Joint Oceanographic Institutions, United States

Sediments from above the bottom-simulating reflector in eight holes drilled during Ocean Drilling Program Leg 204 were analyzed to understand how sediment grain-size compositions correlate with the presence of gas hydrates on Hydrate Ridge. The results showed that all samples studied fall into the 1- to 148-mu m size range. Silt is the dominant component (60%-75%), with <35% clay and <5% sand. The presence of gas hydrate generally correlates to sediment layers with >0.5%- 5% sand; however, the presence of gas hydrate is also related to layers containing <0.5% sand but with increasing silt. Abundant statistical data were obtained from correlations between 71 grain size intervals and the infrared thermal anomalies in Leg 204 holes. Values of Pearson's correlation coefficients are low (<0.478), probably indicating that variation in grain size is of certain importance for controlling the presence of gas hydrates. The statistical results showed the trend that the coarser the grain size, the more abundant the presence of gas hydrate, and vice versa, within correlated grain-size intervals at most Leg 204 sites. The exception to this is at Sites 1249 and 1250, where the presence of gas hydrate is significantly controlled by rapid supply of gas and fluid flow. Statistically, at most Leg 204 sites the presence of gas hydrate is related to a grain-size interval of 10-148 mu m. Three groups of Leg 204 sites were recognized. At the sites on the southern summit area, presence of gas hydrate correlates with coarse silt to fine sand, whereas at the two sites in the slope basin area, gas hydrate is related to the very fine silt fraction. This implies that there are more lithologic control factors (e.g., structural and sedimentary histories) controlling the presence of gas hydrate in the study area.