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

摘要

Methane and nitrous oxide are important greenhouse gases which show a strongincrease in atmospheric mixing ratios since pre-industrial time as well aslarge variations during past climate changes. The understanding of theirbiogeochemical cycles can be improved using stable isotope analysis.However, high-precision isotope measurements on air trapped in ice cores arechallenging because of the high susceptibility to contamination andfractionation.Here, we present a dry extraction system for combined CH and NOstable isotope analysis from ice core air, using an ice grating device. Thesystem allows simultaneous analysis of δD(CH) or δC(CH), together with δN(NO), δO(NO) and δN(NO fragment) on a single icecore sample, using two isotope mass spectrometry systems. The optimumquantity of ice for analysis is about 600 g with typical "Holocene" mixingratios for CH and NO. In this case, the reproducibility(1σ ) is 2.1‰ for δD(CH), 0.18‰ for δC(CH), 0.51‰ for δN(NO), 0.69‰ forδO(NO) and 1.12‰ for δN(NO fragment). For smaller amounts of ice the standard deviation increases,particularly for NO isotopologues. For both gases, small-scaleintercalibrations using air and/or ice samples have been carried out incollaboration with other institutes that are currently involved in isotopemeasurements of ice core air. Significant differences are shown between thecalibration scales, but those offsets are consistent and can therefore becorrected for.