Anomalous Concentration of Methane Emissions at the Continental Shelf Edge of the Northern Cascadia Margin

Abstract

A recent compilation of methane plumes detected offshore Washington State includes 1,772 individual bubble streams issuing from 491 discrete vent sites. The majority of these plume sites form a narrow 10‐km‐wide band located shallower than 250‐m water depth, with most sites located near the 175‐m‐deep continental shelf break that tracks the head scarps of large submarine canyons. Archive multichannel seismic profiles over the Cascadia shelf and uppermost margin that were co‐located within a few hundred meters with active emission sites show that methane bubble streams arise from listric/normal faults and triangular‐shaped regions of disturbed seismic reflectors that intersect the seafloor and extend several kilometers into the subsurface. Geological processes were evaluated for producing the narrow emission site depths including nonuniform distribution of methane within the Cascadia accretionary sediment wedge and horizontal transfer of groundwater from onshore subaerial sources. A model of enhanced sediment permeability arising from a contrasting response between the inner and outer portions of the accretionary wedge deformation during a megathrust earthquake cycle appears the most likely mechanism. This faulting is generated during extension of the overriding plate during megathrust earthquake cycles, with semicontinuous permeability enhancement of the fluid pathways from excitation by contemporary incident seismic waves.

Controls on Gas Emission Distribution on the Continental Slope of the Western Black Sea

“…The fact that apparently most gas venting is seen on the shelf adds to other studies world-wide (e.g., Schmale et al, 2005;Shakhova et al, 2010;Borges et al, 2016;Ruppel and Kessler, 2017;Johnson et al, 2019) proposing that gas emission on continental shelf regions are a significant contributor of methane to the atmosphere as gas is not fully consumed or dissolved into the ocean on its path through the water column. A nearly E-W trending fault cuts the pockmark and is identified by high backscatter signal along which gas escape is seen NW of the pockmark, tightly linked to small-offset fault-branches.…”

The Aquitaine Shelf Edge (Bay of Biscay): A Primary Outlet for Microbial Methane Release

“…In light of our study and recent discoveries in particular along the U.S. Atlantic and Cascadia margins, continental shelf edges around the world may be seen as primary outlets for microbial methane releases, independently of the prevalent tectonic regime and the presence of gas hydrates (Johnson et al, 2015; Prouty et al, 2016). Microbial methane circulation might play a predominant role in the shaping and evolution of the shelf edge in interactions with offshore groundwater discharge (Hong et al, 2019; Pierre et al, 2017; Pierre, Demange, et al, 2017) and possibly but not necessary with gas hydrate dissociation (Johnson et al, 2019; Skarke et al, 2014).…”

“…Although the Aquitaine Shelf microbial methane system presents some similarities with other shallow water sites (Baraza & Ercilla, 1996;García-García et al, 2007;Johnson et al, 2019;Jones et al, 2012;Naudts et al, 2006;Prouty et al, 2016;Schattner et al, 2012;Yun et al, 1999), it represents a unique case at continental shelves with regard to (1) the nature and origin of the fluids (pure microbial methane not related to gas hydrates); (2) the presence of seeps and related MDACs over a large area (375 km 2 ) with a good state of preservation over time, attesting to the persistence of methane circulation through time; (3) and the amount of released methane at the seafloor (144 Mg/yr). The discovery of the Aquitaine Shelf microbial methane system identifies and highlights the edge of the shelf as a privileged area for microbiogeological processes.…”

“…The lack of seafloor seep‐related morphologies on bathymetric maps of many shelves (Römer et al, 2017; Yun et al, 1999) may therefore simply be an apparent absence due to data resolution. Surveys conducted along the continental margins of the U.S. Atlantic (Brothers et al, 2014; Skarke et al, 2014), Cascadia (Johnson et al, 2019; Riedel et al, 2018) and Black Sea (Naudts et al, 2006) have revealed numerous seeps and seep‐related structures at the shelf and upper slope domains.…”

A record of seafloor methane seepage across the last 150 million years

“…Natural methane leakage from the seafloor (i.e. cold seeps) is a well-known phenomenon occurring in a wide range of geologic and geodynamic settings, including deep-sea fans, convergent margins, and polar regions, with new seepage sites steadily discovered across all oceans [1][2][3] . Marine cold seepage systems are complex and highly dynamic, and can respond rapidly to external perturbations such as variations in pressure and temperature 4 .…”