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Carbon isotopic studies on methane in polar ice cores as a clue to past methane sources

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Behrens, M. , Möller, L. and Fischer, H. (2009): Carbon isotopic studies on methane in polar ice cores as a clue to past methane sources , Jahrestagung der Arbeitsgemeinschaft Stabile Isotope e.V., 2009, Potsdam. .
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Abstract:

Methane is recognised as an important greenhouse gas that contributes ~15 % to the current anthropogenic warming. With the help of isotopic analysis the contributions of different sources and sinks to the atmospheric burden can be estimated. Air enclosures in ice cores are the only direct paleoatmospheric archive and allow an assessment of natural changes of methane in the past. Ice core reconstructions of atmospheric methane concentrations show significant increases for both glacial/interglacial transitions as well as for rapid warming events (Dansgaard/Oeschger events). Due to the different carbon isotopic signature of different methane sources high-precision measurements of delta13CH4 in ice cores supply clues about the global methane cycle.We developed a highly automated (continuous-flow) gas chromatography combustion isotope ratio mass spectrometry technique (GC/C/IRMS) for ice core samples of ~200 g. The methane is melt-extracted using a purge and trap method, then separated from the main air constituents, combusted and measured as CO2 by a conventional isotope ratio mass spectrometer. A CO2 working standard, a CH4 and two air reference gases are used to identify potential sources of isotope fractionation within the entire sample preparation process and to enhance the stability, reproducibility and accuracy of the system. We show the performance of our set-up and data from the Antarctic EDML ice core.

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