UC Berkeley

The history of atmospheric O₂ partial pressures (Po₂) is inextricably linked to the coevolution of life and Earth's biogeochemical cycles. Reconstructions of past Po₂ rely on models and proxies but often markedly disagree. We present a record of Po₂ reconstructed using O₂/N₂ ratios from ancient air trapped in ice. This record indicates that Po₂ 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 declining Po₂ provides distinctive evidence that a silicate weathering feedback stabilizes Pco₂ on million-year time scales.