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Cascadia Margin

Methane seeps along the Cascadia margin
Yellow and red dots represent new methane seeps discovered along the Cascadia margin on the Nautilus expedition. Blue dots are ROV dive sites. Click image for larger map.

An unknown amount of methane is fluxing through continental margins, and up to ~5000 gigatons of carbon are stored in icy methane hydrate deposits within their sediment. Sources of this methane include biogenic production, disassociation of the methane hydrate layers, and deep thermogenic sources. Methane is one of a group of carbon species that have been recognized as important greenhouse gases and thus the methane geochemical cycle is important to climate modeling. Only in the past several years has sonar technology been available to rapidly locate and map these submarine methane sources over large areas of the seafloor. The Cascadia margin is of particular interest because the methane is stored within the accretionary wedge of a major subduction zone. It represents a geologic end member that has a strong tectonic overprint, in contrast to the thickly sedimented passive margins represented in the U.S. Atlantic and Gulf of Mexico, which also have extensive methane seeps. There is increasing evidence for accumulation of seep-derived carbon species both from known seeps and in regional surface water along the Cascadia margin, which potentially has an environmental impact on the upper water column. A baseline characterization of methane seeps along the Cascadia margin is critical to complete to assess methane input into the water column, and is timely because the continued rise in ocean temperatures could potentially impact the rate of release of methane from the hydrate layers into the ocean and possibly the atmosphere.

 

Read more about the methane seeps and the expedition in Oceanography (2017): Water column and cold seep exploration of the Cascadia Margin.

Frozen and gas phase methane at Astoria Canyon Gas bubbles in 3D view over the seafloor at Smith River Ling cod in the methane bubbles at Nehalem Bank site.
Exposed methane hydrate (frozen methane) and methane gas bubbles found on the seafloor near Astoria Canyon. Methane gas bubble streams in 3D view over the seafloor at Smith River. Lingcod fish swimming through the methane bubbles while sampling gas with red funnel at Nehalem Bank.


Methane hydrate in the Cascadia Margin.
Frozen methane. (Click for video).

Video Collection:

EOI's YouTube collection of videos from the Cascadia Margin.

 


2016 E/V Nautilus Expedition

Methane seeps along the Cascadia margin
Methane gas released when ROV temperature probe punctures the sediment at Coquille site.

Discovery of 4x's the number of known methane seeps off the coast:

An exploration effort in June 2016 along the Cascadia margin (offshore Washington, Oregon, and N. California) was conducted on the Ocean Exploration Trust Inc. ship E/V Nautilus, with NOAA-PMEL and Oregon State University CIMRS scientists aboard. The expdition was funded by OET and the NOAA Ocean Exploration Research (OER) Program. The expedition used the latest midwater sonar technology to map out methane bubble streams and made dives with a remotely operated vehicle at selected sites to sample the methane bubbles. The results from this expedition are a game changer - more than 900 new methane sources were discovered in water depths from 105 to 2045 m, more than four times the number previously known. This large number of seeps from just this limited survey indicates that a substantial revision of the methane flux is needed and that cold seeps and the extensive carbonate hard grounds associated with them are an important ecosystem that needs to be factored into future management decisions concerning essential fish habitat. One exciting discovery was finding an exposure of methane hydrate and gas bubbles streaming out of the seafloor in Astoria canyon at a depth of 850 m. The hydrate, a mixed water/methane ice phase, is present over extensive areas of continental margins, but has only rarely been observed exposed at the seafloor along the Cascadia margin.

 

View the expedition cruise report here (pdf).