Gas hydrates form under conditions of low temperature, high pressure, and an adequate supply of gas (usually methane). Global methane hydrate deposits in sediments are estimated to contain ~104 Gt of carbon, approximately twice that estimated for all other global fossil fuel deposits (Kvenvolden, 1988). In addition, the solid hydrate strata may act as a seal, resulting in the accumulation of considerable volumes of free gas below the bottom-simulating reflector (BSR). Methane is a potentially significant energy source for bacteria; thus, gas hydrates may provide a globally significant energy source for deep sediment bacteria in marine environments.
Previous investigation into microbial activity in gas hydrate-bearing sediments in the Cascadia Margin (Ocean Drilling Program [ODP] Leg 146) demonstrated that both bacterial populations and their activity increased significantly in association with the presence of a discrete zone of gas hydrate, such that within the hydrate zone deep bacterial activity was greater than at the sediment surface (Cragg et al., 1995). Rates of anaerobic methane oxidation increased in a discrete hydrate zone (Site 889/890) to approximately nine times the rate at other depths, coincident with an order of magnitude increase in the total bacterial population. However, rates of bacterial methanogenesis from H2:CO2 were five orders of magnitude lower than oxidation rates (Cragg et al., 1996), suggesting either a significant local flow of methane into the sediment or a source of methane in addition to H2:CO2 methanogenesis.
ODP Leg 164 was dedicated to improving knowledge of the in situ characteristics and amounts of natural gas hydrates in sediments, via a drilling regime at three sites (Sites 994, 995, and 997) on the Blake Ridge to 750 m depths, which included drilling through the hydrate-stability field and the BSR and into the sediments below. In Blake Ridge sediments, the bulk (98%+) of gas in the hydrates is methane, and isotopic and compositional data indicate a microbial origin (Brooks et al., 1983; Galimov and Kvenvolden, 1983). Thus, Leg 164 offered an opportunity (1) to demonstrate whether the results from Leg 146 were representative of other gas hydrate sites, (2) to investigate further the importance of deep bacterial processes for driving geochemical changes, and (3) to investigate the origin of methane in the gas hydrate deposits.