Environmental Science and Engineering Seminar
High precision analyses of the isotopic composition of methane can be used to discriminate between different methane sources. I will discuss two recent examples of this technique in different environments.
Two methane isotope instruments (IRMS and QCLAS) were deployed for 5 months at the Cabauw experimental site for atmospheric research in the Netherlands. The combined high resolution dataset with more than 2500 measurements of both d13C and dD reveals the overwhelming contribution of isotopically depleted agricultural CH4 emissions from ruminants at the Cabauw site, but also allows the identification of specific events with elevated contributions from more enriched sources such as natural gas and landfills. The dataset was compared to model calculations using the global model TM5 and the mesoscale model FLEXPART-COSMO. The results of both models agree better with the measurements when the TNO-MACC emission inventory is used in the models than when the EDGAR inventory is used. Thus, high-resolution isotope measurements allow evaluating emission inventories and thus further constraining the regional methane budget.
As a second example, we measured CH4 concentration and isotopic composition of gas extracted from sediment and water sampled over the East Siberian Arctic Shelf (ESAS) between 2007 to 2013. We find high concentrations (up to 500μM) of CH4 in the pore water of the partially thawed subsea permafrost of this region. For all sediment cores, both hydrogen and carbon CH4 isotope data reveal the predominant presence of CH4 that is not of thermogenic/natural gas origin as it has long been thought, but resultant from microbial CH4 formation, using as primary substrate glacial water and old organic matter preserved in the subsea permafrost or below. Radiocarbon data demonstrate that the CH4 present in the ESAS sediment is of Pleistocene age or older, but a small contribution of highly 14C-enriched CH4 prohibits precise age determination at some sites. Our data suggest that at locations where bubble plumes have been observed, CH4 can escape near-total anaerobic oxidation in the surface sediment. CH4 will then rapidly migrate through the water column to escape to the atmosphere generating a positive radiative feedback.