The combined results of studies carried out during and in conjunction with Expedition 311 reveal a very complex sedimentological, geochemical, and geophysical regime that controls the formation and distribution of gas hydrate on the northern Cascadia margin. Results from Expedition 311 have significantly augmented our understanding of the geologic controls on the occurrence of gas hydrate. The main conclusions that can be drawn from the studies associated with Expedition 311 are as follows:
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Methane within sediment and recovered gas hydrate along the expedition transect were produced primarily by microbial CO2 reduction and secondarily by acetate fermentation; all hydrocarbon sources are microbial, and no thermogenic gases were detected.
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Isotope fractionation during microbial generation of methane results in a progressive isotopic enrichment of the carbon in the methane, CO2, and DIC with increasing sediment depth and distance from the deformation front.
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Pore fluid composition (e.g., chlorinity) demonstrates a significant component of pervasive upward fluid migration across the accretionary complex.
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Fluid expulsion is nonuniform across the margin, with expulsion rates being highest ~15 km away from the deformation front. The systematic change in expulsion rates is the main cause of the apparent separation into saltier and fresher pore fluids at depth because it controls the amount of upward-migrating fresher pore fluids that mix with those pore waters affected by salt-generating diagenetic processes.
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Diffuse upward pore fluid migration is likely overprinted by flow through conduits such as fractures, faults, or permeable strata.
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Evidence for a deep-migrated source of methane was observed in shallow (<50 mbsf) gas hydrate accumulations at the cold vent Site U1328 (Bullseye vent), where near-vertical fracture systems delivered methane from a deep source to the surface.
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Some component of regional dissolved methane advection through the GHSZ is required to explain the presence of gas hydrate occurrence within the accretionary prism and not within the abyssal plain sediments. This advection, in combination with methane production and sedimentation rates, defines the overall thickness of the gas hydrate occurrence zone.
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Gas hydrate occurs preferentially in coarser grained sediments (sandy/silty turbidites) and is mainly formed from methane produced in situ.
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Gas hydrate concentrations in the pore space of the sediments are low (<5%) when averaged over the entire gas hydrate occurrence zone. However, locally, gas hydrate concentrations within sand layers can be as high as 50% of the pore space.
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Gas hydrate distribution is highly heterogeneous across the margin at all scales between each site visited as well at each site between adjacent boreholes, making remote gas hydrate detection and quantification challenging.
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Gas hydrate occurs anywhere within the gas hydrate stability zone where favorable conditions occur (sufficient gas concentration above local solubility and the presence of coarse-grained sediments) and not preferably right above the BSR, as previously predicted (Hyndman and Davis, 1992; Hyndman et al., 2001).
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The top of gas hydrate occurrence along the expedition transect (and the overall thickness of gas hydrate occurrence) deepens with distance from the deformation front, likely as a result of a progressive decrease in pore fluid advection and/or decreasing organic matter quality.
