The distribution of hydrate at Site 994, Leg 164, is highly heterogeneous. Gas hydrate in this region occurs in the sediment column between ~180-450 mbsf. However, the hydrate is mainly concentrated in two depth zones: between 185 and 260 mbsf and between 380 and 450 mbsf. The lower hydrate zone can be explained by methane cycling across the phase boundary between free gas bubbles and gas hydrate. The upper zone can now be attributed to a change in lithology, porosity, and, consequently, a change in the ease of formation of methane hydrate.
Chemical analyses of sediment samples indicate a distinct change in lithology in the upper hydrate zone. The CaCO3 content of sediment above and below the hydrate zone is ~25%, as opposed to ~8% within the hydrate zone. Siliceous microfossils increase in abundance, and there is a resulting increase in porosity. An increase in the abundance of siliceous microfossils may (1) decrease the capillary forces between grains and decrease inhibition of hydrate formation, and (2) change the sediment pore shapes and sizes and increase hydrate ease of formation as a result of the large clathrate cage size. Our results from Site 994 are consistent with recent suggestions that the precipitation of gas hydrate in a sediment matrix is related to the size of the pore space in which the hydrate crystal grows (Klausner, 1991; Handa and Stupin, 1992; Clennell et al., 1995; Claypool, 1996; Brewer et al., 1997; Ruppel, 1997). However, it must be noted that sediment permeability plays a significant role in gas hydrate development, and that variations in permeability may also lead to zones of hydrate accumulation.