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

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  • Journal Title:
    Journal Of Geophysical Research: Solid Earth
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    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. Plain Language Summary Recognizing individual components of the global carbon cycle is critical to understand both past and present future climates. The sediments that accumulate on continental margins are an underappreciated part of this cycle, containing 40% of the oceanic reservoir of free carbon. Methane plumes issuing from the seafloor are an unquantified leakage of carbon from this giant reservoir. The current inventory of methane plumes on the northern Cascadia margin now approaches 500 emission sites, or an average of one a bubble emission site every 500m. The majority of these methane emission sites are grouped within a north-south band that follows the continental shelf edge and uppermost margin. The geological process for producing such a narrow band of emissions appears related to the megathrust earthquakes that occur episodically on the Cascadia Subduction Zone that underlies the Washington coast. During these earthquakes, the Juan de Fuca plate is thrust eastward beneath the overlying North American plate. The overlying plate that includes the Washington continental shelf extends rapidly westward during the fault motion. This rapid westward extension of the overlying plate produces faults and diapirs beneath the outer continental shelf, which become pathways for the methane fluid and gas from the margin sediments.
  • Source:
    Journal of Geophysical Research-Solid Earth, 124(3), 2829-2843.
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