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doi:10.2204/iodp.proc.311.213.2010

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 311 was conducted on the northern Cascadia margin (Fig. F1) during September and October 2005 (see the "Expedition 311 summary" chapter). Expedition 311 was the third deep drilling expedition on the Cascadia margin with a major gas hydrate research objective. The first drilling on the Cascadia margin was conducted in 1992, when Ocean Drilling Program (ODP) Leg 146 (Westbrook, Carson, Musgrave, et al., 1994) established three drill sites on the northern Cascadia margin (Sites 888, 889, and 890) as well as Site 892 on Northern Hydrate Ridge, offshore of Oregon (southern Cascadia margin). In 2002, ODP Leg 204 (Tréhu, Bohrmann, Rack, Torres, et al., 2003) followed with a dedicated study of Southern Hydrate Ridge (SHR). During this expedition, nine closely spaced drill sites were established in a short transect across the ridge to study the summit of SHR, where seafloor gas hydrate had been discovered (e.g., Suess et al., 1999).

Expedition 311 was designed to study gas hydrate occurrences along a transect across the northern Cascadia margin, and thus the expedition augmented the geographical range of previous gas hydrate studies on the Cascadia margin. A transect of four sites (U1325, U1326, U1327, and U1329) was established to characterize the different geologic and tectonic settings of gas hydrate occurrence across the margin, spanning a distance of 32 km from Site U1326 near the deformation front to the landward Site U1329 at the eastern limit of the previously inferred gas hydrate occurrence. These drill sites roughly follow multichannel seismic (MCS) Line 89-08, which was acquired in 1989 as part of the presite survey for Leg 146 (Fig. F2). Projecting the previously established deep basin Site 888 onto the end of the new transect extended the profile to >40 km. In addition to the transect sites, a fifth site (U1328) was established at a cold vent with active fluid and gas expulsion. This provided an opportunity to compare regional pervasive fluid-flow regimes to a site of focused fluid flow.

The main goal of Expedition 311 was to test geologic gas hydrate formation models and associated model parameters in subduction zone accretionary prisms, especially those that account for the formation of concentrated gas hydrate occurrences driven by upward fluid and methane transport. The detailed objectives of the expedition were to (1) study the formation of natural gas hydrate in marine sediments; (2) determine the mechanisms that control the nature, magnitude, and distribution of the gas hydrate occurrence; (3) find the pathways of upward fluid migration required to form large concentrations of gas hydrate; (4) examine the effect of gas hydrate on the physical properties of the host sediment; and (5) investigate the microbiology and geochemistry associated with the gas hydrate occurrence.

Prior to Expedition 311, a general model for gas hydrate formation by removal of methane from upwardly expelled fluids was generally accepted for the Cascadia margin (Hyndman and Davis, 1992). In this model, dissolved microbial methane, inferred to be produced over a thick sediment section, migrates vertically and forms gas hydrate when it enters the stability field. Gas hydrate concentration is predicted to be greatest just above the base of gas hydrate stability, seismically defined by the bottom-simulating reflector (BSR). A model was also proposed for how free gas and the resulting BSR are formed as the base of gas hydrate stability moves upward because of post-Pleistocene seafloor warming, uplift, and sediment deposition (e.g., Paull and Ussler, 1997; von Huene and Pecher, 1998). In addition, physical and mathematical models have been developed for the formation of gas hydrate from upward methane advection and diffusion (e.g., Xu and Ruppel, 1999).

Evidence that focused fluid/gas flow and associated gas hydrate formation contrasts with the diffuse, regional fluid-flow regime has been identified on the northern Cascadia margin at various locations. The most studied site, referred to as Bullseye vent, is an active cold vent field associated with near-surface gas hydrate occurrences and was drilled as Site U1328 during Expedition 311. These vents were shown to be associated with fault-related conduits for focused fluid and/or gas migration and with massive gas hydrate formation within the fault zone (Riedel et al., 2002, 2006).

In this synthesis, we first report integrated findings on the distribution and magnitude (concentration, S_h) of gas hydrate, as determined from logging data and core-derived pore water freshening (Chen et al.; Chen, 2006; Malinverno et al., 2008; Ellis et al., 2008). We then show the results of sedimentological and geochemical core studies as well as infrared (IR) imaging of the recovered core, demonstrating the lithological control on gas hydrate occurrences on the northern Cascadia margin (Torres et al., 2008; Hashimoto and Minamizawa; Wang, 2006). These results are the basis for discussion of the underlying fluid-flow model along the Expedition 311 transect. We combine results from downhole logging data and detailed pore fluid and gas chemistry data (Torres and Kastner; Pohlman et al., 2009; Lu et al., 2008; Kim and Lee) to develop an updated model that can explain the occurrences and distribution of gas hydrates along the Cascadia margin.

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