Abstract

Abstract. Fluid flow related seafloor structures and gas seeps were detected in the North Sea in the 1970s and 1980s by acoustic sub-bottom profiling and oil rig surveys. A variety of features like pockmarks, gas vents and authigenic carbonate cements were found to be associated with sites of oil and gas exploration, indicating a link between these surface structures and the underlying, deep hydrocarbon reservoirs. In this study we performed acoustic surveys and videographic observation at Gullfaks, Holene Trench, Tommeliten, Witch's Hole and the giant pockmarks of the UK Block 15/25, to investigate the occurrence and distribution of cold seep ecosystems in the Northern North Sea. The most active gas seep sites, i.e. Gullfaks and Tommeliten, were investigated in detail. At both sites, gas bubbles escaped continuously from small holes in the seabed to the water column, reaching the upper mixed surface layer. At Gullfaks a gas emitting, flat area of 0.1 km2 of sandy seabed covered by filamentous sulfur-oxidizing bacteria was detected. At Tommeliten, we found a patchy distribution of small bacterial mats indicating sites of gas seepage. Below the patches the seafloor consisted of sand from which gas emissions were observed. At both sites, the anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) was the major source of sulfide. Molecular analyses targeting specific lipid biomarkers and 16S rRNA gene sequences identified an active microbial community dominated by sulfur-oxidizing and sulfate-reducing bacteria (SRB) as well as methanotrophic bacteria and archaea. Stable carbon isotope values of specific, microbial fatty acids and alcohols from both sites were highly depleted in the heavy isotope 13C, indicating that the microbial community incorporates methane or its metabolites. The microbial community composition of both shallow seeps shows high similarities to the deep water seeps associated with gas hydrates such as Hydrate Ridge or the Eel River basin.

Highlights

  • The North Sea is a marginal sea of the Atlantic on the European continental shelf

  • As part of the 5th EU framework project METROL “Methane fluxes in ocean margin sediments: microbiological and geochemical control” we studied the distribution, biogeochemistry and microbiology of gas seepage in the North Sea, to identify potential sites of methane emission to the atmosphere, and to better understand the functioning of shallow water seep ecosystems

  • Based on the concentration and stable carbon isotope values of specific lipid biomarkers, as well as by 16S rRNA sequence analysis, we describe the microbial communities of two active shallow water seeps in the North Sea (Gullfaks and Tommeliten)

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Summary

Introduction

The North Sea is a marginal sea of the Atlantic on the European continental shelf. Its sedimentary basin, especially the western and northern areas, hosts large gas and oil fields which are exploited since the 1970s. Based on the concentration and stable carbon isotope values of specific lipid biomarkers, as well as by 16S rRNA sequence analysis, we describe the microbial communities of two active shallow water seeps in the North Sea (Gullfaks and Tommeliten). Their phylogenetic and biogeochemical characteristics are compared with those of known deep water cold seep communities to investigate whether shallow and deep water seeps are populated by different types of methanotrophs

Sampling sites
Seafloor observations
Sediment sampling
Biomarker analysis
Sequencing and phylogenetic analysis
Distribution of active seeps in the Northern North Sea
Microbial methane oxidation and sulfate reduction
Microbial diversity and community composition based on 16S rRNA gene analyses
Methane and sulfate turnover
Biomarker and carbon isotope composition
The North Sea seeps in comparison to deep water cold seeps
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