HR: 09:45h
AN: B41G-08    [Abstracts]
TI: Revising Estimates of the Methane Production Pathway in Peatland Porewater Using Intramolecular Isotopic Analyses of Acetate
AU: * Thomas, B
EM: bthomas@geosc.psu.edu
AF: Department of Geosciences Penn State University, 801 Deike Building, University Park, PA 16802, United States
AU: Arthur, M A
EM: arthur@geosc.psu.edu
AF: Department of Geosciences Penn State University, 801 Deike Building, University Park, PA 16802, United States
AU: Freeman, K H
EM: kate@geosc.psu.edu
AF: Department of Geosciences Penn State University, 801 Deike Building, University Park, PA 16802, United States
AB: Stable isotopic measurements of methane and carbon dioxide are routinely applied to environmental samples to assess the relative importance of methane production by either aceticlastic or hydrogenotrophic methanogenesis. Such estimates rely upon assumptions about isotopic fractionation during methane production and oxidation. Rigorous isotope-based pathway estimates require knowledge of the carbon isotopic composition of both carbon dioxide and acetate. In practice, technical barriers have limited measurements of the isotopic composition of whole acetate in natural samples. Yet, the estimate of whole acetate isotopic values, even when available, may not represent accurately the composition of the methyl carbon, which is, in fact, the precursor to methane. It is exceedingly rare to find carbon isotopic measurements of acetate-methyl in the literature, and, to our knowledge, the d13C of the acetate-methyl precursor to methane has never before been reported from peatland porewater samples. Extremely 13C-depleted methane, -70 permil VPDB, and 13C-enriched carbon dioxide from acidic northern peat bogs are typically interpreted as signatures of hydrogenotrophic methanogenesis. The hypothesized dominance of methane production from hydrogen in acidic bogs contrasts with the vast majority of freshwater wetlands in which aceticlastic methanogenesis dominates. Using a new technique for the online analysis of the intramolecular carbon isotopic composition of acetate in natural samples, we find the acetate-methyl in peat porewaters can be significantly depleted relative to bulk organic matter. In porewater profiles from both winter and summer, acetate is as much as 15 permil depleted relative to bulk carbon. We hypothesize that acetate- methyl isotopic depletion results from conditions that favor autotrophic acetogenesis and subsequent acetate consumption by aceticlastic methanogens. Porewater depth profiles during winter and summer illustrate depth- dependent increases in the fraction of methane derived from carbon dioxide, with deeper peat dominated by hydrogenotrophic methanogenesis, but shallow peat dominated by aceticlastic methanogens. Significant aceticlastic methane production from autotrophically produced acetate challenges the ability of hydrogen isotopic measurements of methane to represent the pathway of methanogenesis. Supplementing our field observations, intramolecular acetate measurements of incubation experiments confirm that an aceticlastic methanogen can facilitate significant acetate-carboxyl exchange with DIC. This novel technique confirms two caveats associated with whole acetate carbon isotopic data: 1, the carboxyl carbon isotopic composition may not accurately reflect the composition of the parent molecule, and 2, the acetate methyl may be derived from inorganic carbon or the fractionation effect of fermentation in acidic porewaters may be significant.
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0475 Permafrost, cryosphere, and high-latitude processes (0702, 0716)
DE: 0490 Trace gases
DE: 0497 Wetlands (1890)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting