COMPOUND SPECIFIC CARBON ISOTOPE ANALYSIS FOR BIOMARKERS ASSOCIATED WITH MARINE METHANOTROPHY IN THE ARCTIC
Dougherty, Mara Ryan
Mignerey, Alice C
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A large reservoir of methane exists in marine sediments. The fate of methane is of particular concern in the Arctic, a region that has already demonstrated sensitivity to climate change. The removal of this potent greenhouse gas from the carbon cycle is largely mediated by microorganisms. In methane bearing ocean sediments where sulfate penetrates the surface sediment, sulfate reducing bacteria (SRB) and archaeal methanotrophs are found and believed to act as a consortium in the anaerobic oxidation of methane (AOM). Despite efforts based on thermodynamic models, rate measurements, and δ13C analysis of microbial biomarkers, the process by which methane is removed from anoxic sediments remains speculative. Sediment samples were collected from the Beaufort Shelf, east of Point Barrow, AK as part of the Methane in the Arctic Shelf/Slope (MITAS) Expedition in 2009. Core PC13 from this cruise was selected for compound specific carbon isotope analysis due the measured sulfate and methane concentrations. Stable carbon isotope analysis of the bacterial biomarkers selected specifically for known SRB phylotypes associated with AOM (i.e., i-C15:0, ai-C15:0 and C16:1 fatty acid methyl esters) resulted in δ13C values ranging from -27.8 to -25.3 /, strongly 13C-enriched relative to the biogenic methane in this core (δ13C = -100.0 to -74.6 /). At AOM sites, the microbial community involved in the process should reflect the carbon isotopic signature of the methane in instances of methanotrophy. In PC13, the bacterial biomarkers were not 13C-depleted like the methane, suggesting the lack of sulfate dependent AOM. The measurement of sulfate reduction rates and phylogenetic investigations corroborated the result from biomarker analysis, that the primary pathway for methanotrophy at this site is not coupled to sulfate reduction. Radiocarbon analyses of the bacterial biomarkers from PC13 were not utilized for the determination of methanotrophic pathways because the biomarkers targeted were for phylotypes whose dominant function at this site is not coupled to methanotrophy. However, the radiocarbon age of the bacterial markers may be useful in determining the sediment deposition rate at this site. For these biomarkers at 396 and 516 cm below the seafloor, the radiocarbon ages are 5805 and 5878 radiocarbon years, respectively. These ages result in an offset of 2500 radiocarbon years older relative to the shell fragments analyzed from the same depth. The biomarker age likely represents the older sediment delivered to the western Arctic via current systems, while the age of the shell fragments were deposited contemporaneously.