Orbital tuning chronology for the Vostok climate record supported by trapped gas composition

We present data on the O2/N2 ratios of trapped gas samples for the entire length of the Vostok climate record. As in other cores, O2/N2 ratios in these samples are less than the atmospheric ratio, by a small and variable amount, because O2 is selectively excluded during the gas trapping process, and because O2 is also preferentially lost in poorly preserved core samples. Samples younger than 150 ka have large and variable O2 depletions. Samples older than 200 ka have O2/N2 ratios that replicate well and vary smoothly with depth. We plot O2/N2 ratios of well replicated samples older than 160 ka, using a chronology derived by matching the δ18O of paleoatmospheric O2 (δ18Oatm) to northern hemisphere June insolation. On this timescale, O2/N2 varies coherently with local (78°S) summertime insolation. Based on time series analysis of the O2/N2 record and the dynamics of snow metamorphism at the surface, we conclude that summertime insolation influences physical properties of ice grains that control the degree of O2 exclusion during bubble closeoff. O2/N2 in Vostok is thus arguably a property that records local summertime insolation and can be used to test independent chronologies for the core. We show that the δ18Oatm chronology, supported by the coincidence of O2/N2 ratios with insolation, is also compatible with recent radiometric dating of corals from high sea stands. We further successfully test the δ18Oatm tuning chronology by showing that it predicts a chronology for the GISP2 core which is essentially indistinguishable from the standard GISP2 chronology and, therefore, in excellent agreement with the radiometric chronology of Hulu Cave, China. An accurate chronology for the Vostok ice core is now in place.