Measurement of stable carbon isotope ratios of methane in ice samples

Methane concentrations and carbon isotope ratios in ice samples offer an approach to study past budgets of atmospheric methane and to investigate changes in source composition or sink processes. Methane concentrations in ice samples also provide a tool for age correlation between different ice cores. Critical for the use of these parameters are measurements with sufficient accuracy and precision on small samples. To this end we developed a micro-extraction gas chromatography-continuous-flow isotope ratio mass spectrometry (GC-IRMS) technique to extract, focus and measure the methane 13C/12C ratio (δ13CH4) from extremely small gas occlusions in ice (ca. 300 pmol of methane from 100 to 250 g of ice). This micro-measurement allows the time resolution (sample stratigraphic thickness) necessary in paleo-records to investigate decadal-range changes in the methane cycle coupled to abrupt warming events during the last glacial period. The accuracy (±0.4‰) and precision (1σ = 0.32‰) of our method was established by in-house ice standards and confirmed with analysis of ice from GISP2 and the Agassiz ice cap. Our δ13CH4 measurements of −49.7‰ between 225 and 330 yr BP agree very well with previously published data. These δ13CH4 ice core results indicate an unexpected 13C-enrichment of the pre-industrial atmospheric methane, compared to predictions by mass balance models.