Laboratory determination of thermal diffusion constants for N2/N2 in air at temperatures from −60 to 0°C for reconstruction of magnitudes of abrupt climate changes using the ice core fossil-air paleothermometer

Rapid temperature change causes fractionation of isotopic gaseous species in air in firn (snow) by thermal diffusion, producing a signal that is preserved in trapped air bubbles as the snow forms ice. Using a model of heat penetration and gas diffusion in the firn, as well as the values of appropriate thermal diffusion constants, it is possible to reconstruct the magnitude of a particular paleoclimate change. Isotopic nitrogen in air serves as a convenient tracer for such paleoreconstruction, because the ratio ²⁹N₂/²⁸N₂ has stayed extremely constant in the atmosphere for ≥10⁶ years. However, prior to this work no data were available for thermal diffusion of ²⁹N₂/²⁸N₂ in air, but only in pure N₂. We devised a laboratory experiment allowing fractionation of gases by thermal diffusion in a small, tightly controlled temperature difference. A mass spectrometer was employed in measuring the resulting fractionations yielding measurement precision greater than was attainable by earlier thermal diffusion investigators.Our laboratory experiments indicate that the value of the thermal diffusion sensitivity (Ω) for ²⁹N₂/²⁸N₂ in air is +(14.7 ± 0.5) × 10⁻³ per mil/°C when the average temperature is -30.0°C. The corresponding value for ²⁹N₂/²⁸N₂ in pure N₂ that we find is +(15.3 ± 0.4) × 10⁻³ per mil/°C at -30.6°C, in agreement with the previously available literature data within their large range of uncertainty. We find that an empirical equation, Ω = (8.656/T_K − 1232/T K2) ± 3% per mil/°C, describes the slight variation of the sensitivity values for ²⁹N₂/²⁸N₂ in air with temperature in the range of -60 to 0°C. A separate set of experiments also described in this paper rules out adsorption as a candidate for producing additional temperature change-driven fractionation of ²⁹N₂/²⁸N₂ in the firn air. The combined newly obtained data constitute a calibration of the fossil-air paleothermometer with respect to isotopic nitrogen and will serve to improve the estimates of the magnitudes of past abrupt climate changes recorded in ice cores.