Simultaneous stable isotope analysis of methane and nitrous oxide on ice core samples
- 1Institute for Marine and Atmospheric research Utrecht, Utrecht Univ., Princetonplein 5, 3584CC Utrecht, The Netherlands
- 2Climate and Environmental Physics, Physics Institute and Oeschger Centre for Climate Change Research, Univ. of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
- 3Earth and Environmental Systems Institute, Geoscience, 237 Deike Building, Pennsylvania State Univ., University Park, PA 16802, USA
- 4Center for Ice and Climate, Niels Bohr Institute, Univ. of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
- 5Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
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.