Tracking and Quantifying Methane Bubble Plumes on the North Cascadia Margin

On the North Cascadia margin off Vancouver Island, multiple bubble plumes from seafloor vents were investigated with acoustic imaging, submersible observations, and water sampling. In the Bullseye vent region at water depths of ~1260 m, surface observations with 12 and 18 kHz echosounders imaged several large plumes, with apparent widths of up to 500 m and heights of up to 800 m. As in previous studies, the plumes are likely hydrate related, since they disappear at depths near the 500-m upper limit of gas hydrate stability. The large observed width of the plumes is partially due to the echosounder’s ~300-m beam width on the seafloor, but it is also due to the combined effect of multiple vent sites. In three submersible dives, acoustic imaging with a 675 kHz sector scanning sonar showed 10 individual bubble plumes distributed over an imaged area of 100 m by 150 m. In two dives, the plumes appeared linear in the horizontal-plane image, with lengths of 50 m or more. A prominent bubble stream was tracked from its first observation 80 m above the seafloor to the vent where it originated, as indicated by visual observations of bubbles venting from a 1-m-long crack. By recognizing that the beam width in the vertical plane was 30 degrees, the dip of the bubble stream could be determined. From the many acoustic images of this bubble stream, the 3D geometry of the plume was reconstructed. Its shallow angle of only 25 degrees to the seafloor indicated a near-seafloor current of ~0.6 knot to the southwest, assuming a bubble rise rate of 20 cm/s. This shallow-dipping plume contrasted with a nearby set of 7 near-vertical bubbles streams, observed acoustically several days later. The difference in behavior could be due to tidal-related changes in current, or it could be due to lateral current variations due to differences in seafloor topography. The acoustic images guided the placement of a NEPTUNE acoustic scanner sonar on the seafloor, connected to the Site 889 junction box. The 260 kHz multibeam sonar images the 3D volume around the instrument out to a maximum range of ~100 m. At the current rate of one 3D image per hour, it is possible to monitor time variations in vent activity. The acoustic images will also guide locations for water sampling and measurements of conductivity-temperature-depth, during a cruise in late September 2010. Measurements of methane concentration in the bubble plume will provide constraints on the magnitude of the methane gas emissions from individual vents. This will calibrate the amplitude measurements from the acoustic sonar, and so help determine the total methane flux from the vent field into the water column.