The history of atmospheric O2 partial pressures (Po2) is inextricably linked to the coevolution of life and Earth's biogeochemical cycles. Reconstructions of pastPo2 rely on models and proxies but often markedly disagree. We present a record ofPo2 reconstructed using O2/N2 ratios from ancient air trapped in ice. This record indicates thatPo2 declined by 7 per mil (0.7%) over the past 800,000 years, requiring that O2 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 (Pco2) but decliningPo2 provides distinctive evidence that a silicate weathering feedback stabilizesPco2 on million-year time scales.