Reports: ND251001-ND2: Compound-Specific Isotopes of Methanogenic Precursors in Coal: Laboratory, Field and Modeling Studies

Jennifer McIntosh, University of Arizona

Improved understanding of microbial methane derived from coal (coalbed methane; CBM) provides fundamental insight into subsurface carbon cycling and may be applied to stimulate methanogenic systems for additional gas yield. This research seeks improved understanding of carbon and hydrogen isotope systematics –what these isotope systems actually record during methanogenesis - through field sampling, laboratory investigations, carbon isotope analysis, and isotope modeling.

Field sampling: Twenty water and 19 gas samples were collected from producing and monitoring wells along a ~25 km long NW-SE transect from the edge of the Powder River Basin (PRB) to the center. These samples bracket a key interface between shallow, sulfate-reducing waters, and deeper methanogenic waters. A wide range of C and H isotope ratios were observed in the PRB, expressed below in standard delta notation as per mil, including variation from edge to center of basin: d13C of methane -78.2 to -56.2 per mil, d13C of CO2 -24.7 to 4.7 per mil, d13C of dissolved inorganic carbon ‑17.5 to 17.5 per mil, d2HCH4 -327 to -249 per mil, and d2H of water ‑165 to ‑129 per mil. Variations in d13C may record competition between sulfate reduction and methanogenesis, with their different fractionation factors, or alternatively might indicate a change of methanogenic pathways (acetoclastic methanogenesis vs. CO2 reduction) as previous studies have implied.

Laboratory incubations: In order to supplement field-collected water and gas samples with experiments in which the metabolic pathway is better constrained, incubations were performed in collaboration with Dr. Elizabeth Jones, microbiologist at the U.S. Geological Survey (USGS). Post-doc David Vinson (supported on the PRF project) traveled to USGS-Reston to undertake the design and participate in the setup of these experiments. These incubations combined coal, a nutrient solution, and a microbial consortium containing known methanogens. A headspace gas sample yielded methane with a d13C value of -49 per mil. Samples treated with BES, an inhibitor of methanogens, yielded significant levels of acetate (high µM to low mM range). d13C of acetate in these BES-controlled incubations (‑45 to ‑30 per mil) is more negative than bulk coal (‑26 to ‑24 per mil), consistent with acetogenesis having occurred in the BES-treated experiments.

A second round of experiments were designed to further constrain mass balance of carbon flow and associated isotopic fractionation during coal biodegradation under methanogenic conditions, and to further increase the quantity of acetate available to be sampled from incubations. These experiments have been designed in collaboration with Dr. Denise Akob, who recently succeeded Dr. Jones at the USGS. Vinson traveled to the USGS-Reston to design the experiments, which began in Spring 2013 and are ongoing.

Compound-specific carbon isotope analysis of acetate: More pathway-specific information on methanogenesis may be derived from compound-specific tracers involving methane precursors, such as d13C of acetate. In collaboration with Dr. Neal Blair, of Northwestern University, we are validating a method for compound-specific d13C determination of acetate. This method involves preconcentration of low-acetate samples typical of coal waters, separation of acetate from the sample by headspace solid-phase microextraction (HS-SPME), and d13C analysis on acetate by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). Initial efforts in the Northwestern University Stable Isotope Laboratory demonstrated precise and accurate measurement of d13C of acetate down to ~500 µM acetate concentrations using HS-SPME. Further methods development of the sample introduction into the gas chromatograph was undertaken at the University of Arizona Laboratory for Emerging Contaminants with Dr. Leif Abrell. Collectively, these efforts have led to significant improvements in the extraction and sample injection procedure that are most crucial in low-concentration samples. Given that few other researchers have attempted HS-SPME analysis of low-concentration samples, our work has involved experiments to optimize the conditions of sample extraction and introduction into the gas chromatograph (temperature, time, injection method, and other run conditions). Efforts to obtain precise and accurate analyses of compound-specific d13C of acetate are ongoing, focused on further optimization of sample extraction time, sample introduction methods, and gas chromatographic conditions.

Presentation of results: Results were presented at the Goldschmidt geochemistry conference (Montreal, June 2012) and the Geological Society of America conference (Charlotte, NC, November 2012). At the Montreal Goldschmidt conference, PI Jennifer McIntosh and post-doc Vinson were co-conveners of a session on "Water-rock-microbial interactions in energy systems," which brought together an international group of researchers. We have also submitted a proposal session for the 2014 Goldschmidt conference in Sacramento, CA on "Microbial bioconversion of hydrocarbons formation and bioconversion for enhanced recovery and remediation." Vinson presented PRF results in 5 talks at geosciences departmental seminars during winter 2013 while interviewing for faculty positions.  McIntosh presented PRF results in 3 department seminars in 2012-2013 at the University of Arizona, Lehigh University and Montana State University.

Two articles are in preparation for submission to peer-reviewed journals: (1) a critical review on the application of C and H isotopes to microbial gas systems derived from coal and shale biodegradation, focused on gas generated in situ from biodegradation; and (2) a isotope model for C mass balance and the competing role of sulfate reduction. This model will be applied to our results from the PRB and will be applicable to other microbial gas systems. The critical review paper has incorporated substantive involvement from several co-authors: Anna Martini of Amherst College; Neal Blair of Northwestern University; Steve Larter of the University of Calgary; and Bill Orem of USGS.

Mentoring and career development: Post-doc Vinson was supported by PRF funds through December 2012, and has continued to collaborate on the project while a post-doc at Northwestern University (April 2013-present). This project has allowed Vinson to expand his background to the biogeochemistry of coal biodegradation and methanogenesis, and Vinson has benefited from numerous mentoring and career advancement opportunities. Activities and results obtained during the PRF-funded research contributed to a successful NSF Earth Sciences Postdoctoral Fellowship proposal to expand Vinson's research on the challenging question of what actually controls the carbon isotopic composition of biogenic methane. Vinson provided mentoring to 5 MS and PhD students in McIntosh's research group doing research in methanogenesis and other areas of low-temperature geochemistry.