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doi:10.2204/iodp.sp.311.2005

INTRODUCTION

The primary objective of IODP Expedition 311, on the northern Cascadia margin, is to constrain geologic models for the formation of gas hydrate in subduction zone accretionary prisms. Natural gas hydrate occurs in marine continental slope and onshore Arctic permafrost environments. The Arctic occurrences can exhibit very high gas hydrate concentrations but appear to contain less total gas than marine gas hydrate occurrences. Recent studies have indicated that the largest occurrences of gas hydrate may lie in nearly horizontal layers several hundred meters beneath the seafloor of continental slopes, especially in the large subduction zone accretionary sedimentary prisms. Gas hydrate and underlying free gas produce the ubiquitous bottom simulating reflectors (BSRs) along numerous continental margins of the world. Gas hydrates do occur on passive margins, but they are less common and appear to usually contain lower concentrations.

The two marine gas hydrate areas that have received the most detailed scientific study, including previous drilling during the Ocean Drilling Program (ODP), are the Blake Ridge region off the east coast of the United States (a passive margin setting) and the Cascadia margin off the coast of Oregon and Vancouver Island (subduction zone accretionary prism). Important new information on Arctic gas hydrate occurrences have been obtained from the Mallik-1998 and Mallik-2002 drilling projects in northern Canada (Dallimore et al., 1999, 2002). If our conclusions for the occurrence and distribution of gas hydrate in nature are correct, gas hydrate formed within accretionary prisms is the most important both for the long-term energy potential of gas hydrate and for the role that natural gas hydrate plays in climate change. Within accretionary prisms, the largest amount of gas hydrate appears to occur in a very widespread layer located just above the BSR. However, also important is the focusing of a portion of the upward methane flux into localized migration conduits or channels to form concentrated near-seafloor gas hydrate accumulations. The amount of gas hydrate in the widespread layer above the BSR, compared to that in local concentrations near the seafloor, is especially important for understanding the response of marine gas hydrate to climate change. Near-seafloor gas hydrate accumulations will respond much more quickly to ocean temperature changes compared to gas hydrate several hundred meters below the seafloor. For the region of ODP Site 889/890, Taylor et al. (2002) calculated that a 30 m thick hydrate deposit lying at the base of the stability field would dissociate due to seabed warming in approximately 8000 years. However, near-seafloor hydrate could dissociate much more quickly and be much more responsive to human-induced global warming (e.g., Wood et al., 2002).

Off Vancouver Island a gas hydrate–related BSR occurs in a 30 km wide band parallel to the coast beneath much of the continental slope (Fig. F1). Gas hydrate is believed to be concentrated in a layer 50–100 m thick, just above the base of the gas hydrate stability field, which is located 200–300 m below the seafloor (mbsf). The gas hydrate concentrations, estimated from downhole logging data collected during Leg 146, may reach ~30% of pore space. The surveys and studies that have been carried out and the evidence for the presence and content of gas hydrate are summarized in two recent comprehensive review articles (Hyndman et al., 2001; Spence et al., 2000).

Models for Formation of Widespread Gas Hydrate Layer

A general model for deep-sea gas hydrate formation by removal of methane from upwardly expelled fluids was developed earlier for the Expedition 311 area (Fig. F2) (Hyndman and Davis, 1992). Mainly microbial methane, inferred to be produced over a thick sediment section, migrates vertically to form gas hydrate when it enters the stability field. The gas hydrate concentration is predicted to be greatest just above the BSR. A model has also been proposed for how free gas and the resulting BSR will be formed as the base of the gas hydrate stability moves upward due to 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 involving from upward methane advection and diffusion (e.g., Xu and Ruppel, 1999).

Testing these models and determining the appropriate model parameters requires