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The history of atmospheric O 2 partial pressures (Po 2 ) is inextricably linked to the coevolution of life and Earth's biogeochemical cycles. Reconstructions of past Po 2 rely on models and proxies but often markedly disagree. We present a record of Po 2 reconstructed using O 2 /N 2 ratios from ancient air trapped in ice. This record indicates that Po 2 declined by 7 per mil (0.7%) over the past 800,000 years, requiring that O 2 sinks 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 (Pco 2 ) but declining Po 2 provides distinctive evidence that a silicate weathering feedback stabilizes Pco 2 on million-year time scales.

A Pleistocene ice core record of atmospheric O2 concentrations