Abstract
This study focuses on seafloor methane seep sites and their distribution in the northwestern part of the German North Sea. Methane seepage is a common phenomenon along marine shelves and known to occur in the North Sea, but proof of their existence was lacking in the study area. Using a ship-based multibeam echosounder we detected a minimum of 166 flares that are indicative for free gas releases from the seafloor in the German “Entenschnabel” area, which are not related to morphologic expressions at the seafloor. However, a group of small depressions was detected lacking water column anomalies but with indications of dissolved fluid release. Spatial analysis revealed that flares were not randomly distributed but show a relation to locations of subsurface salt diapirs. More than 60% of all flares were found in the vicinity of the salt diapir “Berta”. Dissolved methane concentrations of ∼100 nM in bottom waters were ten times the background value in the “Entenschnabel” area (CH4< 10 nM), supporting the finding of enhanced seepage activity in this part of our study area. Furthermore, locations of flares were often related to acoustic blanking and high amplitude reflections in sediment profiler echograms, most prominently observed at location Berta. These hydroacoustic signatures are interpreted to result from increased free gas concentrations in the sediments. Electromagnetic seabed mapping depicts local sediment conductivity anomalies below a flare cluster at Berta, which can be explained by small amounts of free gas in the sediment. In our area of interest, ten abandoned well sites were included in our mapping campaign, but flare observations were spatially not related to these wells. Naturally seeping methane is presumably transported to the seafloor along sub-vertical faults, which have formed concurrently to the updoming salt. Due to the shallow water depths of 30 to 50 m in the study area, flares were observed to reach close to the sea surface and a slight oversaturation of surface waters with methane in the flare-rich northeastern part of the working area indicates that part of the released methane through seepage may contribute to the atmospheric inventory.
Highlights
Seafloor methane (CH4) seepage is widely known to occur along almost all continental margins
Weak appearing anomalies or anomalies with anomalous shapes were classified as uncertain. This uncertainty increases during bad weather conditions causing enhanced noise in the echogram or when gas emissions occur in pulses of bubble release that show up as single anomalies within the water column instead of continuous linear flares that are connected to the seafloor
Our results show that methane seepage is not uncommon in the German “Entenschnabel” region in the North Sea
Summary
Seafloor methane (CH4) seepage is widely known to occur along almost all continental margins. This includes the diffusion or advection of dissolved methane from the sediment into the bottom water or the expulsion of free methane gas bubbles. Since the Pleistocene, the area has been effected by climate and sea level variations, leading to deposits of glacial-interglacial sediments that are characterized by abundant subglacial tunnel valleys related to melt water flows (Lutz et al, 2009). Several clusters of bright spots were identified above known salt diapirs, suggesting that salt diapirism lead to fracturing of the overburden strata and formation of migration pathways as well as anticlinal structures for hydrocarbon accumulation (Müller et al, 2018)
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