Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Comparison of the North Sea seep site sediments of Tommeliten & Gullfaks by molecular and geochemical tools

The purpose of this work was to investigate if active seeps of the Central and Northern North Sea host communities of methanotrophs utilising methane and thus partly controlling the methane efflux. The hypothesis was that anaerobic methane-oxidizing communities associated with methane seepage in the North Sea are similar to those known from deep-water seeps. We analyzed two types of seep systems in the North Sea, the Gullfaks seep field, and the Tommeliten area.

The Gullfaks seep field, located at about 61°10 N, 2°14 E lies above one of the three giant oil fields of the North Sea Plateau in the western limb of Viking Graben at a water depth of about 150 meters. The commercially exploited reservoirs are located in two to three kilometres depth in Jurassic sedimentary rocks and are charged by deeper, organic rich sediments. From these shallow reservoirs, methane gas constantly leaks into the overlying sand layers. This sand was deposited during the last glacial maximum when the sea level reached a low with about 120m less than today. Because of the relatively high permeability of the sediments no pockmarks are formed upon gas release. As part of the METROL project, we were able to visit the gas seeps of Gullfaks on the RV Heincke cruise 169 and 208.

The first exploratory visit during He169 allowed to map the field and to detect many small gas seeps in an area of about 100x200m covered by mats of giant sulfide oxidizing bacteria. Gas flares in the water column were detected via a high resolution sediment echo sounder system (SES) and observations of the sea ground. Small streams of rising gas bubbles were observed with a towed camera approximately every 5 square metres.

The main objective of the cruise He208 in May 2004 was to investigate the microbial community, which is fuelled by the rising gas. The bacterial mats were identified to be composed of Beggiatoa filaments. First experiments with radioactive tracer indicated that a high proportion of the methane is oxidized anaerobically with sulphate at Gullfaks, producing high fluxes of sulfide. The sulfide is then oxidized by the giant sulphur oxidizing bacteria belonging to the Beggiatoa group. Molecular analyses of the sediments by FISH and 16S rDNA clone libraries showed that members of the ANME 2a and 2c group in consortium with sulfate reducing bacteria of the Desulfosarcina/Desulfococcus clade as responsible methane-oxidizing microorganisms. The most intriguing question is as to how an anoxic setting is maintained in the upper 20 cm of the sand, a prerequisite as most methanotrophs are highly sensitive to oxygen.

The Tommeliten field in the Central North Sea has also been a site of active exploitation, and its natural gas seeps are known since more than 20 years. Here, cracks in a buried marl horizon allow methane to migrate into overlying sediments. Our observations from vibrocorer sampling show that the marl sediments represent a barrier to the advective methane flow from a deeper reservoir. A shallower depth of the marl boundary in the vicinity of plumes indicates that subsurface gas pressure lifts sediments. As a consequence, methane is advected into the overlying, comparably soft, clayish silt. At several spots coinciding with the apex of the marl domes, methane is released into the water column as shown by hydroacoustic sediment echosounding. In the vicinity of the gas seeps, sea floor observations show small mats of giant sulfide-oxidizing bacteria above patches of black sediments. In the gassy subsurface sediments, the presence of ANME1 communities and their associated partner sulfate reducing bacteria was shown. Anaerobic ethane oxidation was low, but higher in this layer compared to sediments above and below the gassy zone. Furthermore, pieces of carbonates were found in association with the gassy subsurface sediments. Close by to the seepage structures, carbonate crusts are exposed up to 20-50 cm above seafloor forming small reef systems. The subsurface and surface carbonates contain 13C-depleted, archaeal lipids indicating previously high gas seepage at Tommeliten. High amounts of sn2-hydroxyarchaeol relative to archaeol in the crusts and low abundances of biphytanes give evidence that ANME2 archaea were the potential mediators of AOM at the time the carbonates formed.

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Alfred-Wegener-Institute Bremerhaven
Celsiusstr. 1
28359 Bremen
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