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 N₂ O stable isotope analysis from ice core air, using an ice grating device. The system allows simultaneous analysis of δD(CH₄ ) or δ¹³ C(CH₄ ), together with δ¹⁵ N(N₂ O), δ¹⁸ O(N₂ O) and δ¹⁵ N(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 N₂ O. In this case, the reproducibility (1σ ) is 2.1[per thousand] for δD(CH₄ ), 0.18[per thousand] for δ¹³ C(CH₄ ), 0.51[per thousand] for δ¹⁵ N(N₂ O), 0.69[per thousand] for δ¹⁸ O(N₂ O) and 1.12[per thousand] for δ¹⁵ N(NO⁺ fragment). For smaller amounts of ice the standard deviation increases, particularly for N₂ O 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.

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