Simultaneous stable isotope analysis of methane and nitrous oxide on ice core samples

Abstract. Methane and nitrous oxide are important greenhouse gases which show a strong increase in atmospheric mixing ratios since pre-industrial time as well as large variations during past climate changes. The understanding of their biogeochemical cycles can be improved using stable isotope analysis. However, high-precision isotope measurements on air trapped in ice cores are challenging because of the high susceptibility to contamination and fractionation.

Here, we present a dry extraction system for combined CH₄ and N₂O stable isotope analysis from ice core air, using an ice grating device. The system allows simultaneous analysis of δD(CH₄) or δ¹³C(CH₄), together with δ¹⁵N(N₂O), δ¹⁸O(N₂O) and δ¹⁵N(NO⁺ fragment) on a single ice core sample, using two isotope mass spectrometry systems. The optimum quantity of ice for analysis is about 600 g with typical "Holocene" mixing ratios for CH₄ and N₂O. In this case, the reproducibility (1σ ) is 2.1‰ for δD(CH₄), 0.18‰ for δ¹³C(CH₄), 0.51‰ for δ¹⁵N(N₂O), 0.69‰ for δ¹⁸O(N₂O) and 1.12‰ for δ¹⁵N(NO⁺ fragment). For smaller amounts of ice the standard deviation increases, particularly for N₂O isotopologues. For both gases, small-scale intercalibrations using air and/or ice samples have been carried out in collaboration with other institutes that are currently involved in isotope measurements of ice core air. Significant differences are shown between the calibration scales, but those offsets are consistent and can therefore be corrected for.