Ice core evidence for atmospheric oxygen decline since the Mid-Pleistocene transition

The history of atmospheric oxygen (PO₂) and the processes that act to regulate it remain enigmatic because of difficulties in quantitative reconstructions using indirect proxies. Here, we extend the ice-core record of PO₂ using 1.5-million-year-old (Ma) discontinuous ice samples drilled from Allan Hills Blue Ice Area, East Antarctica. No statistically significant difference exists in PO₂ between samples at 1.5 Ma and 810 thousand years (ka), suggesting that the Late-Pleistocene imbalance in O₂ sources and sinks began around the time of the transition from 40- to 100-ka glacial cycles in the Mid-Pleistocene between ~1.2 Ma and 700 ka. The absence of a coeval secular increase in atmospheric CO₂ over the past ~1 Ma requires negative feedback mechanisms such as Pco₂-dependent silicate weathering. Fast processes must also act to suppress the immediate Pco₂ increase because of the imbalance in O₂ sinks over sources beginning in the Mid-Pleistocene.

A Pleistocene ice core record of atmospheric O2 concentrations

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.