Laboratory-determined air thermal diffusion constants applied to reconstructing the magnitudes of past abrupt temperature changes from gas isotope observations in polar ice cores (Greenland)

Assessing the magnitude of past abrupt climate change has important implications for predictions of future climate change. Thermal diffusion captures a signal of abrupt temperature change in air trapped in polar ice, whose magnitude is related to the magnitude of temperature change by a coefficient known as the thermal diffusion factor. In this work, thermal diffusion factors (alpha T) of 29N2/28N2 and of 40Ar/36Ar in air have been measured in the laboratory for the first time. A novel thermal diffusion apparatus and a mass spectrometer were employed in experiments. The mean values of alpha T x 103 that we find at -30.0°C are 3.59 +/- 0.10 for isotopic nitrogen and 9.85 +/- 0.04 for isotopic argon in air. The value for 29N2/28N 2 in pure N2 that we find is 3.71 +/- 0.09 at -30.6°C. For 40Ar/36Ar the value in pure argon is 11.25 +/- 0.03. Both pure gas values are in agreement with the data available in the literature, while having a precision 5--10 times greater.; We find that the temperature dependence of the thermal diffusion factors in air in the range -60 to -10°C can be described by the following empirical equations: alphaT x 103 = 8.656 - 1232/ <T> (+/-3%) for nitrogen, and alpha T x 103 = 26.08 - 3952/ <T> (+/-1%) for argon, where <T> is the effective average temperature. A complementary set of experiments employed a chamber with a porous medium (snow firn). The experiments ruled out the possibility of adsorption producing additional fractionation of 29N 2/28N2 in air in response to an abrupt warming. Results of our laboratory study provide a foundation for the use of the fossil air ice core paleothermometer.; The thermal diffusion constants presented here allowed an improvement of the previously published estimate for the end of the Younger Dryas warming (∼11.6 kyrs BP), with the new estimate being 10 +/- 4°C. The thermal diffusion results were then applied to new 29N 2/28N2 and 40Ar/36Ar data from the Greenland (GISP2) ice core, obtained as part of this work. The magnitudes of the warmings, which occurred in several decades, are found (tentatively) at ∼10.9°C for Dansgaard-Oeschger event 19 (∼68.8 kyr BP), and ∼10.7°C for Dansgaard-Oeschger event 20 (∼73.1 kyr BP).