A Pleistocene ice core record of atmospheric O₂ concentrations

The history of atmospheric O₂ partial pressures (P_O2) is inextricably linked to the coevolution of life and Earth's biogeochemical cycles. Reconstructions of past P_O2 rely on models and proxies but often markedly disagree. We present a record of P_O2 reconstructed using O₂/N₂ ratios from ancient air trapped in ice. This record indicates that P_O2 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 (P_CO2) but declining P_O2 provides distinctive evidence that a silicate weathering feedback stabilizes P_CO2 on million-year time scales.