An automated setup to measure paleoatmospheric δ13C-CH4, δ15N-N2O and δ18O-N2O in one ice core sample

Abstract

Air bubbles in ice core samples represent the only opportunity to study the isotopic variability of paleoatmospheric CH4 and N2O. The highest possible precision in isotope measurements is required to maximize the resolving power for CH4 and N2O sink and source reconstructions. We present a new setup to measure δ13C-CH4, δ15N-N2O and δ18O-N2O isotope ratios in one ice core sample, with a precision of 0.09‰, 0.6‰ and 0.7‰, respectively, as determined on 0.6–1.6 nmol CH4 and 0.25–0.6 nmol N2O. The isotope ratios are referenced to the VPDB scale (δ13C-CH4), the N2-air scale (δ15N-N2O) and the VSMOW scale (δ18O-N2O). Ice core samples of 200–500 g are melted while the air is constantly extracted to minimize gas dissolution. A helium carrier gas flow transports the sample through the analytical system. A gold catalyst is used to oxidize CO to CO2 in the air sample without affecting the CH4 and N2O sample. CH4 and N2O are then separated from N2, O2, Ar and CO2 before they get pre-concentrated and separated by gas chromatography. While the separated N2O sample is immediately analysed in the mass spectrometer, a combustion unit is required for δ13C-CH4 analysis, which is equipped with a constant oxygen supply as well as a post-combustion trap and a post-combustion GC-column (GC-C-GC-IRMS). The post combustion trap and the second GC column in the GC-C-GC-IRMS combination increase the time for δ13C-CH4 analysis which is used to measure δ15N-N2O and δ18O-N2O first and then δ13C-CH4. The analytical time is adjusted to ensure stable conditions in the ion-source before each sample gas enters the IRMS, thereby improving the precision achieved for measurements of CH4 and N2O on the same IRMS. After the extraction of the air from the ice core sample, the analysis of CH4 and N2O takes 42 min. The setup is calibrated by analyzing multiple isotope reference gases that were injected over bubble-free-ice samples. We show a comparison of ice core sample measurements for δ13C-CH4 that are of excellent reproducibility and accuracy, and in good agreement with previously published data.