Methane and minor oil macro-seep systems — Their complexity and environmental significance

Hydrocarbon-related microbial processes in the deep sediments of the Eastern Mediterranean Levantine Basin

“…Although the geological process forming the pockmark field in Acre location is unknown, previous affiliation of pockmark fields with hydrocarbon seepage (Hovland et al ., , ; Judd & Hovland, ; Cathles et al ., ) implies that hydrocarbon seepage in this location is possible. Moreover, not only methane, but larger hydrocarbons/oil seepage can cause such large TOC enrichments (Hovland et al ., ). The dominance of ANME‐3 in Acre sediments may be a result of the presence of larger hydrocarbons, as this group is mainly restricted to submarine mud volcanoes (Knittel & Boetius, ).…”

Increased Fluid Flow Activity in Shallow Sediments at the 3 km Long Hugin Fracture in the Central North Sea

“…This areal extent is likely to be a minimum estimate because, as discussed above, precipitation of sulfides may prevent the formation of bacterial mats at the seafloor. This area is slightly smaller than other known methane seep areas in the North Sea, such as the “Heincke” site (0.1 km 2 ) and Tommeliten (0.12 km 3 ); however, active seepage only occurs over a fraction of their surface area (Hovland et al, , Judd & Hovland, ) so the Hugin Fracture may be of similar importance as a chemosynthetic hotspot as other fluid flow structures in the North Sea.…”

“…The presence of methane‐ and sulfide‐containing, reduced fluids in shallow sediments can have broad ecological and biological implications for the marine environment, providing chemosynthetic energy to usually energy‐depleted parts of the ocean, but potentially also releasing a potent greenhouse gas into the water column and the atmosphere. In the Central and Northern North Sea, the presence of methane and its oxidation products in shallow subsurface sediments and methane release to the water column has been demonstrated at seabed pockmarks (Hovland et al, ) and at abandoned hydrocarbon exploration wells (e.g., Vielstädte et al, ). The discovery of reduced fluids and evidence for the presence of shallow methane (high sulfide concentrations, bacterial mats) in the Hugin Fracture sediments suggests that fractures may also be important fluid migration pathways in the North Sea.…”

“…The chemical composition of the porewaters is strongly dependent on their origin, their migration and, as they are often highly reactive, their chemical transformation during transport. Finally, the presence of white microbial mats at the seafloor formed by giant filamentous sulfide oxidizing bacteria is often used as a proxy for the presence of reduced, methane‐containing fluids in the shallow subsurface (e.g., Hovland & Svensen, ; Hovland et al, ). These microbial mats live on the sulfide produced by the microbial turnover of methane with sulfate deeper in the sediment (anaerobic oxidation of methane: CH 4 + ‐> HS – + + H 2 O, (equation (1); Boetius et al, ) and can be detected by video‐imaging and biogeochemical analyses of sediments.…”

Evidence for extensive methane venting on the southeastern U.S. Atlantic margin

“…Understanding the factors that govern the distribution of such communities has been a scientifi c pursuit since the discovery of the fi rst cold seep (Paull et al, 1984). Despite the potential signifi cance of seabed methane escape, our knowledge of the frequency and intensity of active seabed fl uid fl ux is constrained by a limited number of deep-sea observations (Hovland et al, 2012).…”