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

We present data on the O-2/N-2 ratios of trapped gas samples for the entire length of the Vostok climate record. As in other cores, O-2/N-2 ratios in these samples are less than the atmospheric ratio, by a small and variable amount, because O-2 is selectively excluded during the gas trapping process, and because O-2 is also preferentially lost in poorly preserved core samples. Samples younger than 150 ka have large and variable O-2 depletions. Samples older than 200 ka have O-2/N-2 ratios that replicate well and vary smoothly with depth. We plot O-2/N-2 ratios of well replicated samples older than 160 ka, using a chronology derived by matching the delta(18)O of paleoatmospheric O-2 (delta(18)O(atm)) to northern hemisphere June insolation. On this timescale, O-2/N-2 varies coherently with local (78degreesS) summertime insolation. Based on time series analysis of the O-2/N-2 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 O-2 exclusion during bubble closeoff. O-2/N-2 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 delta(18)O(atm) chronology, supported by the coincidence of O-2/N-2 ratios with insolation, is also compatible with recent radiometric dating of corals from high sea stands. We further successfully test the delta(18)O(atm) 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. (C) 2002 Elsevier Science B.V. All rights reserved.