During Ocean Drilling Program (ODP) Leg 164, gas hydrate sediments were recovered at seven drill sites on the Blake Ridge and on the Carolina Rise (Fig. 1). On the Blake Ridge, Sites 994, 995, and 997 were drilled down to 750 meters below seafloor (mbsf), at about 3000 m of water depth. A geophysical survey of this area (Holbrook et al., 1996; Wood et al., Chap. 27, this volume) has shown that the bottom-simulating reflector (BSR), which marks the lower limit of gas hydrate stability, is located at about 450 mbsf. The depth distribution of the major chemical components of interstitial waters, as well as the direct measurements with the pressure core sampler (PCS) of the gas contained in the sediments (Paull, Matsumoto, Wallace, et al., 1996), indicate that the gas hydrates are mostly concentrated within the sediments from depths between 200 and 450 mbsf, where they occupy up to 9% of the pore space (Dickens et al., 1997). However, chemical data indicate that the formation of gas hydrates starts immediately below the sulfate reduction zone (i.e., near 25 mbsf) (Egeberg and Dickens, in press). Below the BSR, free gas bubbles fill up to 12% of the pore space. In the Cape Fear Diapir, the BSR that surrounds the diapir is interrupted by the piercement structure, which is thought to be salt (Paull, Matsumoto, Wallace, et al., 1996). Sites 991, 992 and 993 were drilled to within about 50 mbsf of the top of the diapir at water depths of 2635 m, 2648 m, and 2704 m, respectively.
Diagenetic carbonates are present as nodules and lamina above and below the BSR, but there is no sedimentological or mineralogical discontinuity crossing the BSR. The mineralogy and stable isotopic compositions of these diagenetic carbonates are used to evaluate the conditions of formation of these minerals, especially their relationships with the occurrence of gas hydrates within the sediments.
Sulfide minerals, mostly pyrite, are generally present as a minor component (a few weight percent [wt%]) disseminated in the sediments as small cubic crystals or framboids. They rarely become concentrated in nodules or infilling of burrows. The sulfur isotopic composition of these sedimentary sulfides may help in understanding the extent of the process of bacterial sulfate reduction from which the reduced sulfide was probably produced.