Hydrocarbon gases, particularly methane, are ubiquitous in oceanic sediment worldwide, and our study of the southeastern North American continental margin represents a continuation of our research on gaseous hydrocarbons in sediment drilled by the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP). The precedent for this work was established in 1971 during DSDP Leg 18 (McIver, 1973). In that early study, oceanic sediment samples from offshore Oregon were sealed in cans, and the composition of the gas, released into the headspace of the cans by shaking, was determined by gas chromatography. This basic procedure has been modified and used in systematic studies of hydrocarbon gases in sediment samples from DSDP Leg 76 (Blake Outer Ridge, Atlantic Ocean; Kvenvolden and Barnard, 1983) and Leg 84 (inner slope, Middle America Trench, Pacific Ocean; Kvenvolden and McDonald, 1985), and during ODP Leg 104 (Vøring Plateau, Norwegian Sea; Kvenvolden et al., 1989), Leg 112 (shelf and slope deposits, Peru-Chile Trench, Pacific Ocean; Kvenvolden and Kastner, 1990), Leg 141 (inner slope, Chile Trench; Froelich et al., 1995), and now Leg 164 (Carolina Rise and Blake Ridge, Atlantic Ocean).
At some drilling locations, methane is so abundant that it takes the form of methane hydrate, a solid, icelike clathrate structure in which a crystalline lattice of water molecules encases molecules of gas, mainly methane. For example, during the investigation of hydrocarbon gases in sediment, methane hydrate was observed on Leg 76 (Kvenvolden and Barnard, 1983), Leg 84 (Kvenvolden and McDonald, 1985), Leg 112 (Kvenvolden and Kastner, 1990), and Leg 164 (Paull, Matsumoto, Wallace, et al., 1996).
In this paper we describe the hydrocarbon gas distribution in sediment cored at seven sites on Leg 164 (Fig. 1) and compare the results with those obtained fifteen years earlier at nearby Site 533 (Fig. 1) from Leg 76 (Kvenvolden and Barnard, 1983). Methane is shown to be the most abundant hydrocarbon gas in these sediments, and the concentration of methane and its distribution with depth are similar at ODP Site 997 (Leg 164) and DSDP Site 533 (Leg 76).