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Glacial/interglacial variations in atmospheric carbon dioxide

Abstract

Twenty years ago, measurements on ice cores showed that the concentration of carbon dioxide in the atmosphere was lower during ice ages than it is today. As yet, there is no broadly accepted explanation for this difference. Current investigations focus on the ocean's ‘biological pump’, the sequestration of carbon in the ocean interior by the rain of organic carbon out of the surface ocean, and its effect on the burial of calcium carbonate in marine sediments. Some researchers surmise that the whole-ocean reservoir of algal nutrients was larger during glacial times, strengthening the biological pump at low latitudes, where these nutrients are currently limiting. Others propose that the biological pump was more efficient during glacial times because of more complete utilization of nutrients at high latitudes, where much of the nutrient supply currently goes unused. We present a version of the latter hypothesis that focuses on the open ocean surrounding Antarctica, involving both the biology and physics of that region.

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Figure 1: The history of atmospheric CO2 back to 420 kyr ago as recorded by the gas content in the Vostok ice core from Antarctica4.
Figure 2: A simplified view of the Holocene (pre-industrial) carbon cycle.
Figure 3: The p CO 2 of surface sea water at 20 °C and a salinity of 35 parts per thousand, as set by its dissolved inorganic carbon (DIC) content and alkalinity (ALK).
Figure 4: The time-dependent response of the CYCLOPS model to a sudden halving in the CaCO3/Corg ratio of the export flux out of the low-latitude surface ocean33.
Figure 5: The effect on atmospheric CO2 of the biological pump in a region of deep-ocean ventilation.
Figure 6: The modern ocean (a, b) and a Southern Ocean-based hypothesis for reduced levels of atmospheric CO2 during glacial times ( c, d).

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Acknowledgements

We thank R. F. Anderson, M. L. Bender, M. A. Brzezinski and J. R. Toggweiler for discussions. We are indebted to P. G. Falkowski, G.M. Henderson and C. Prentice for comments on the manuscript.

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Sigman, D., Boyle, E. Glacial/interglacial variations in atmospheric carbon dioxide. Nature 407, 859–869 (2000). https://doi.org/10.1038/35038000

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