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

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 CH 4 and N 2 O stable isotope analysis from ice core air, using an ice grating device. The system allows simultaneous analysis of δD(CH 4 ) or δ 13 C(CH 4 ), together with δ 15 N(N 2 O), δ 18 O(N 2 O) and δ 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 typical "Holocene" mixing ratios for CH 4 and N 2 O. In this case, the reproducibility (1σ ) is 2.1 ‰ for δD(CH 4 ), 0.18 ‰ for δ 13 C(CH 4 ), 0.51 ‰ for δ 15 N(N 2 O), 0.69 ‰ for δ 18 O(N 2 O) and 1.12 ‰ for δ 15 N(NO + fragment). For smaller amounts of ice the standard deviation increases, particularly for N 2 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.

Online technique for isotope and mixing ratios of CH<sub>4</sub>, N<sub>2</sub>O, Xe and mixing ratios of organic trace gases on a single ice core sample

“…3 for the injection scheme) without producing significant memory effects. This allows for the measurement of isotopic ratios of different isobaric species within a run, while previous studies measuring CH 4 and N 2 O on a single sample either reserved a specific IRMS for each species (Sapart et al, 2011) or held one species until the IRMS was preconditioned for the second species . Currently, the CH 4 and N 2 O Gaussian peaks within each chromatogram serve only to monitor the system performance and for troubleshooting.…”

“…Ice samples from the Antarctic ice core B34 (our δ 13 C-CH 4 value: −47.10 ± 0.05 ‰) have also been measured previously by our colleagues at the Institute for Marine and Atmospheric Research Utrecht (IMAU) and at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI); Sapart et al (2011) report δ 13 C-CH 4 values of −46.46 ± 0.21 ‰ for IMAU and −46.57 ± 0.13 ‰ for AWI. Both values were measured before the awareness of the Kr issue.…”

“…Previous methods determining both CH 4 and N 2 O isotopes on a single ice core sample either used two separate mass spectrometers for CH 4 and N 2 O (Sapart et al, 2011) or two individual acquisitions on a single mass spectrometer . To our knowledge we are the first to combine these two isobaric species (CH 4 -derived CO 2 and N 2 O) in a single acquisition run.…”

“…To analyse isotopes of CH 4 and N 2 O on ice core samples, several methods using different extraction techniques have been developed (Sowers et al, 2005;Schaefer and Whiticar, 2007;Behrens et al, 2008;Melton et al, 2011;Sapart et al, 2011;Sperlich et al, 2013;Bock et al, 2014). In contrast, only a few techniques for ice core samples have been established to analyse trace gas species at the ppt level, such as ethane, propane, and methyl chloride (Aydin et al, 2002;Saito et al, 2006;Aydin et al, 2007).…”

An automated GC-C-GC-IRMS setup to measure palaeoatmospheric δ<sup>13</sup>C-CH<sub>4</sub>, δ<sup>15</sup>N-N<sub>2</sub>O and δ<sup>18</sup>O-N<sub>2</sub>O in one ice core sample

“…Brand, 1995;Behrens et al, 2008 The uncertainty estimates are based on the standard deviation (1σ ) apart from a , which is described by . The b indicates when the referencing method of NEEM for the isotopic composition of N 2 O is explained by Sapart et al (2011).…”