Hole 994C was drilled at 2799 meters below sea level (mbsl) to a depth of 703 meters below seafloor (mbsf). The sediment sequence is part of a large drift deposit created by deposition from the thermohaline induced Western Boundary Undercurrent (Paull, Matsumoto, Wallace, et al., 1996). Sediments at Hole 994C were deposited from the late Miocene to the Holocene (6 to 0 Ma), and are dominantly composed of a mixture of terrigenous silicates and biogenic carbonates deposited by hemipelagic accumulation. Terrigenous silicates are predominantly clay with lesser amounts of quartz and feldspar; biogenic carbonates are predominantly nannofossils (coccoliths) but include foraminifers. All nannofossil zones and subzones are present without obvious hiatuses (Paull, Matsumoto, Wallace, et al., 1996). Sedimentation rates increase with depth, reaching about 400 m/m.y. at the bottom of the hole (Paull, Matsumoto, Wallace, et al., 1996). Shipboard examination of smear slides indicate that clays, calcite, and quartz are the dominant mineral components; feldspars, dolomite, and pyrite are minor components. Siliceous microfossils are present primarily as diatoms (Pl. 1A, B), although there are some sponge spicules and radiolarians.
The upper zone of gas hydrate in Hole 994C exists between about 185 and 260 mbsf (boundaries are not well defined), whereas the lower zone exists between about 380 and 450 mbsf (Paull, Matsumoto, Wallace, et al., 1996; Dickens et al., 1997). Each zone is highly heterogeneous and contains hydrate nodules, possible fracture fillings, and thin layers embedded in clay with sharp hydrate to clay boundaries (Paull, Matsumoto, Wallace et al., 1996).
One hundred and seventy-four sediment samples were taken from Hole 994C for this investigation (Table 1). Samples were taken at intervals ranging from 20 to 800 cm between 102 and 332 mbsf. Sampling was somewhat random because it was dictated by sample restrictions and core recovery; for example, the uneven spacing between samples is because core recovery with the extended core barrel (XCB) and depth intervals with relatively high gas concentrations was often less than 60% (Paull, Matsumoto, Wallace, et al., 1996). Forty-nine of the samples were from within the upper hydrate zone and 125 samples were from above and below the upper hydrate zone. Samples taken were standard shipboard 10 cm3 plugs of sediment. All samples analyzed were dark gray (5YR 4/1) to dark greenish gray (5GY 4/1 to 5Y 4/1) in color (Paull, Matsumoto, Wallace, et al., 1996).