Organic carbon content, bacterial methanogenesis, and accumulation processes of gas hydrates in marine sediments.

Gas hydrate volume% filled in pore space of sediments by in situ bacterial methane production is calculated as a function of total organic carbon (TOC) contents in sediments, assuming that all the excess amount of methane beyond the solubility in pore water can form the hydrate under ordinary conditions of outer continental margins. The results suggest that at least 0.5% TOC is required for the hydrate formation. Average volume of gas hydrates filled in pore space of hydrate-bearing sediments is estimated to be about 5∼6% at a site on the Blake Ridge (Matsumoto et al., 1996). TOC required to fill the 5% pore volume as gas hydrate is calculated to be about 2% in the case of water depth of 3000 m and utilizable organic carbon for methanogenesis of 10%. This TOC value is comparable to the measured TOC values in the sediments at the site (0.8 to 2.3%, average 1.4%; Shipboard Scientific Party, 1996b). Therefore, hydrate formation by in situ bacterial methanogenesis can roughly explain the average amount of gas hydrates in the sediments. However, for the formation of locally concentrated massive gas hydrates, some accumulation processes are required. Accumulation of gas hydrates near the base of gas hydrate stability zone (BGHS) is possible by the recycling of methane and migration of methane from depths below the BGHS.