Texas A&M University, United States

A transect of four sites (U1325, U1326, U1327, and U1329) across the northern Cascadia margin was established during Integrated Ocean Drilling Program Expedition 311 to study the occurrences and formation of gas hydrate in accretionary complexes. In addition to the transect sites, a fifth site (U1328) was established at a cold vent with active fluid and gas flow. The four transect sites represent different stages in the evolution of gas hydrate across the margin from the earliest occurrence on the westernmost first accreted ridge (Site U1326) to its final stage at the eastward limit of gas hydrate occurrence on the margin in shallower water (Site U1329). Logging while drilling and measurement while drilling carried out prior to coring provided a set of measurements that guided subsequent coring and special tool deployments at all five sites. Additional wireline logging at each site and two vertical seismic profiles at Sites U1327 and U1328 were completed. A total of 1217.76 m of sediment core was recovered using the advanced piston corer and extended core barrel systems. Standard coring was interspersed with 24 (16 successful) pressure core sampler runs for onboard degassing experiments and 19 HYACINTH deployments, of which four were stored under in situ pressure for subsequent shore-based studies. Indirect evidence of the presence of gas hydrate included increased electrical resistivities and P-wave velocities on downhole logs and low-salinity interstitial water anomalies, numerous infrared cold spots, and decreases in void gas C1/C2 ratios, as well as gas hydrate-related sedimentological moussy/soupy textures in recovered cores. Gas hydrate was also observed directly in the recovered cores, and >31 gas hydrate samples were preserved in liquid nitrogen for shore-based studies. The combined observations show that gas hydrate at these sites mainly occurs within coarser-grained turbidite sands and silts. The occurrence of gas hydrate appears to be controlled by several key factors, and the concentration of gas hydrate changes significantly as those factors vary in the sediments along the margin. The key controlling factors are Local methane solubility linked with pore water salinity, Fluid/gas advection rates, and Availability of suitable host material (coarse-grained sediments). In the previous model for gas hydrate formation in an accretionary margin, the highest concentrations of gas hydrate were expected to occur near the base of the gas hydrate stability zone (GHSZ) above the bottom-simulating reflector (BSR), with concentrations gradually decreasing upward as a result of pervasive fluid advection from tectonically driven fluid expulsion. However, the results of Expedition 311 show that this model is too simple and that there are additional controlling factors. Although evidence for widespread gas hydrate-related BSRs was observed in the data, by far the largest concentrations of gas hydrate were observed at the top of the gas hydrate occurrence zone, at a point where the amount of methane in the pore fluid exceeded the local methane solubility threshold. This condition was most evident at Sites U1326 and U1327, where gas hydrate was observed in sections several tens of meters thick at a shallow depth of approximately 100 meters below seafloor (mbsf); concentrations exceed 80% of the pore volume.