Abstract:
Geophysical and geological studies of gas hydrate on the N. Cascadia continental slope off Vancouver Island include numerous seismic surveys, heat flow, seafloor imaging, sediment coring, electrical surveys, seafloor compliance, geological studies, and the core sampling and downhole measurements of ODP Site 889/890. Seismic velocity and electrical resistivity data, along with downhole geophysical and geochemical data indicate regional hydrate of 20-30% of the pore space in a 100 m interval above the BSR. Underlying free gas concentrations are less than 1% in a thin layer. Regional upward advection in the sedimentary prism is suggested by landward velocity changes, by heat flow data and by core evidence for seafloor carbonate concentrations. Recent work has concentrated on chimney and vent structures that are well resolved in DTAGS near-seafloor seismic lines and mapped by a 3-D high resolution seismic survey. The structures are up to several 100 m in diameter and commonly rise from below the BSR. They blank most of the seismic stratigraphy. Some are elongate, probably associated with small faults. Strong seismic reflectivity below the BSR near the chimneys suggests free gas feeding upward. The largest structure has a strong seafloor expression. Recent piston coring of this structure recovered massive and disseminated hydrate a few meters below the seafloor. Heat flow probe measurements show only a small difference over the vent structure compared to the adjacent area suggesting there is no present strong upward fluid or gas flow. Expulsion may be episodic. However, there is evidence of temperature anomalies in the overlying ocean from CTD profiles. Analyses of the hydrate, sediments and pore fluids are in progress, and a ROPOS remotely operated vehicle survey and sampling of the vents is scheduled for August, 2000.