HR: 0800h
AN: B11A-1027    [Abstracts]
TI: Physical and Biological Carbon Isotope Fractionation in Methane During Gas-Push-Pull-Tests
AU: Gonzalez-Gil, G
B11A-1027 AF: Swiss Federal Institute of Technology (ETH) Zurich, ETH Zentrum, CHN G50.2 Unversitaetsstr. 16, Zurich, CH-8092 Switzerland
AU: * Schroth, M H
EM: martin.schroth@env.ethz.ch
AF: Swiss Federal Institute of Technology (ETH) Zurich, ETH Zentrum, CHN G50.2 Unversitaetsstr. 16, Zurich, CH-8092 Switzerland
AU: Gomez, K
B11A-1027 AF: Swiss Federal Institute of Technology (ETH) Zurich, ETH Zentrum, CHN G50.2 Unversitaetsstr. 16, Zurich, CH-8092 Switzerland
AU: Zeyer, J
B11A-1027 AF: Swiss Federal Institute of Technology (ETH) Zurich, ETH Zentrum, CHN G50.2 Unversitaetsstr. 16, Zurich, CH-8092 Switzerland
AB: Stable isotope analyses have become a common tool to assess microbially-mediated processes in subsurface environments. We investigated if stable carbon isotope analysis can be used as a tool to complement gas push-pull tests (GPPTs), a novel technique that was recently developed and tested for the in-situ quantification of CH4 oxidation in soils. During a GPPT a gas mixture containing CH4, O2 and nonreactive tracer gases is injected into the soil, where CH4 is oxidized by indigenous microorganisms. Thereafter, a blend of injected gas mixture and soil air is extracted from the same location, and CH4 oxidation is quantified from an analysis of extracted CH4 and tracer gases. To assess the magnitude of physical isotope fractionation due to molecular diffusion during GPPTs, we conducted laboratory experiments in the absence of microbial activity in a 1m-high, 1m-diameter tank filled with dry sand. During the GPPTs' extraction phase, the isotopic composition of methane was analyzed. Results indicated strong carbon isotope fractionation (>20 per mil) during GPPTs. To assess the combined effect of physical and biological isotope fractionation, numerical simulations of GPPTs were conducted in which microbial CH4 isotope fractionation was simulated using first-order rate constants and microbial kinetic isotope fractionation factors previously reported for methane oxidation in landfill environments. Results of these simulations indicated that for small CH4 oxidation rates, overall isotope fractionation in CH4 is dominated by physical fractionation. Conversely, for high CH4 oxidation rates, overall fractionation is dominated by biological fractionation. Thus, CH4 isotope fractionation data alone from a single GPPT cannot be used to assess microbial CH4 oxidation. However, biological fractionation may be quantified if physical fractionation due to diffusion is known. This can be achieved by conducting two sequential GPPTs, with microbial activity being inhibited in the second test.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
SC: Biogeosciences [B]
MN: Fall Meeting 2005