Over the past eight hundred thousand years, glacial-interglacial cycles oscillated with a period of one hundred thousand years ('100k world' 1 ). Ice core and ocean sediment data have shown that atmospheric carbon dioxide, Antarctic temperature, deep ocean temperature, and global ice volume correlated strongly with each other in the 100k world [2][3][4][5][6] . Between about 2.8 and 1.2 million years ago, glacial cycles were smaller in magnitude and shorter in duration ('40k world' 7 ). Proxy data from deep-sea sediments suggest that the variability of atmospheric carbon dioxide in the 40k world was also lower than in the 100k world 8-10 , but we do not have direct observations of atmospheric greenhouse gases from this period. Here we report the recovery of stratigraphically discontinuous ice more than two million years old from the Allan Hills Blue Ice Area, East Antarctica. Concentrations of carbon dioxide and methane in ice core samples older than two million years have been altered by respiration, but some younger samples are pristine. The recovered ice cores extend direct observations of atmospheric carbon dioxide, methane, and Antarctic temperature (based on the deuterium/hydrogen isotope ratio δD ice , a proxy for regional temperature) into the 40k world. All climate properties before eight hundred thousand years ago fall within the envelope of observations from continuous deep Antarctic ice cores that characterize the 100k world. However, the lowest measured carbon dioxide and methane concentrations and Antarctic temperature in the 40k world are well above glacial values from the past eight hundred thousand years. Our results confirm that the amplitudes of glacial-interglacial variations in atmospheric greenhouse gases and Antarctic climate were reduced in the 40k world, and that the transition from the 40k to the 100k world was accompanied by a decline in minimum carbon dioxide concentrations during glacial maxima.Earth has been cooling, and ice sheets expanding, over approximately the past 52 million years (Myr) 11 . Superimposed on this cooling are periodic changes in the Earth's climate system driven by variations in the eccentricity (with periods of 400 and 100 kyr) and precession (23 and 19 kyr) of the Earth's orbit around the Sun, and the tilt of the spin axis (about 41 kyr). From around 2.8 to 1.2 Myr ago (Ma), the Earth's climate system oscillated between glacial and interglacial states with a period of about 40 kyr (the 40k world 7 ). Between 1.2 and 0.8 Ma, an interval known as the 'mid-Pleistocene transition' (MPT), the period of glacial cycles lengthened to about 100 kyr and glacial periods became colder 12 , for reasons that are poorly understood. The post-MPT glacial cycles are characterized by a quasi-100-kyr period (the 100k world 1 ).Studies of stratigraphically continuous ice cores have shown that atmospheric CO 2 is directly linked to Antarctic and global temperature over the last 800 kyr 5,6 . The coupling of the Earth's climate and carbon cycle in earlier times, however, has not be...

Here, we present direct measurements of atmospheric composition and Antarctic climate from the mid-Pleistocene (∼1 Ma) from ice cores drilled in the Allan Hills blue ice area, Antarctica. The 1-Ma ice is dated from the deficit in 40 Ar relative to the modern atmosphere and is present as a stratigraphically disturbed 12-m section at the base of a 126-m ice core. The 1-Ma ice appears to represent most of the amplitude of contemporaneous climate cycles and CO 2 and CH 4 concentrations in the ice range from 221 to 277 ppm and 411 to 569 parts per billion (ppb), respectively. These concentrations, together with measured δD of the ice, are at the warm end of the field for glacial-interglacial cycles of the last 800 ky and span only about one-half of the range. The highest CO 2 values in the 1-Ma ice fall within the range of interglacial values of the last 400 ka but are up to 7 ppm higher than any interglacial values between 450 and 800 ka. The lowest CO 2 values are 30 ppm higher than during any glacial period between 450 and 800 ka. This study shows that the coupling of Antarctic temperature and atmospheric CO 2 extended into the mid-Pleistocene and demonstrates the feasibility of discontinuously extending the current ice core record beyond 800 ka by shallow coring in Antarctic blue ice areas.climate change | glacial cycles | atmospheric CO 2 | ice cores | greenhouse gases

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

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Odd oxygen (O[ 3 P], O[ 1 D], and O 3 ) is a key component of the atmosphere's oxidizing capacity. As such, tracing its evolution over time may provide better constraints on greenhouse-gas lifetimes, stratosphere-troposphere coupling, biosphere-atmosphere interactions, and radiative forcing in the past. Moreover, elevated odd-oxygen concentrations in the upper troposphere and stratosphere mean that a globally integrated record of odd-oxygen chemistry would be a unique window on the high-altitude atmosphere of the past, not just in terms of chemistry but also climate (