial Maximum deoxygenation, which occurs despite overall reduced production and respiration of organic matter in the glacial ocean. Our results challenge the notion that deep ocean glacial deoxygenation was caused by a stronger biological pump or more sluggish circulation, and instead highlight the importance and previously underappreciated role of O2 disequilibrium. Lower than modern dissolved oxygen levels in the deep ocean during the Last Glacial Maximum were the result of greater disequilibrium between the atmosphere and ocean, according to proxy record-constrained Earth system modelling.