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The history of atmospheric oxygen ( P O 2 ) 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 P O 2 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 P O 2 between samples at 1.5 Ma and 810 thousand years (ka), suggesting that the Late-Pleistocene imbalance in O 2 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 CO 2 over the past ~1 Ma requires negative feedback mechanisms such as P co 2 -dependent silicate weathering. Fast processes must also act to suppress the immediate P co 2 increase because of the imbalance in O 2 sinks over sources beginning in the Mid-Pleistocene.