Leg 164 was the first Ocean Drilling Program (ODP) leg to have the study of the occurrence of natural gas hydrates in sediments as its principal objective. A series of sites were drilled on the Blake Ridge, a Neogene and Quaternary sediment drift (Tucholke et al., 1977) comprising hemipelagic silt and contourites, and on a sector of the southern Carolina Rise penetrated by one of a series of diapirs that form a linear array extending for more than 200 km along the base of the rise. Gas hydrate had been previously recovered in Deep Sea Drilling Project (DSDP) drilling on the Blake Ridge (Gradstein and Sheridan, 1983). A bottom-simulating reflector (BSR), interpreted as indicating the base of an interval of gas hydrate accumulation, is prominently developed on the Blake Ridge and Carolina Rise (Shipley et al., 1979; Dillon and Paull, 1983).
Extending from the Carolina Rise to the Hatteras Abyssal Plain is the Cape Fear Slide, one of the largest continental margin slide features in the world (Popenoe et al., 1993). Near its headwall scarp, the sole of the Cape Fear Slide is breached by the Cape Fear Diapir. Leg 164 drilled three sites (Sites 991, 992, and 993) near the Cape Fear Diapir (Shipboard Scientific Party, 1996c) to investigate the relationship (if any) between the formation of the slide and the intrusion of the diapir, and to test the hypothesis that decomposition of the base of a hydrate layer might have fluidized sediments to mobilize the base of the slide (Carpenter, 1981; Cashman and Popenoe, 1985; Popenoe et al., 1993; Schmuck and Paul, 1993).
At the top of the Blake Ridge, where it merges with the Carolina Rise, the Blake Ridge diapir produces a topographic bulge in the overlying sediments, and is marked by a pockmarked seafloor with actively venting faults and associated chemosynthetic communities (Paull et al., 1995). Objectives at Site 996 were to investigate methane migration and gas hydrate formation in a fault zone where the methane may be sourced from decomposition of hydrate below. Site 996 also provided abundant samples of hydrate recovered in cores (Shipboard Scientific Party, 1996e).
Rock-magnetic investigations on samples from Leg 164 have focused on the bacterial mediation of the diagenesis of magnetic oxides and sulfides. At the Blake Ridge sites (Sites 994, 995, and 997), these studies have confirmed the rock-magnetic signature of enhanced bacterial activity at the BSR and have indicated a close correspondence between rock magnetic parameters that reflect oxidation state and grain size in the magnetic mineralogy (magnetite and greigite) and the patterns of biological activity in bacterial cultures isolated from these samples (Musgrave, Goodman, Parkes, et al., 1997).
The diapir sites, Sites 991, 992, 993, and 996, provide a different setting from the Blake Ridge sites. No BSR is present at the Cape Fear Diapir sites, and drilling at Site 996 was restricted to shallower than 63 m below seafloor (mbsf). Hence none of the diapir sites involves penetration of a BSR. In contrast to the Blake Ridge sites, however, all four diapir sites involve substantial disturbance to the sequence of sediment accumulation and early magnetic diagenesis typical of reduced marine sediments. At Sites 991, 992, and 993, this disturbance is physical, and results from diapir emplacement, from deformation related to repeated slumping, and from exhumation following the slumping of the Cape Fear Slide (Shipboard Scientific Party, 1996b). At Site 996, methane is being vented to the seafloor along a fault that taps a BSR that is domed upwards around the Blake Ridge Diapir. The fault-enhanced transport of methane into the zone of seawater diffusion appears to result in very active sulfate reduction, including the production of large quantities of H2S, similar to the situation observed at Site 892 on Leg 146 (Shipboard Scientific Party, 1994; Housen and Musgrave, 1996).
For each of these diapir sites, rock-magnetic studies were directed toward contrasting the disturbed rock-magnetic depth profile with the undisturbed profile observed at the Blake Ridge sites.