Abstract
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.
Highlights
The atmospheric mixing ratios of methane (CH4) and nitrous oxide (N2O) have increased since pre-industrial time, which has contributed significantly to the increased radiative forcing since 1750 (Forster et al, 2007)
The extraction device consists of a 6 l stainless steel (SS) pot equipped with a perforated SS cylinder with sharp edges, the so-called “ice-grater” (Etheridge et al, 1988), where an ice core sample is grated under its own weight by sliding back and forth over the grater
The SS pot is reconnected to the vacuum extraction system (Fig. 1) and the air evacuated from the ice is processed through Trap 1 (T1), which traps H2O at −80 ◦C, and Trap 2 (T2), which traps CO2, N2O and most higher molecular weight hydrocarbons at −196 ◦C, to Trap 3 (T3)
Summary
The atmospheric mixing ratios of methane (CH4) and nitrous oxide (N2O) have increased since pre-industrial time, which has contributed significantly to the increased radiative forcing since 1750 (Forster et al, 2007). For both gases, large variations are observed during past climate changes (Spahni et al, 2005). The advent of continuous-flow isotope ratio mass spectrometry (IRMS) (Merritt et al, 1995), has led to the development of numerous analytical systems that only require small amounts of sample Sapart et al.: Simultaneous stable isotope analysis of methane and nitrous oxide on ice core samples
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