About 150 km southeast of the Sites 994, 995, 997 transect is a transect of three other sites (102, 103, and 104) from DSDP Leg 11 (Fig. 1). These three sites were drilled in 1970 (Ewing and Hollister, 1972) to investigate the causes of BSRs seen on seismic profiles along the Blake Outer Ridge. Although the effects of high concentrations of gas were observed, no obvious gas hydrate was recovered. Nevertheless, the occurrence of gas hydrate in sediments at these sites was postulated on the basis of the concurrence of geophysical and geochemical evidence (Stoll et al., 1971; Ewing and Hollister, 1972; and Lancelot and Ewing, 1972).
The postulation for gas-hydrate occurrence in sediment of the Blake Outer Ridge led to the drilling in 1980 of Site 533 during DSDP Leg 76, about 70 km northwest of Sites 102, 103, and 104 (Fig. 1); thus, Site 533 is located about 80 km from the newly occupied Sites 994, 995, and 997. Site 533 was intentionally placed on the periphery of a major gas-hydrate accumulation (mapped distribution of BSRs), and drilling to 399 m did not pass through sediment showing significant seismic blanking and did not penetrate the calculated base of the gas-hydrate stability zone at ~600 mbsf (Kvenvolden and Barnard, 1983). The distribution of methane concentrations with depth is shown in Figure 3, based on data summarized in Kvenvolden et al. (1990). A gas-hydrate sample was recovered at 238 mbsf at Site 533. These methane concentrations and depth distributions compare favorably with those reported for Site 997.