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Geophysical and geochemical signatures associated with gas hydrate–related venting in the northern Cascadia margin

Michael Riedel^,1, Ivana Novosel^,2, George D. Spence^,2, Roy D. Hyndman^#,3, Ross N. Chapman^,4, Richard C. Solem⁵ and Trevor Lewis^,6

¹ Natural Resources Canada, Geological Survey of Canada–Pacific, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada
² School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, British Columbia V8W 3P6, Canada
³ Natural Resources Canada, Geological Survey of Canada–Pacific, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada
⁴ School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, British Columbia V8W 3P6, Canada
⁵ Scripps Institution of Oceanography, University of California–San Diego, La Jolla, California 92093-0212, USA
⁶ Sidney Geophysical Consultants Ltd., 1107 Maple Road, Sidney, British Columbia V8L 5P5, Canada

This paper presents a comprehensive, multidisciplinary study of cold vents associated with near-seafloor gas hydrate. Several cold vents characterized by seismic blank zones have been identified on the northern Cascadia margin near Ocean Drilling Program (ODP) Site 889/890. The most prominent vent site (Bullseye vent) has been the subject of intense geophysical and geochemical studies, including two- and three-dimensional (2D/3D) seismic imaging, heat flow measurements, piston coring with measurements of sediment physical properties and pore-fluid geochemistry, seafloor video observation, and sampling with the unmanned submersible ROPOS. The main seismically derived constraining observations are: (1) blanking increases with seismic frequency, (2) at low frequencies, layers can be traced through the zones, (3) blank zones widen with depth, (4) blank zones are underlain by a bottom simulating reflector (BSR), and (5) no velocity anomalies were detected across the vents. Constraints from piston core and thermal probe analyses are: (1) massive hydrate was recovered just below the seafloor at Bullseye vent, and (2) chemical alteration of sediments was observed by reduced magnetic susceptibility, increased thermal conductivity, and an elevated sulfate/methane interface. Additional constraints are: (1) no thermal anomaly was observed, (2) widespread carbonates and active chemosynthetic communities were found, and (3) elevated levels of methane were detected in the water column above Bullseye vent.

We present a model for the seismic blanking at Bullseye vent that honors the constraints from all observations. The cold vents represent channels or networks of filamentous fractures containing hydrate and/or free gas. Free gas can be present within the hydrate stability field only in fractures, which may be coated with hydrate that prevents the inflow of water. The overall concentration of hydrate or gas within the vent must be small, because there was no observable velocity anomaly.

Key Words: gas hydrate • cold vent • seismic 2D/3D • piston coring • geochemistry • physical properties