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44729-AC2
Light Methane in Cold Environments: Insights from the Intramolecular Carbon Isotopic Composition of Acetate
Stable C and H isotopic measurements of methane and carbon dioxide are now routinely applied to environmental samples as an in situ means of assessing the relative importance of two main methane production pathways, aceticlastic and hydrogenotrophic methanogenesis. Isotopic pathway estimates rely heavily upon assumptions about the in situ isotopic fractionation during methane production and oxidation. Moreover, isotope-based pathway estimates require knowledge of the isotopic composition of both methane precursors: carbon dioxide and acetate-methyl. In practice, technical barriers have limited measurements of the isotopic composition of whole acetate in natural samples. Even when whole acetate data is available, it is exceedingly rare to find carbon isotopic measurements of acetate-methyl. To our knowledge, the d13C of the acetate-methyl precursor to methane has never before been reported from peatland porewater samples.
We have significantly improved a method for determining the intramolecular carbon isotopic composition of acetate using direct injection of aqueous samples. This system builds upon prior work that established pyrolytic conditions for online analysis and represents a significant advance in that it requires minimal preparation for samples containing as little as 1 mM sodium acetate in aqueous solution. This technique is applicable for analysis of oilfield brines, culture samples, biological samples and natural porewaters. We have demonstrated the accuracy of the technique by use of a stable isotope dilution series. We also have shown that addition of a base and cryogenic preconcentration may cause the carboxyl carbon to exchange with dissolved inorganic carbon. This exchange does not appear to affect the measured methyl carbon isotope value although it can significantly alter the measured isotopic composition of the carboxyl carbon of acetic acid. Our preconcentration experiments demonstrate that the method is suitable for carbon isotopic measurements of acetate methyl carbon in natural samples at concentrations as low as 90 uM, considerably broadening potential applications.
Using this 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 in the upper 40 cm that favor autotrophic acetogenesis and subsequent acetate consumption by aceticlastic methanogens. Significant aceticlastic methane production from autotrophically produced acetate challenges the ubiquity with which hydrogen isotopic measurements of methane can determine the pathway of methanogenesis.
Supplementing our field observations, intramolecular acetate measurements of incubation experiments confirm that aceticlastic methanogenesis can facilitate significant acetate-carboxyl exchange with DIC. This novel technique confirms two caveats associated with whole acetate carbon isotopic data: 1) it is possible that the carboxyl carbon isotopic composition does not accurately reflect a simple fractionation relative to the parent molecule, and 2) the acetate methyl may not be derived from larger fermentation products or the fractionation effect of fermentation in acidic porewaters may be large.
