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Methane emission and consumption at a North Sea gas seep (Tommeliten area)
1Max Planck Institute for Marine Microbiology, 28 359 Bremen, Germany
2Alfred Wegener Institute for Polar and Marine Research, 27 515 Bremerhaven, Germany
3Research Center Ocean Margins, University of Bremen, 28 359 Bremen, Germany
4Statoil, 4001 Stavanger, Norway
5University of Georgia, Athens, Georgia 30602-3636, USA
6Wilderspool House, High Mickley, Stocksfield, Northumberland, NE43 7LU, UK
7FILAX Gesellschaft f¨ur Wissenschaftliche Datenverarbeitung mbH, 27 568 Bremerhaven, Germany
8University of Aarhus, 8000 Aarhus, Denmark
9International University Bremen, 28 759 Bremen, Germany
Abstract. The Tommeliten seepage area is part of the Greater Ekofisk area, which is situated above the Tommeliten Delta salt diapir in the central North Sea (56°29.90' N, 2°59.80' E, Norwegian Block 1/9, 75 m water depth). Here, cracks in a buried marl horizon allow methane to migrate into overlying clay-silt and sandy sediments. Hydroacoustic sediment echosounding showed several venting spots coinciding with the apex of marl domes where methane is released into the water column and potentially to the atmosphere. In the vicinity of the gas seeps, sea floor observations showed small mats of giant sulphide-oxidizing bacteria above patches of black sediments as well as carbonate crusts, which are exposed 10 to 50 cm above seafloor forming small reefs. These Methane-Derived Authigenic Carbonates (MDACs) contain 13C-depleted, archaeal lipids indicating previous gas seepage and AOM activity. High amounts of sn2-hydroxyarchaeol relative to archaeol and low abundances of biphytanes in the crusts give evidence that ANaerobic MEthane-oxidising archaea (ANME) of the phylogenetic cluster ANME-2 were the potential mediators of Anaerobic Oxidation of Methane (AOM) at the time of carbonate formation. Small pieces of MDACs were also found subsurface at about 1.7 m sediment depth, associated with the AOM zone. This zone is characterized by elevated AOM and Sulphate Reduction (SR) rates, increased concentrations of 13C-depleted tetraether derived biphytanes, and specific bacterial Fatty Acids (FA). Further biomarker and 16S rDNA based analyses of this horizon give evidence that AOM is mediated by archaea belonging to the ANME-1b group and Sulphate Reducing Bacteria (SRB) most likely belonging to the Seep-SRB1 cluster. The zone of active methane consumption was restricted to a distinct horizon of about 20 cm. Concentrations of 13C-depleted lipid biomarkers (e.g. 500 ng g-dw−1 biphythanes, 140 ng g-dw−1 fatty acid ai-C15:0), cell numbers (1.5×108 cells cm−3), AOM and SR rates (3 nmol cm−3 d−1) in the Tommeliten AOM zone are 2–3 orders of magnitude lower compared to AOM zones of highly active deep water cold seeps such as Hydrate Ridge or the Gulf of Mexico.
Final Revised Paper (PDF, 9846 KB) Discussion Paper (BGD)
Citation: Niemann, H., Elvert, M., Hovland, M., Orcutt, B., Judd, A., Suck, I., Gutt, J., Joye, S., Damm, E., Finster, K., and Boetius, A.: Methane emission and consumption at a North Sea gas seep (Tommeliten area), Biogeosciences, 2, 335-351, 2005. Bibtex EndNote Reference Manager
