Expedition 311 Synthesis: scientific findings

Abstract

Integrated Ocean Drilling Program Expedition 311 was conducted to study gas hydrate occurrences and their evolution along a transect spanning the entire northern Cascadia accretionary margin. A transect of four research sites (U1325, U1326, U1327, and U1329) was established over a distance of 32 km, extending from Site U1326 near the deformation front to Site U1329 at the eastern limit of the inferred gas hydrate occurrence zone. In addition to the transect, a fifth site (U1328) was established at a cold vent setting with active fluid and gas expulsion, which provided an opportunity to compare regional pervasive fluid-flow regimes to a site of focused fluid advection. In this synthesis, a revised gas hydrate formation model is proposed based on a combination of geophysical, geochemical, and sedimentological data acquired during and after Expedition 311 and from previous studies. The main elements of this revised model are as follows:1. Fluid expulsion by tectonic compression of accreted sediments at nonuniform expulsion rates along the transect results in the evolution of variable pore water regimes across the margin. Sites closer to the deformation front are characterized by pore fluids enriched in dissolved salts at depth, where zeolite formation from ash diagenesis is dominant. In contrast, the landward portion of the margin shows a freshening of pore fluids with depth as a result of the progressive overprinting of diagenetic salt generation with freshwater generation from the smectite-to-illite transition at greater depth. 2. In situ methane produced by microbial CO 2 reduction within the gas hydrate stability zone is the prevalent gas source for gas hydrate formation. 3. Some minor methane advection from depth is required overall to explain the occurrence of gas hydrate (and the associated downhole isotopic signatures of CH 4 and CO 2 ) within the sediments of the accretionary prism and the absence of gas hydrate within the abyssal plain sediments. In contrast, methane migrating from depth is a dominant source for gas hydrate formation at the cold vent Site U1328 (Bullseye vent). 4. Gas hydrate preferentially forms in coarser grained sandy/silt turbidites, resulting in very high local gas hydrate concentrations. Typically, gas hydrate occupies <5% of the pore space throughout the gas hydrate stability zone. Higher gas hydrate saturations were observed in intervals with abundant coarse-

Bayesian inversion of marine controlled source electromagnetic data offshore Vancouver Island, Canada

“…Gas hydrate dissociation during recovery causes freshening of the pore water and low chlorinity values, as observed between 150 and 220 mbsf above the base of the GHSZ (especially in the 10 m above the assumed base of the GHSZ at 220 mbsf). However, between 5 and 20 mbsf, high chlorinity values indicate a recent and rapid formation of gas hydrates, faster than the assumed diffusion rate of 5 × 10 −6 cm 2 s −1 (Riedel et al 2006a). Rapid gas hydrate formation is possible in scenarios of high methane supply, for example, along fractures (Nimblett & Ruppel 2003).…”

“…Hydrates were discovered at 92 mbsf in one core (maximum 38 per cent), but do not seem to be laterally continuous, suggesting hydrate formation along fractures. Free gas concentrations of 58 per cent below the GHSZ were found in one pressure core, but could not be confirmed by wireline acoustic logging or by vertical seismic profiling (Riedel et al 2006a;Expedition 311 Scientists 2006a).…”

“…A geological interpretation of the survey areas is drawn from the Integrated Ocean Drilling Program (IODP) Expedition 311 (X311) by Expedition 311 Scientists (2005) and Riedel et al (2006a). The two CSEM survey areas are located on the upper slope around X311 Site U1329 (Area 1), and on the middle slope, around the Bullseye vent and Site U1328 (Area 2) as shown in Fig.…”

“…Reflection seismic data reveal turbidite sequences and erosional surfaces (Expedition 311 Scientists 2005; Scherwath et al 2006). The detailed analyses at Site U1329 [drilled to ∼220 meters below seafloor (mbsf)] reveal three main lithostratigraphic units (Riedel et al 2006a). Units 1 and 2 consist of Holocene to Pleistocene clay to silty clay to ∼140 mbsf.…”

“…For example, gas hydrates have been found to accumulate at the base of the GHSZ (possible mechanism: gas hydrate recycling) as well as in sand layers. Gas hydrates have also been found laterally variable, accumulating within fractures along major fluid conduits (Riedel et al 2006a;Haacke et al 2007;Zühlsdorff & Spiess 2004). While drilling provides very localized, fine-scaled information and seismic data are usually sensitive to impedance contrasts associated with velocity and/or density changes, electromagnetic studies allow the evaluation of bulk volumes of sediments containing gas hydrates and/or free gas (Edwards et al 2010).…”