Gas hydrate is an icelike compound that contains methane and/or other low molecular weight gases in a lattice of water molecules. Gas hydrates are stable under the temperature and pressure conditions generally found in the Arctic and near the seafloor at water depths >300 m. They are quite common beneath the slope of both active and passive continental margins where methane originates from the decomposition of organic matter by biogenic and/or thermogenic processes. International interest in gas hydrates has increased considerably in the past several years because of increasing recognition that the large volumes of gas stored in these structures represent a significant fraction of the global carbon budget (see review by Kvenvolden and Lorenson, 2001) and may be a potential energy resource for the future (e.g., Milkov and Sassen, 2002). Several authors have also suggested that sudden widespread dissociation of subseafloor gas hydrates in response to changing environmental conditions may have had a significant effect on past climate (e.g., Revelle, 1983; Nisbet, 1990; Paull et al., 1991; Katz et al., 1999; Dickens, 2001). These effects remain speculative, as the volume of gas stored in the global gas hydrate reservoir and its behavior during changing environmental conditions are currently poorly constrained.
In order to evaluate the economic potential of hydrates, their role as a natural hazard, and their impact on climate, we need to know the following:
These questions were the focus of Ocean Drilling Program (ODP) Leg 204, which was dedicated to understanding the biogeochemical factors controlling the distribution and concentration of gas hydrates in an accretionary margin setting. A three-dimensional (3-D) seismic site survey (Tréhu and Bangs, 2001; Tréhu et al., 2002) and logging-while-drilling (LWD) data acquired at the beginning of the leg provided "road maps" to guide coring and sampling. These data enabled us to anticipate the depths at which gas hydrates should be expected and select targets for special sampling tools. Accurate quantification of in situ gas hydrate and free gas concentrations is difficult because of hydrate dissociation and gas loss during core retrieval (Paull and Ussler, 2001). A major focus of Leg 204 was therefore to acquire samples under pressure using the ODP pressure core sampler (PCS) system and the recently developed Hydrate Autoclave Coring Equipment (HYACE) system, which includes a laboratory transfer chamber for maintaining pressure while making physical property measurements. Extensive use was made of infrared (IR) cameras immediately after core retrieval to rapidly identify potential hydrate-bearing samples and preserve them for careful study. Special attention was also given to making high-resolution measurements of the chemistry of interstitial waters (IWs), resulting in a large number of IW samples from this cruise. We also deployed tools to measure in situ temperature and pore pressure, especially in zones where LWD data indicated rapid changes in the physical properties of the sediments, and acquired downhole and two-ship seismic data.