Ocean Drilling Program, United States

Gas hydrate on the crest of the Blake Ridge is concentrated over two depth zones: between 185 and 260 mbsf, and between 380 and 450 mbsf. Although the abundance of hydrate in the lower zone may be explained by methane cycling across the phase boundary between free gas bubbles and gas hydrate, the upper zone lacks a satisfactory explanation. Chemical analyses of sediment samples from Hole 994C (31 degrees 47.139'N, 75 degrees 32.753'W) were performed to determine if the relatively high hydrate accumulation between 185 and 260 mbsf coincides with an observable change in sediment composition and microporosity. Our analyses indicate a distinct change in lithology across the upper hydrate zone: the carbonate content decreases from about 25% to about 8% with a corresponding increase in siliceous microfossils and bulk porosity. An increase in the abundance of siliceous microfossils increases the size and roundness of pore spaces. Large and round pores should provide nucleation sites for gas hydrate that are uninhibited by capillary forces between grains. Upward advecting fluids that are supersaturated with methane may deposit gas hydrate as they pass through the diatom-rich depth interval. Results presented here are consistent with the hypothesis that gas hydrate distribution is influenced by sediment lithology and microporosity.