Pockmarks in the Witch Ground Basin, Central North Sea

Abstract

Marine sediments host large amounts of methane (CH4), which is a potent greenhouse gas. Quantitative estimates for methane release from marine sediments are scarce, and a poorly constrained temporal variability leads to large uncertainties in methane emission scenarios. Here, we use 2‐D and 3‐D seismic reflection, multibeam bathymetric, geochemical, and sedimentological data to (I) map and describe pockmarks in the Witch Ground Basin (central North Sea), (II) characterize associated sedimentological and fluid migration structures, and (III) analyze the related methane release. More than 1,500 pockmarks of two distinct morphological classes spread over an area of 225 km2. The two classes form independently from another and are corresponding to at least two different sources of fluids. Class 1 pockmarks are large in size (>6 m deep, >250 m long, and >75 m wide), show active venting, and are located above vertical fluid conduits that hydraulically connect the seafloor with deep methane sources. Class 2 pockmarks, which comprise 99.5% of all pockmarks, are smaller (0.9–3.1 m deep, 26–140 m long, and 14–57 m wide) and are limited to the soft, fine‐grained sediments of the Witch Ground Formation and possibly sourced by compaction‐related dewatering. Buried pockmarks within the Witch Ground Formation document distinct phases of pockmark formation, likely triggered by external forces related to environmental changes after deglaciation. Thus, greenhouse gas emissions from pockmark fields cannot be based on pockmark numbers and present‐day fluxes but require an analysis of the pockmark forming processes through geological time.

Resolving hydromechanical coupling in two and three dimensions: spontaneous channelling of porous fluids owing to decompaction weakening

“…Decompaction weakening is the principal driver owing to flow channeling in the reported results, suggested to be a viable mechanism leading to the formation of fluid escape pipes (Böttner et al 2019). Porosity waves with a symmetric bulk rheology would fail to produce the vertically elongated tubular features strikingly well reproducing the subsurface flow features mapped and interpreted as fluid escape pipes (e.g.…”

“…Cathles et al 2010;Cartwright & Santamarina 2015, and references therein); however, they mainly involve capillary forces and fracture mechanics that lead to a potential blowout scenario. An alternative mechanism suggests that these subsurface channel-like flow features are the result of localized creep of the porous matrix coupled to non-linear Darcian flow (Räss et al 2018;Böttner et al 2019). The solitary waves limit of hydromechanically coupled model provides in this case the underlying physical mechanism leading to the occurrence of flow instability in viscously deforming porous media (Räss et al 2018).…”

Complex Eyed Pockmarks and Submarine Groundwater Discharge Revealed by Acoustic Data and Sediment Cores in Eckernförde Bay, SW Baltic Sea

“…Seeping fluids frequently result in authigenic carbonate precipitation due to the microbially driven anaerobic oxidation of methane once methane enters the SMTZ (e.g., Boetius et al, 2000; Ritger et al, 1987). These carbonates are often associated with high‐amplitude, positive polarity reflections and a high backscatter signal in pockmarks (Böttner et al, 2019; Dandapath et al, 2010; Ho et al, 2012; Judd & Hovland, 2009). Similarly, the suspension of fine‐grained material due to fluid escape, leaving the coarser‐grained material behind as well as enhancing biological activity due to seeping fluids (skeleton remains, dead and living shells, etc.…”

CO2 Release From Pockmarks on the Chatham Rise‐Bounty Trough at the Glacial Termination

“…Pockmarks are found on other continental margins as well (Hovland & Judd, ). At some of these sites, modern pore water geochemistry has been analyzed and found to contain large amounts of methane (Andreassen et al, ; Böttner et al, ; Feldens et al, ) implying that destabilization of methane clathrates at depth resulted in a release of methane that caused the deformation. But at other sites, it is less clear what the source of subsurface gas is because direct geochemical measurements from the pockmarks themselves are not available (e.g., de Mahiques et al, ; Somoza et al, ).…”