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Geochemical observations on Hydrate Ridge, Cascadia Margin during R/V BROWN-ROPOS cruise : August 1998 Public Deposited

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  • A massive release of methane on the Cascadia Hydrate Ridge was documented during a ROPOS program in August 1998, consistent with previously reported observations in 1996. An extensive survey of the seafloor revealed that the seeps lie within a narrow band trending 109 degrees. This feature parallels larger mounds imaged by Seabeam as well as larger structures of the accretionary prism such as the Daisy bank. The area of intense bubbling is characterized by extensive bacterial mats. Large clam fields were observed ten's of meters away from the gas seeps. A third province with carbonate blocks but no clams or bacterial mats was mapped approximately 200 meters away from the seeps. To constrain fluid flow through the sediments, we deployed 8 osomotic flow meters. The areas of gas discharge are discrete and highly focussed within conduits with an approximate cross-sectional area of 5 cm2. We estimate the gas flow rate to be on the order of 5 liters/minute. While the subsurface plumbing is unknown, the high flow rate of the sampled gas seep suggests a very short transit time from the gas source (presumably the base of the BSR at 70 mbsf) to the sea floor. The Rn/CH4 ratio in gas samples collected from the gas vents is very high, approximately 50 dpm/liter (stp) CH4. Using these values, we estimate that the time required for the fluids to transit 70 m is approximately 1 hour. To further constrain the nature of the discharging fluids, we will analyze samples for their elemental and isotopic composition. Methane hydrate should be stable at the temperature and pressure conditions at the seafloor on Hydrate Ridge. Indeed, solid hydrate was observed to form within the gas samplers as well as on the camera itself, supporting the conclusion that methane is rapidly transported to the seafloor from beneath the BSR within discrete conduits, most likely separated from significant amounts of pore water. When discharged at the seafloor, some of the methane precipitate as hydrate and some continues to rise within the water column. Bubbles were observed with the ROV up to 50 meters above the seafloor. This methane generates a plume in the water column, which was first documented during the 1996 GEOMAR survey. The most pronounced methane plumes observed during 1998 occur nearest to the active discharge sites, where methane concentrations up to 800 nmol/l were recorded.
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