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. 2017 Feb 23:7:42997.
doi: 10.1038/srep42997.

Widespread methane seepage along the continental margin off Svalbard - from Bjørnøya to Kongsfjorden

S Mau  1 , M Römer  1 , M E Torres  2 , I Bussmann  3 , T Pape  1 , E Damm  3 , P Geprägs  1 , P Wintersteller  1 , C-W Hsu  1 , M Loher  1 , G Bohrmann  1
Affiliations

Widespread methane seepage along the continental margin off Svalbard - from Bjørnøya to Kongsfjorden

S Mau et al. Sci Rep. .

Abstract

Numerous articles have recently reported on gas seepage offshore Svalbard, because the gas emission from these Arctic sediments was thought to result from gas hydrate dissociation, possibly triggered by anthropogenic ocean warming. We report on findings of a much broader seepage area, extending from 74° to 79°, where more than a thousand gas discharge sites were imaged as acoustic flares. The gas discharge occurs in water depths at and shallower than the upper edge of the gas hydrate stability zone and generates a dissolved methane plume that is hundreds of kilometer in length. Data collected in the summer of 2015 revealed that 0.02-7.7% of the dissolved methane was aerobically oxidized by microbes and a minor fraction (0.07%) was transferred to the atmosphere during periods of low wind speeds. Most flares were detected in the vicinity of the Hornsund Fracture Zone, leading us to postulate that the gas ascends along this fracture zone. The methane discharges on bathymetric highs characterized by sonic hard grounds, whereas glaciomarine and Holocene sediments in the troughs apparently limit seepage. The large scale seepage reported here is not caused by anthropogenic warming.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Flare locations along the Svalbard margin.
(a) Flares observed during HE449 and HE450 (gray dots) as well as flares described by Sahling et al. off Prins Karls Forland (PKF) (white dots). The map shows the main structural features in red according to Faleide et al.; the striped fields indicate the stretched continental crust (stripes from left top to right bottom) and the Tertiary volcanic province (stripes from left bottom to right top). (b) Flares at Hornsundbanken reach the sea-surface (orange arrow marks the location in (a)). The map was generated using ArcGIS 10.2 (©ESRI).
Figure 2
Figure 2. Contour plot of salinities observed in the 100–200 m water depth layer.
The contour of a salinity of 34.9 indicates the Arctic front between saline, warm Atlantic Water and fresher, cold Arctic Surface Water. The Atlantic Water is carried by the Norwegian Atlantic Current (NwAC) and the West Spitsbergen Current (WSC) (dark gray arrows). The Arctic Surface Water is carried by the East Spitsbergen Current (ESC) and the Coastal Current (CC) (black arrows). The locations of the transects of Supplementary Figure S2 online are shown as blue lines. The water mass Melt/Surface Water (MSW) occurred in depths shallower than 100 m and is, thus, not shown here. The plot was generated using Ocean Data View Version 4.5.7 (https://odv.awi.de) and DIVA gridding.
Figure 3
Figure 3. South to north transects of dissolved methane concentration anomalies.
Methane anomalies were derived by subtracting the atmospheric methane equilibrium concentrations from the measured methane concentrations. The maps indicate the transect location and the red star marks the starting point. Above each contour plot, the approximate location along the transect is indicated. The abbreviations stand for Hornsundb. – Hornsundbanken, Isfjordenb. – Isfjordenbanken, and PKF – Prins Karls Forland. The plot was generated using Ocean Data View Version 4.5.7 (https://odv.awi.de) gridding weighted averages.
Figure 4
Figure 4. δ13C vs. 1/CH4 concentration for water samples.
Samples are grouped according to their location at the Svalbard shelf. The stippled black line marks a mixing line between the sample with the highest methane concentration and background methane values of the ocean. The gray lines indicate microbial oxidation trends. The best fit to all data has an R2 of 0.36 with a δ13C-value of the source methane of −64‰.
Figure 5
Figure 5. The relative activity of methane oxidizing microorganisms (k′) versus methane concentrations at Hornsundbanken (encircled) and Isfjordenbanken (without outline).
The color code shows the resulting methane oxidation rates (MOx). Highest MOx-rates correlate with sites of gas emissions, i.e. highest methane concentrations. In addition, k′ and MOx increase down-current from Hornsundbanken to Isfjordenbanken. The plot was generated using Ocean Data View Version 4.5.7 (https://odv.awi.de).
Figure 6
Figure 6. Surface methane anomalies, wind speed at 10 m above sea-surface during the time when water samples were collected, and sea-air flux of methane.
The resulting sea-air flux of methane was elevated at the seepage sites off PKF, Hornsundbanken, and Sørkappbanken as well as in areas of wind speeds >5 m s−1. The plot was generated using Ocean Data View Version 4.5.7 (https://odv.awi.de) gridding weighted averages.
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