Gas hydrate is an icelike substance that contains methane or other low molecular weight gases in a lattice of water molecules. Methane hydrates are stable under the temperature and pressure conditions generally found in the Arctic and near the seafloor at water depths >500 m. They are quite common beneath the slope of both active and passive continental margins, where methane originates from the decomposition of organic matter. International interest in this material 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 methane budget and may therefore be a potential energy resource for the future (see review by Kvenvolden and Lorenson, 2001). 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 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:

1. How are hydrates and underlying free gas distributed vertically and horizontally in the sediment?
2. What controls their distribution (i.e., the sources of gas, fluid migration, and the physical chemistry of hydrate formation)?
3. What are the effects of this distribution on the mechanical properties of the seafloor?
4. How can hydrate and gas distribution be mapped regionally using remote-sensing geophysical techniques?
5. How does hydrate respond to changes in pressure and temperature resulting from tectonic and oceanographic perturbation?
6. How can we use the isotopic record as a proxy for past tectonic and climate changes?

In addition, the question of whether the hydrate system harbors a rich biosphere is of broad interest, particularly given the recent recognition that the biosphere extends deeper into the earth and that it has a larger impact on the geologic record than previously thought.

The Ocean Drilling Program (ODP) has a critical role to play in addressing the above questions because it provides the only means of directly sampling gas hydrates and underlying sediments containing free gas. Hydrates have been sampled during several ODP cruises. Leg 164 to the Blake Ridge was the first (and, to date, only) leg focused primarily on understanding the dynamics of hydrate formation. Hydrates were secondary objectives of ODP cruises to the Chile (Leg 141) and Oregon (Leg 146) accretionary complexes, which were focused on understanding the mechanics and hydrology of accretionary wedges. Results from these expeditions have highlighted the need to (1) dedicate a leg to exploring gas hydrate formation in active accretionary wedges and (2) develop new tools and techniques to better estimate in situ hydrate and gas concentrations. Accurate quantification of hydrate and gas concentrations has been elusive so far, due to hydrate dissociation and gas loss during core retrieval, unless core is retrieved at in situ pressure (Paull and Ussler, 2001). Furthermore, commonly used geochemical proxies for estimating the in situ hydrate concentration of sediments are not adequate because the initial composition of pore waters is not known and can be very variable. Consequently, a major focus of Leg 204 will be to acquire samples under pressure using the ODP pressure core system (PCS) and the recently developed hydrate autoclave coring equipment (HYACE, HYACINTH) (http://www.tu-berlin.de/fb10/MAT/hyace.html), which includes a laboratory transfer chamber for maintaining pressure while making physical properties measurements (http://www.geotek.co.uk/hyace.html).

Drilling results to date also suggest that there are other factors controlling the depth to which gas hydrates are stable in addition to temperature and pressure (e.g., Ruppel, 1997) and that hydrate may persist in a metastable state outside the stability field (Guerrin et al., 1999; Buffett and Zatsepina, 1999). To address these outstanding issues, we will frequently deploy tools to measure in situ temperature and pressure, especially in zones where logging-while-drilling (LWD) data indicate rapid changes in the physical properties of the sediments.

Because of the recognition that estimation of hydrate and free gas concentrations using geophysical remote sensing techniques is more complicated than previously thought (e.g., MacKay et al., 1994; Holbrook et al., 1996), we have also incorporated an extensive suite of downhole and two-ship seismic experiments into our drilling plan.