Oxygen/nitrogen ratios of the GISP2 and Vostok ice cores

We present a record of O2/N2 variations in trapped gases of the GISP2 core based on the analysis of 229 depths, and present O2/N2 analyses of 271 new depths in the Vostok ice core. We then compare the records for these two cores. In GISP2 as in Vostok, there are strong signals centered at the orbital frequencies, with O2/N2 in anti-phase with local summer insolation. However, differences are observed in the sub-Milankovitch frequencies. While the GISP2 record shows signals that are in phase with the local temperature record of rapid climate change, the Vostok record shows no apparent relation to the local temperature record. The mechanisms by which local summer insolation induces orbital variability in both δO2/N2 records, and by which temperature (or related properties) induce high frequency signals only in the GISP2 core, but not in Vostok remain to be identified. Nevertheless, our new data establish a strong empirical relationship between O2/N2 and local summer insolation, strengthening the use of O2/N2 variations to establish an orbital chronology for deep Antarctic ice cores. Based on the observation described above, we assume the direct control by local summer insolation on snow property(s) which is critical for oxygen/nitrogen fractionation during bubble close-off, and construct a chronology for the Vostok ice core. In this presentation, this chronology will be compared to other absolutely dated paleoclimate records and their relations will be discussed.