GEOLOGICAL SETTING

Site 996 is located above the crest of the Blake Ridge Diapir on the northwestern end of the Blake Ridge (Fig. 1). The Blake Ridge is a large Neogene and Quarternary sediment drift deposited by the Western Boundary Undercurrent (WBUC) that consists of hemipelagic silt- and clay-rich contourite deposits (Tucholke and Mountain, 1979). The Blake Ridge Diapir is the southernmost in a linear array of about 20 diapiric structures that rise out of the base of the Carolina Trough from as much as 12 km below sea level (e.g., Dillon et al., 1982). The region around the diapir displays a strong bottom-simulating reflector (BSR; Paull and Dillon, 1981) at an approximate depth of 450 mbsf, but curves upward around the flanks of the diapir. Although the cause of this curvature is unclear, it may be related to changes in gas hydrate stability induced by increased heat or salt around the flanks of the diapir.

Seafloor venting of microbial gases on top of the Blake Ridge Diapir was discovered by Paull et al. (1995b). Acoustically identified, gas-rich plumes extend as much as 320 m above a pockmarked seafloor displaying active chemosynthetic communities. These plumes apparently emanate from a seafloor pockmark that is located above a fault extending down towards a dome in the BSR (Fig. 2). Side-scan sonar data reveal a reflective seafloor along the trace of this fault zone. Photographic surveys and sampling (Paull et al., 1995b) indicate that these reflective patches are associated with biological communities and methane-derived carbonate cements.

Solid gas hydrates were recovered from all holes drilled at Site 996. The presence of gas hydrates near the sediment surface indicates that, at present, gas hydrates are stable under seafloor pressure/temperature (P/T) conditions. Pore-fluid data and the presence of active chemosynthetic communities at Site 996 are indicative of upward vertical transport of methane-rich fluids. The methane content of headspace gases in sediments near the pockmark increases from 1300 to 11,000 ppm with increasing depth to 60 mbsf. Sulfate concentrations approach 0 mM near the seafloor (Paull, Matsumoto, Wallace, et al., 1996). This rapid SO₄^2- depletion, together with high concentrations of interstitial CH₄, low amounts of sedimentary organic matter (<0.5%; Paull, Matsumoto, Wallace, et al., 1996), and high levels of dissolved HS^- indicate that anaerobic methane oxidation is occurring at Site 996 (Borowski et al., 1997). Given the low level of sedimentary organic matter, these observations are consistent with advective transport of CH₄, which supports the chemosynthetic community on the seafloor above the Blake Ridge Diapir. Intense microbial oxidation of CH₄ increases alkalinity, thus promoting carbonate precipitation. Sharp decreases in Ca²⁺, Mg²⁺, and S^r2+ concentrations immediately below the sediment/water interface suggests uptake of these elements into authigenic carbonates (Paull, Matsumoto, Wallace, et al., 1996).