Atmospheric CH4 along the Trans-Siberian railroad (TROICA) and river Ob: Source identification using stable isotope analysis

The concentration and stable isotope (13C, D) signatures of methane (CH4) were determined in air samples collected in Russia, summer of 1999 plus 2001 and spring 2004, using the Trans-Siberian railway (TROICA project) and a boat as atmospheric measurement platforms. CH4 concentrations over wetlands in Western Siberia, especially in the Middle Ob area, are significantly enhanced relative to the background mixing ratios. The source isotopic signature of the samples collected in Western Siberia is δ13Csource=−62.9±0.7‰ (n=5) and δDsource=−369±11‰ (n=5) in June–July 1999, confirming that biogenic CH4 from wetlands dominated the substantial CH4 excess along the Siberian transect and on the Ob river. Source estimates (δ13Csource) are in the range between −54.1±2.8‰ and −67.4±1.6‰ for different regions in June–July 2001. It the spring expedition the individual source signatures are more uncertain as deviations of CH4 concentration from a background are rather small. The strongest deviations of the isotopic composition from the mean expedition values occur in vicinity of Perm (1999, 2001) and Novosibirsk (2004), where the isotopic signature of natural gas in atmospheric CH4 is evident.