Synchronous changes in atmospheric CH4 and Greenland climate between 40 and 8 kyr BP

Air trapped in ice core bubbles provides our primary source of information about past atmospheres. Air isotopic composition (15N/14N and 40Ar/36Ar) permits an estimate of the temperature shifts associated with abrupt climate changes because of isotope fractionation occurring in response to temperature gradients in the snow layer on top of polar ice sheets. A rapid surface temperature change modifies temporarily the firn temperature gradient, which causes a detectable anomaly in the isotopic composition of nitrogen and argon. The location of this anomaly in depth characterizes the gas age - ice age difference (Δage) during an abrupt event by correlation with the δD (or δ18O) anomaly in the ice. We focus this study on the marine isotope stage 5d/5c transition (108 kyr B.P.), a climate warming which was one of the most abrupt events in the Vostok (Antarctica) ice isotopic record [Petit et al., 1999]. A step-like decrease in δ15N and δ40Ar/4 from 0.49 to 0.47‰ (possibly a gravitational signal due to a change in firn thickness) is preceded by a small but detectable δ15N peak (possibly a thermal diffusion signal). We obtain an estimate of 5350 ± 300 yr for Δage, close to the model estimate of 5000 years obtained using the Vostok glaciological timescale. Our results also suggest that the use of the present-day spatial isotope-temperature relationship slightly underestimates (but by no more than 20 ± 15%) the Vostok temperature change from present day at that time, which is in contrast to the temperature estimate based on borehole temperature measurements in Vostok which suggests that Antarctic temperature changes are underestimated by up to 50% [Salamatin et al., 1998]. Copyright 2001 by the American Geophysical Union.