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...

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Dissolved noble gases are ideal conservative tracers of physical processes in the Earth system due to their chemical and biological inertness. Although bulk concentrations of dissolved Ar, Kr, and Xe are commonly measured to constrain physical models of atmosphere, ocean, and terrestrial hydrosphere processes, stable isotope ratios of these gases (e.g. 136 Xe/ 129 Xe) are seldom used because of low signal-to-noise ratios. Here we present the first results from a new method of dissolved gas sampling, extraction and analysis that permits measurement of stable Ar, Kr, and Xe isotope ratios at or below ∼5 per meg amu −1 precision (1σ), two orders-of-magnitude below conventional Kr and Xe isotopic measurements. This gain in precision was achieved by quantitative extraction and subsequent purification of dissolved noble gases from 2-L water samples via helium sparging and viscous dual-inlet isotope ratio mass spectrometry. We have determined the solubility fractionation factors (α sol) for stable Ar, Kr, and Xe isotope ratios between ∼2 and 20 • C via laboratory equilibration experiments. We have also conducted temperaturecontrolled air-water gas exchange experiments to estimate the kinetic fractionation factors (α kin) of these isotope ratios. We find that both α sol and α kin , normalized by isotopic mass difference (m), decrease in magnitude with atomic number but are proportional to m for isotope ratios of the same element. With the new ability for high precision isotopic measurements, we suggest that dissolved Kr and Xe isotope ratios in groundwater represent a promising, novel geochemical tool with important applications for groundwater modeling, water resource management, and paleoclimate.

Past studies of noble gas concentrations in the deep ocean have revealed widespread, several percent undersaturation of Ar, Kr, and Xe. However, the physical explanation for these disequilibria remains unclear. To gain insight into undersaturation set by deep‐water formation, we measured heavy noble gas isotope and elemental ratios from the deep North Pacific using a new analytical technique. To our knowledge, these are the first high‐precision seawater profiles of 38Ar/36Ar and Kr and Xe isotope ratios. To interpret isotopic disequilibria, we carried out a suite of laboratory experiments to measure solubility fractionation factors in seawater. In the deep North Pacific, we find undersaturation of heavy‐to‐light Ar and Kr isotope ratios, suggesting an important role for rapid cooling‐driven, diffusive air‐to‐sea gas transport in setting the deep‐ocean undersaturation of heavy noble gases. These isotope ratios represent promising new constraints for quantifying physical air‐sea gas exchange processes, complementing noble gas concentration measurements.