Chronology of the Vostok ice core constrained by O2/N2 ratios of occluded air, and its implication for the Vostok climate records

We present a timescale for the Vostok ice core that is derived by orbitally tuning to O2/N2 ratios in occluded air for depths deeper than 1550 m (>112 ka), and by gas correlation to the GISP2 chronology for the section shallower than 1422 m (<102 ka). Our chronology of the deeper section rests on the assumption that, during the bubble close off process, local summer insolation indirectly controls the extent of O2 exclusion and hence the O2/N2 ratio in trapped gases. The newly derived O2/N2 chronology is consistent with absolutely dated speleothem records. The O2/N2 chronology differs from previously published orbital tuning chronologies (CH4 and δ18Oatm) by up to ∼±6 kyr, and from the original GT4 chronology by up to ∼15 kyr. The difference between the O2/N2 chronology and the δ18Oatm chronology varies in time with strong signals centered at 1/100 and 1/41 kyr−1. The ages for the last four glacial terminations in Vostok correspond to high obliquity (>23.7° at terminations’ midpoints). They also correspond with decreasing precession index, corresponding to increasing boreal summer insolation. The Vostok temperature record, boreal summer insolation, and the rate of change of the SPECMAP property (reflecting planktonic foram δ18O) with respect to time are highly coherent at precession and obliquity periods. These three properties vary almost synchronously, with the possibility that Vostok temperature lags behind the other two. Our new timescale supports the widespread view that boreal summer insolation played an important role in glacial–interglacial cycles.