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

Nature Geoscience volume 14, pages 43–50 (2021)Cite this article

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A Publisher Correction to this article was published on 01 March 2021

An Author Correction to this article was published on 11 January 2021

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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.

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Fig. 1: Comparison of surface O2 saturation from a compilation of the World Ocean Atlas 2018 (WOA18 (ref. 48))+float data (University of Washington Argo O2 reanalysis49, and quality-controlled data from the SOCCOM (Southern Ocean Carbon and Climate Observations and Modeling) programme50).
Fig. 2: Schematic of the O2 decomposition used in this study.
Fig. 3: The components of O2 in the PIC and LGM relative to PIC.
Fig. 4: Response of PIC O2 components to LGM perturbations and assessment of the robustness of the response of O2 components to perturbations.
Fig. 5: Relationship between ΔO2, ΔAOU and biological carbon storage.

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Data availability

The model output that support the findings of this study are available from https://doi.org/10.5281/zenodo.4078981. World Ocean Atlas data were obtained from https://www.nodc.noaa.gov/OC5/woa18/, and float data from the SOCCOM website https://soccom.princeton.edu/.

Code availability

Model codes are available from https://github.com/samarkhatiwala/tmm.

Change history

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Acknowledgements

This study was supported by UK NERC grants NE/M020835/1 and NE/T009357/1 to S.K., and US National Science Foundation grant OCE-1924215 to A.S. E.C. acknowledges support from the Rhodes Trust. Computing resources were provided by the Climate Simulation Laboratory at NCAR’s Computational and Information Systems Laboratory (ark:/85065/d7wd3xhc), sponsored by the National Science Foundation and other agencies, and the University of Oxford Advanced Research Computing (ARC) facility (https://doi.org/10.5281/zenodo.22558). Data shown in Fig. 1 were collected and made freely available by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) Project funded by the National Science Foundation, Division of Polar Programs (NSF PLR -1425989), supplemented by NASA, and by the International Argo Program and the NOAA programmes that contribute to it. The Argo Program is part of the Global Ocean Observing System.

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  1. Department of Earth Sciences, University of Oxford, Oxford, UK

    Ellen Cliff & Samar Khatiwala

  2. College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA

    Andreas Schmittner

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  1. Ellen Cliff
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S.K. and A.S. designed the study; S.K. carried out the experiments; all the authors analysed the results; E.C. wrote the manuscript with input from S.K. and A.S.

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Correspondence to Ellen Cliff.

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Supplementary Figs. 1–26 and discussion.

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Cliff, E., Khatiwala, S. & Schmittner, A. Glacial deep ocean deoxygenation driven by biologically mediated air–sea disequilibrium. Nat. Geosci. 14, 43–50 (2021). https://doi.org/10.1038/s41561-020-00667-z

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