Gas hydrates have been recovered in gravity cores within 10 m of the seafloor in sediment of the Gulf of Mexico (Brooks et al., 1984, Brooks et al., 1989, MacDonald et al., 1994, Sassen and MacDonald, 1994, Sassen and MacDonald, 1997), the offshore portion of the Eel River Basin of California (Brooks et al., 1991), the Black Sea (Yefremova and Zhizhchenko, 1974), the Caspian Sea (Ginsburg et al., 1992), the Sea of Okhotsk (Ginsburg et al., 1993), offshore Norway in the Barents Sea (Ginsburg et al., 1997), and the eastern Mediterranean Sea (Woodside et al., 1996). Also, gas hydrates have been recovered at greater sub-bottom depths along the southeastern coast of the United States on the Blake Outer Ridge (Kvenvolden and Barnard, 1983, Shipboard Scientific Party, 1983), in the Gulf of Mexico (Bouma, Coleman, Meyer, et al., 1986), in the Cascadia Basin near Oregon (Shipboard Scientific Party, 1994), the Middle America Trench (Kvenvolden and McDonald, 1985; Shipley and Didyk, 1982), offshore Peru (Kvenvolden and Kastner, 1990), and on both the eastern and western margins of Japan (Shipboard Scientific Party, 1990, 1991).
Even though gas hydrates are known to occur in numerous arctic and marine sedimentary basins, little is known about the geologic parameters controlling their distribution or natural gas content. One of the major objectives of ODP Leg 164 was to establish the gas content of the Blake Ridge and an important part of that assessment is the gas content of the recovered gas hydrate.
In this paper we describe the gas content, composition, and carbon isotopic composition in recovered gas hydrate from three sites on Leg 164 (Fig. 1) and compare the results with those obtained 15 yr earlier at nearby Site 533 from Deep Sea Drilling Project (DSDP) Leg 76 (Kvenvolden and Barnard, 1983); from the Gulf of Mexico (Brooks et al., 1984, Brooks et al., 1989, and Davidson et al., 1986); from the Middle America Trench Leg 84 (Kvenvolden and McDonald, 1985); from Leg 112 offshore Peru (Kvenvolden and Kastner, 1990); and from Leg 146 offshore Oregon (Kastner et al., 1998).
The Atlantic continental margin of the United States is a classic "passive" margin and is generally used as an example of a geologic feature developed during continental rifting (Bally, 1981). A major geologic feature of the eastern margin of the United States is the Blake Ridge, a sediment drift deposit that was built upon transitional continental to oceanic crust by the accretion Tertiary to Quaternary hemipelagic muds and silty clays. The thickness of the methane-hydrate stability zone in this region ranges from zero along the northwestern edge of the continental shelf to a maximum thickness of about 700 m along the eastern edge Blake Outer Ridge.
Seven sites (Sites 991-997) were drilled during Leg 164 (Fig. 1). Sites 991 through 993 were drilled in upper Pleistocene to upper Miocene nannofossil clays and silty clays on the crest and flanks of the Cape Fear Diapir of the Carolina Rise. No gas hydrate was recovered, nor were there any proxy indicators of gas hydrate occurrences here. Site 996 was drilled in upper Pleistocene to lower Pleistocene nannofossil-bearing and nannofossil-rich clays on the crest of the Blake Ridge Diapir. Gas hydrate was recovered in abundance here. At Sites 994, 995, and 997, Holocene to upper Miocene nannofossil-rich clay and nannofossil clay were drilled over the crest of the Blake Ridge on a transect of holes that penetrated below the base of gas hydrate stability over a distance of 9.6 km. Gas hydrate was recovered from each site along the transect, although the majority was recovered at Sites 994 and 997.