A transect of four sites (U1325, U1326, U1327, and U1329) across the Northern Cascadia margin was cored during Integrated Ocean Drilling Program Expedition 311 to study gas hydrate occurrences and formation models for accretionary complexes. In addition to the transect sites, a fifth site (U1328), representing a cold vent with active fluid and gas flow, was visited.
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/measurement while drilling carried out at the start of the expedition 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, interspersed with 24 (16 successful) pressure core sampler runs for onboard degassing experiments and 19 Fugro piston corer/HYACE deployments; 4 of these pressure cores 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 hydraterelated sedimentological moussy/soupy textures in recovered cores. Gas hydrate was also observed directly in the recovered cores, and >30 gas hydrate samples were preserved in liquid nitrogen for shore-based studies. The combined observations show that gas hydrate 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 (1) local methane solubility linked with pore water salinity, (2) fluid/gas advection rates, and (3) 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 localized near the base of the gas hydrate stability zone above the bottom-simulating reflector (BSR), with concentrations gradually decreasing upward as a result of pervasive fluid advection from overall tectonically driven fluid expulsion. However, the results of Expedition 311 show that this model is too simple and that there are additional overprinting factors. Although evidence for widespread gas hydraterelated 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 exceeds the local methane solubility threshold. This was especially observed at Sites U1326 and U1327, where gas hydrate deposits several tens of meters thick occur at a shallow depth of ~100 meters below seafloor (mbsf) with concentrations locally exceeding 80% of the pore volume. Another site of very large gas hydrate concentrations was found at the cold vent Site U1328, where beds of massive gas hydrate occur within the top ~40 mbsf with concentrations exceeding 80% of the pore space as a result of focused fluid/gas migration from underneath.
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