CONCLUSIONS

Gas hydrates were recovered from sediments at Sites 994, 995, 996, and 997. Disseminated gas hydrate was not recovered, although indirect observations such as Cl^- dilution in pore water implied its presence. Gas hydrates from Site 996 were associated with active gas venting and chemosynthetic communities and typically occurred as nodules or vertical veins. Gas hydrates from Sites 994, 995, and 997 were mainly disseminated in sediment pore space of marine silt and clay or, as at Site 997 (331 mbsf), occurred as a massive hydrate presumably filling fracture or fault. Gas and water released from gas hydrate decomposition yielded 18 to 154 volumes of gas per volume of water at STP. When correcting for excess Cl^- in gas hydrate water, the volumetric ratios were from 29 to 204, as compared to a likely maximum value in nature of about 195 to 204. Gas analyses showed that the hydrocarbon gas included in the gas hydrates was mainly C₁ with minor amounts of CO₂ and C₂. C₃⁺ hydrocarbons were present, but in lower concentrations than in samples of sediment gas from sediments near where gas hydrates were found. C₂ appears to be preferentially included in the gas hydrate. CO₂ carbon isotopic composition in gas hydrate is lighter by about 15 relative to CO₂ sources in the surrounding sediment. It is unclear if this fractionation occurs in situ or is an artifact of sample processing.