HR: 1340h
AN: B43B-0164    [Abstracts]
TI: Two Dimensional Distributions of Microbial Methane Oxidation in Marine Gas Seeps Interpreted from „$^{13}$C-CH$_{4}$
AU: * Kinnaman, F S
EM: frank\_kinnaman@umail.ucsb.edu
AF: Department of Geological Sciences, UC Santa Barbara, Building 526, UC Santa Barbara, Santa Barbara, CA 93106-9630
AU: Valentine, D L
EM: valentine@geol.ucsb.edu
AF: Department of Geological Sciences, UC Santa Barbara, Building 526, UC Santa Barbara, Santa Barbara, CA 93106-9630
AB: The Southern California Bight is well known for its \"cold\" hydrocarbon vents, areas of intense leakage of underlying thermogenic natural gas. Isotopic studies of methane in sediments at Coal Oil Point, offshore UC Santa Barbara, were performed around small (~.5 mol of gas h$^{-1}$) marine gas seeps. Seep gas was found to consist of 80% methane, 15% CO$_{2}$, and 5% C$_{2}$-C$_{4}$ hydrocarbons. In order to assess the subsurface bacterial consumption of methane, we devised a two-dimensional, in-situ sampling system to collect gas samples. The system consists of a stainless-steel, square sampling grid that is inserted vertically into the sediment. Samples are collected by syringe through holes in the plate, after sediment has been removed from one side of the plate and the grid has been exposed. Samples are immediately injected into serum bottles, which are crimp-sealed at depth. The „$^{13$C-CH$_{4}$ in each sample is analyzed by isotope ratio mass spectrometry upon return to the lab. Progressive enrichments of „$^{13$C-CH$_{4}$ in sediments surrounding several gas seeps were observed; methane was enriched by as much as 60% compared to the original seep gas „$^{13$C-CH$_{4}$ (typical seep gas „$^{13$C-CH$_{4}$ = -51%). The maximum enrichment was typically observed at a lateral distance of 3-4 cm from the seep. The isotopic enrichment is attributed to the preferential utilization of $^{12}$C-CH${_4}$ by methane-oxidizing bacteria and provides an estimate of the extent of methane oxidation - which is up to 95%. This work also provides insights into the manner of subsurface methane flow in gas seeps, and minor channels of seepage can be inferred from plumes of $^{12}$C methane.
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting