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

Abstract

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 13 C-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 13 C-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 belongCorrespondence to: H. Niemann (hniemann@mpi-bremen.de) ing 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 13 Cdepleted lipid biomarkers (e.g. 500 ng g-dw −1 biphythanes, 140 ng g-dw −1 fatty acid ai-C 15:0 ), cell numbers (1.5×10 8 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.

Regulation of anaerobic methane oxidation in sediments of the Black Sea

“…Microbial process rates in core P806GC determined by radiotracer experiments were indeed low, especially for SRR. However, in diffusion dominated systems at other continental margins AOM rates are mostly similarly low, and peak rates from the Black Sea, in particular in core P771GC with a very pronounced methane tailing, were in the same range (3-6 nmol cm −3 d −1 ) as maximum AOM rates measured in the North Sea (Niemann et al, 2005) and Skagerrak (Parkes et al, 2007), where methane tailing did not occur. The combined presence of methane and sulfate in an extended SMTZ should facilitate AOM activity and sulfate reduction also in shallower horizons.…”

“…The process of anaerobic oxidation of methane (AOM) is widespread in continental margin sediments and occurs in a variety of different environments, including diffusion controlled sediments (Iversen and Jørgensen, 1985), sediments containing shallow gas accumulations (Niemann et al, 2005;Treude et al, 2005b), gas-hydrate bearing sediments (Treude et al, 2003;Joye et al, 2004;Orcutt et al, 2004), and mud volcanoes (Niemann et al, 2006), In the western part of the Black Sea, which is dominated by an extensive shelf in the northwest and by river deltas of the Danube and Dnjepr rivers, the sediment contains large amounts of methane, and numerous active methane seeps (Popescu et al, 2001). These seeps are mainly located above the gas hydrate stability zone, at the transition from the continental shelf to the upper slope, and are common in the area of the Danube canyon and the Dnjepr paleo-delta (Naudts et al, 2006).…”

Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

“…High-acoustic backscatter is caused by the enhanced acoustic impedance or roughness contrast between certain regions of the seafloor and their surroundings (Blondel and Murton, 1997). At methane seeps, this contrast is primarily caused by the presence of methane-derived authigenic carbonates (MDACs), chemosynthetic "cold seep" communities (clams, tube worms), bubbles or gas hydrates in the sediment (Hovland et al, 1985;Ritger et al, 1987;Paull et al, 1992;von Rad et al, 1996;Greinert et al, 2001;Peckmann et al, 2001;Fonseca et al, 2002;Greinert et al, 2002b;Orange et al, 2002;Johnson et al, 2003;Niemann et al, 2005;Holland et al, 2006;Ivanov et al, 2007). Multibeam and side scan sonar surveys also detect changes in the seafloor morphology, which can mark the location of gas seeps (e.g.…”

The Geomorphology and Nature of Seabed Seepage Processes

“…However, these midwater hydroacoustic "flares" (named thus because they are highly visible on echosounder recordings) are not only caused by various sized bubbles, but possibly also by water density contrasts caused by high concentrations of dissolved methane in the water. Thus, Niemann et al (2005), reported up to 2 orders of magnitude higher concentrations (500 nM) of methane within the acoustic plumes (flares), compared to the background methane concentration (5 nM). Other possibilities for the formation of large, acoustic flares will be discussed later in this chapter.…”

“…Within the central and northern North Sea, there are three, fairly well-studied methane seep locations: the Tommeliten seep area (56°29.90' N, 2°59.80'E) (Hovland and Sommerville, 1985;Hovland and Judd, 1988;Niemann et al, 2005;Wegener et al, 2008;Schneider von Deimling, 2010), the Scanner pockmark seeps (58°28.5' N, 0°96.7'E) (Hovland and Sommerville, 1985;Hovland and Thomsen, 1989;Dando and Hovland, 1992), and the Gullfaks seeps (61°10.1' N, 2°15.8'E) (Hovland, 2007;Wegener et al, 2008). Although each of them are located in different geological settings, they have one main aspect in common: -they occur as continuous macro-methane seeps (Fig.…”