| Summary: | Copyright © 2016 by the American Association for the Advancement of Science; all rights reserved. The history of atmospheric O2partial pressures (PO2) is inextricably linked to the coevolution of life and Earth's biogeochemical cycles. Reconstructions of past PO2rely on models and proxies but often markedly disagree. We present a record of PO2reconstructed using O2/N2ratios from ancient air trapped in ice. This record indicates that PO2declined by 7 per mil (0.7%) over the past 800,000 years, requiring that O2sinks were ∼2% larger than sources. This decline is consistent with changes in burial and weathering fluxes of organic carbon and pyrite driven by either Neogene cooling or increasing Pleistocene erosion rates. The 800,000-year record of steady average carbon dioxide partial pressures (PCO2) but declining PO2provides distinctive evidence that a silicate weathering feedback stabilizes PCO2on million-year time scales.
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