Methane emission and consumption at a North Sea gas seep (Tommeliten area)

H. Niemann (1, 2) , M. Elvert (3) , M. Hovland (4) , B. Orcutt (5) , A. Judd (6) , I. Suck (2) , J. Gutt (2) , S. Joye (5) , E. Damm (2) , K. Finster (7) , A. Boetius (1, 8)

1 Max Planck Institute for Marine Microbiology
2 Department of Bentho-pelagic processes
3 Research Center Ocean Margins
4 Statoil A.S.A. [Norway]
5 University of Georgia [USA]
6 Wilderspool House
7 Aarhus University [Aarhus]
8 International University Bremen

The North Sea hosts large coal, oil and gas reservoirs of commercial value. Natural leakage pathways of subsurface gas to the hydrosphere have been recognized during geological surveys (Hovland and Judd, 1988). 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. In this study, we report of an active seep site (56°29.90'N, 2°59.80'E) located in the Tommeliten area, Norwegian Block 1/9, at 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 during deep mixing situations. In the vicinity of the gas seeps, sea floor observations showed small mats of giant sulphide-oxidizing bacteria above patches of black sediments and 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 Sulphate-Methane Transition Zone (SMTZ). The SMTZ of Tommeliten 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 give evidence that AOM at the Tommeliten SMTZ 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-C_15:0), cell numbers (1.5x10⁸ cells cm^-3), AOM and SR rates (3 nmol cm^-3 d^-1 in the SMTZ are 2-3 orders of magnitude lower compared to AOM zones of highly active cold seeps such as Hydrate Ridge or the Gulf of Mexico.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

H. Niemann, M. Elvert, M. Hovland, B. Orcutt, A. Judd, et al.. Methane emission and consumption at a North Sea gas seep (Tommeliten area). Biogeosciences Discussions, 2005, 2 (4), pp.1197-1241. ⟨hal-00297796⟩