The δ¹⁸O of atmospheric O₂ from air inclusions in the Vostok Ice Core: Timing of CO₂ and ice volume changes during the penultimate deglaciation

Abstract

One important question about the CO 2 ‐climate connection is how increasing atmospheric pCO 2 levels affect climate and thereby the mass balance of continental ice sheets. A record of atmospheric CO 2 variations over the last 160,000 years has recently been constructed by analyzing the trapped gas in the Vostok ice core [Barnola et al., 1987]. The relationship between changes in atmospheric CO 2 and the size of the continental ice sheets has been difficult to ascertain because the CO 2 record is obtained from ice cores while the ice volume record has been constructed from the stable isotopic composition of biogenic CaCO 3 in deep‐sea sediment cores. In order to compare these two records in a more precise manner, we present a record of the isotopic composition (δ 18 O) of atmospheric O 2 trapped in the Vostok ice core, and propose that it may be considered a proxy for the δ 18 O of seawater and hence ice volume. Having a record of atmospheric CO 2 along with a continental ice volume proxy in trapped air in the same ice core allows us to compare the timing of changes in these two parameters with little uncertainty in the relative ages of important events. Our results suggest that, during the penultimate glacial termination, atmospheric CO 2 began to increase at least 3 kyr before the initial introduction of meltwater to the oceans. Possible errors in the relative age‐depth curve of the Vostok ice core and uncertainties in the influence of biological and hydrologic processes on the δ 18 O of atmospheric O 2 introduce some uncertainty into our conclusions. However, our results are in general agreement with the observed phase relationship between atmospheric CO 2 and ice volume [Imbrie et al., 1984; Imbrie et al., 1989] (inferred from changes in the δ 13 C difference between contemporaneous planktonic and benthic forams deposited in deep‐sea cores and the SPECMAP ice volume record).