Gas hydrate is a crystalline substance in which many of the cavities in the H2O lattice contain low molecular weight gases (mostly methane) under conditions of moderate to high pressure and low temperature (Sloan, 1990). The total amount of methane in gas hydrate is thought to be very large, perhaps as much as 1019 g of methane carbon (Kvenvolden, 1993). Gas hydrate is a potentially important fossil fuel resource, can affect global climate, and plays a role in submarine geohazards (Kvenvolden, 1988a, 1988b).
Currently, the presence of gas hydrate in deep-sea sediments can be detected mainly based on the presence of a bottom-simulating reflector (BSR) on seismic profiles, which corresponds to the base of the gas hydrate stability zone (GHSZ). In addition, the bulk and isotopic composition of pore fluids and authigenic carbonates are useful evidence for the presence of gas hydrate in sediments. In this chapter, we describe the chemical and isotopic composition of interstitial water, headspace gas, and authigenic siderite recovered from Site 1146 of Ocean Drilling Program (ODP) Leg 184 and discuss the possible relationships between these geochemical anomalies and the presence of gas hydrate.