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

摘要

The Tommeliten seepage area is part of the Greater Ekofisk area, which issituated 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 intooverlying clay-silt and sandy sediments. Hydroacoustic sediment echosoundingshowed several venting spots coinciding with the apex of marl domes wheremethane is released into the water column and potentially to the atmosphere.In the vicinity of the gas seeps, sea floor observations showed small matsof giant sulphide-oxidizing bacteria above patches of black sediments aswell as carbonate crusts, which are exposed 10 to 50 cm above seafloorforming small reefs. These Methane-Derived Authigenic Carbonates (MDACs)contain 13C-depleted, archaeal lipids indicating previous gas seepageand AOM activity. High amounts of sn2-hydroxyarchaeol relative to archaeol andlow abundances of biphytanes in the crusts give evidence that ANaerobicMEthane-oxidising archaea (ANME) of the phylogenetic cluster ANME-2 were thepotential mediators of Anaerobic Oxidation of Methane (AOM) at the time ofcarbonate formation. Small pieces of MDACs were also found subsurface atabout 1.7 m sediment depth, associated with the AOM zone. This zone ischaracterized by elevated AOM and Sulphate Reduction (SR) rates, increasedconcentrations of 13C-depleted tetraether derived biphytanes, andspecific bacterial Fatty Acids (FA). Further biomarker and 16S rDNA basedanalyses of this horizon give evidence that AOM is mediated by archaeabelonging to the ANME-1b group and Sulphate Reducing Bacteria (SRB) mostlikely belonging to the Seep-SRB1 cluster. The zone of active methaneconsumption was restricted to a distinct horizon of about 20 cm.Concentrations of 13C-depleted lipid biomarkers (e.g. 500 ng g-dw−1biphythanes, 140 ng g-dw−1 fatty acid ai-C_15:0), cellnumbers (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 lowercompared to AOM zones of highly active deep water cold seeps such as HydrateRidge or the Gulf of Mexico.