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

Methane and nitrous oxide are important green- house gases which show a strong increase in atmospheric mixing ratios since pre-industrial time as well as large varia- tions 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 anal- ysis of D(CH4) or 13 C(CH4), together with 15 N(N2O), 18 O(N2O) and 15 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 typi- cal "Holocene" mixing ratios for CH4 and N2O. In this case, the reproducibility (1) is 2.1 ‰ for D(CH4), 0.18 ‰ for 13 C(CH4), 0.51 ‰ for 15 N(N2O), 0.69 ‰ for 18 O(N2O) and 1.12 ‰ for 15 N(NO + fragment). For smaller amounts of ice the standard deviation increases, particularly for N 2O isotopologues. For both gases, small-scale intercalibrations using air and/or ice samples have been carried out in col- laboration 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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