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Seafloor Methane Seeps at the Edge of Hydrate Stability

In June 2019, USGS scientists led a 22-day deep-sea research expedition aboard the R/V Falkor to examine methane seep dynamics and processes along the Cascadia Margin offshore of Washington and Oregon.

Map of the Cascadia subduction zone
The Cascadia subduction zone stretches along the Pacific coastline from offshore Vancouver Island in Canada to offshore northern California. Red circles denote known locations of seafloor methane seeps. White and yellow crosses are locations of ROV dives to explore these methane seeps, with yellow crosses corresponding to sites visited by the R/V Falkor and ROV SuBastian in summer 2019.  The orange line denotes the 1000 m depth contour.

This expedition, the Cascadia Margin Methane Research Action (CAMERA), was led by the USGS and conducted in partnership with Schmidt Ocean Institute, British Geological Survey, GEOMAR, the University of North Carolina-Chapel Hill (UNC), and NOAA-Pacific Marine Environmental Laboratory (PMEL). Through the advanced technology of telepresence, scientists shipboard and onshore participated in exploration of the seafloor and communicated with the public through outreach activities including live streaming and interactive interviews. Read the Schmidt Ocean Institute blogs for more information.

Using the remotely operated vehicle (ROV) SuBastian, the scientists directed 24 submersible dives at sites including Astoria Canyon, Grays Canyon, Heceta, and Coquille. The dives studied the physics, chemistry, ecology, and geology of seafloor methane seeps landward of the deformation front where the Juan de Fuca tectonic plate subducts beneath the North American plate. Advanced technologies were deployed to measure methane concentrations at the seafloor (METS sensors) and aerobic methane oxidation rates (UNC mini landers) and to image methane bubble emissions and estimate the flux of methane from the seafloor (Bubble Box) and GasQuant landers (provided by GEOMAR). In addition, the ROV acquired images of the seafloor and water column at the seep sites and collected samples of water, the benthos (sediments and megafaunal invertebrates), and carbonate rocks that were formed in place as a result of microbial activity.

The expedition amassed a huge dataset that included physical measurements of methane bubble emission rates and sizes; chemical measurements of dissolved gas concentrations in the water column, sediment pore water compositions, and isotopic composition of biological and geological samples; and biogeochemical measurements to constrain the rate of methane oxidation at the seafloor. In addition, the expedition conducted seafloor mapping, video surveys, and quantitative sampling to characterize seafloor habitats and the associated benthic communities near seeps. Discrete sediment samples enable the chemistry of fluids and sediment porefluids at seep locations to be characterized, and the carbonate rock samples allow scientists to determine the timing of seepage events. Taken together, these samples and analysis are providing valuable information about seep dynamics and processes at different locations and on several timescales. See the data on Schmidt Ocean Institute’s web site.

This expedition made some notable advances for deep-sea research. See the YouTube summary.

The scientists established a mini-observatory at a single seep site close to the landward limit of gas hydrate stability on the Cascadia margin upper slope and conducted detailed scientific investigations of several seep sites along depth transects (from shallower than the updip limit of gas hydrate stability to deep water and well within the hydrate stability zone). The expedition provided the first test of the UNC mini-landers capable of conducting in-situ aerobic methane oxidation experiments using advanced chemical sensors; the most rigorous test to date of GEOMAR’s GasQuant II lander to track the spatiotemporal pattern of methane emissions; new samples of methane-derived authigenic carbonates for extensive geochronologic studies focused on determining the timing of methane emissions; the first quantitative collections of animal communities at these seeps; and acquisition of several gas samples to expand PMEL studies that have already provided provocative evidence for the involvement of deep subduction zone fluids in gas emissions on this margin.

Photograph of 2 UNC mini landers surrounded by Sablefish
 Two UNC mini landers, surrounded by Sablefish (Anoplopoma fimbria) and pink sea urchins, incubate seawater in situ to enable calculations of methane oxidation rates.
Photograph of SuBastian’s manipulator jaw stabilizes GEOMAR’s glowing bubble box
ROV SuBastian’s manipulator jaw stabilizes GEOMAR’s glowing bubble box that is capturing high-resolution images of methane bubbles rising from the seafloor.

Funding for this research was derived from several sources, exemplifying the power of partnering to conduct offshore research. The shiptime and ROV support was awarded to Carolyn Ruppel, Ph.D., USGS WHCMSC, by Schmidt Ocean Institute during a national proposal competition in 2017. Dr. Ruppel invited several partner institutions (Federal and academic) to execute the expedition, and each of these partners provided science support in the form of labor, supplies, and research gear. In addition, the U.S. Department of Energy’s National Methane Hydrates R&D Program provided support for some aspects of the science through its interagency agreement with the USGS Gas Hydrates Project. That project, along with the USGS Environments Program, also supported some USGS components of the research and contributes to the EXpanding Pacific Research and Exploration of Submerged Systems (EXPRESS) campaign, a multiyear collaborative effort to inform offshore activities and natural resource-management decisions off the west coast.

Photograph of Red plumes burst out from the tops of these chemosynthetic tubeworms
Red plumes burst out from the tops of these chemosynthetic tubeworms, capturing hydrogen sulfide and oxygen from the surrounding water to feed their bacterial endosymbionts. The tubeworm tubes provide a habitat for several benthic animals, including the pale pink branching octocorals seen here

Multidisciplinary expeditions like CAMERA link observations of seafloor habitats and benthic communities with studies of gas fluxes, the timing of seep emissions, the composition of emitted gas streams, and the rates at which microbes consume methane in the water column. Such research provides information about the interdependency among physical, chemical, and biological systems at methane seeps and how methane seeps differ in space and time along U.S. continental margins. Ultimately, this information can be used to predict where these systems occur elsewhere in the Pacific, which will in turn inform management and conservation of these sensitive environments and links between fluid expulsion and geohazards.

Watch the video.

Learn more on the Schmidt Ocean Institute web site!



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