Delta15N measurements as a calibration tool for the paleothermometer and gas-ice age differences:: A case study for the 8200 BP event on GRIP ice

Isotopic composition measurement on atmospheric nitrogen (delta(15)N) trapped in ice cores is established as a gravitational enrichment indicator. Recently, it was discovered that thermal diffusion has a significant influence on this parameter during periods of large temperature fluctuations [Severinghaus et al., 1998]. Here we present results for the 8200 B.P. event, the largest temperature excursion during the Holocene, which underpin this view and extend it to medium-range temperature changes. The thermal diffusion component is about one third of the gravitational enrichment for this event. The shortness of this cooling event does not significantly affect the gravitational enrichment as documented by rather constant close-off depths. Therefore the delta(15)N residuals can be directly used to estimate temperature changes, hence calibrating the paleothermometer. Since our delta(15)N record can resolve temperature variations as short as a few decades, it complements the calibration technique of borehole temperature measurements, for which the resolution decreases rapidly with increasing age. We obtained a relation delta(15)O(ice) - temperature of 0.30+/-0.11 parts per thousand/K, which agrees nicely with the temporal dependence for cold periods (about 0.30 parts per thousand/K) and is significantly lower than today's Greenland spatial relation (0.67 parts per thousand/K). This corresponds to a range of the surface temperature shift for the 8200 B.P. event in Central Greenland between 5.4 and 11.7 K, with a best estimate of 7.4 K. By matching the delta(15)N variations with delta(18)O(ice) variations we estimate the mean gas-ice age difference to be -209+/-15 years, which is in good agreement with model estimates.