Site 996 is located on the crest of the Blake Ridge Diapir, the southernmost in a series of ~20 diapiric structures rising from deep within sediments of the Carolina Trough (Fig. 1). The objectives at this site were to investigate (1) methane migration and gas hydrate formation in a pockmarked fault zone where methane is leaking out of the continental rise, (2) the source of fluids and gases in a seafloor seep, and (3) the influence of these fluids on the host sediments.
Five short holes were drilled at Site 996 in sediments overlying the Blake Ridge Diapir (Fig. 13 and14). Holes 996A, 996B, and 996C are located within a seafloor pockmark that contains an active chemosynthetic community dominated by mussels. Holes 996D and 996E were drilled on the flanks of this pockmark. Overall core recovery was poor. The sedimentary sequence consists primarily of nannofossil-bearing clay and nannofossil-rich clay, both with varying amounts of foraminifers (0%-15%) and diatoms (0%-25%). Contorted and steeply dipping beds in parts of the sequence provide evidence of soft-sediment deformation that is probably related to diapir uplift. The uppermost 2 m of sediment within the pockmark consist of nannofossil-rich aragonitic clayey silt with abundant bivalve shell fragments. The shell fragments and surrounding sediments commonly show initial stages of calcite and aragonite cementation. Indurated carbonate zones occur from ~5 to 15 mbsf and from 30 to 50 mbsf. Rapid decreases in the calcium and magnesium concentrations of interstitial waters within the upper 10 mbsf and a corresponding increase in the alkalinity indicates the precipitation of carbonate within the uppermost sediments.
All of the sediments recovered at Site 996 are Quaternary in age. The lowermost cores from Holes 996A, 996D, and 996E are early Pleistocene, with a maximum age of about 1.0 Ma. Because of poor recovery, the magnetostratigraphy is very poorly defined. In Hole 996E, the Matuyama/Brunhes boundary is tentatively placed at about 33 mbsf, and the top of the Jaramillo subchron at ~40 mbsf.
Gas hydrate was recovered from all five holes (Holes 996A through E). The hydrate was white and occurred in three different forms: (1) massive pieces, cylindrical to round in shape and as much as 5 to 8 cm long, were found in sediments recovered from the uppermost 9 mbsf; (2) platy, 1-4 mm thick veins that filled wavy vertical fractures; and (3) vertically oriented rod-shaped nodules about 1 cm in diameter and 3-12 cm in length that tapered downcore. Numerous pieces of hydrate were sampled both for shipboard analyses and for storage in pressure vessels for shore based studies.
Magnetic susceptibility and rock-magnetic studies suggest that the zone of bacterial magnetite authigenesis, present in the uppermost few meters at all other Leg 164 sites, is absent from Site 996. High-resolution gamma-ray attenuation porosity evaluator (GRAPE) data for sediments from Hole 996E indicate little variation in wet-bulk density with depth, with values averaging 1.8 g/cm3. Porosities decrease from 75% to 50% in the uppermost 12 m of the hole, but water content decreases exponentially over the same interval, which is similar to the pattern observed in the uppermost 100 m at Site 995.
The methane (C1) content of headspace gases in sediments from Holes 996A, 996C, and 996D increases from 1300 to 11000 ppm with increasing depth to 60 mbsf. Ethane (C2) concentrations ranged from 1 to 10 ppm, yielding C1/C2 ratios near 1000. In contrast, near-surface sediments from Hole 996E, which is located outside of the pockmark, were relatively poor in C1. However, at depths below about 10 mbsf, C1 concentrations and C1/C2 ratios were comparable to those from Holes 996A, 996B, and 996C. The free gases consist almost entirely of C1 (greater than 90%), CO2, and H2S in the upper sections of each Hole. H2S concentrations were as high as 50,000 ppm. Maximum values for C2 and C3 were 940 and 25 ppm, respectively.
Chloride concentrations (Fig. 4) increase to 1.8 times that of seawater with increasing depth (57.7 mbsf) at Site 996, suggesting the influence of evaporites at depth. Increasing Na/Cl ratios and a two-fold increase in K+ concentrations are consistent with a salt source. Large (20%-30%) negative anomalies in chloride concentration also occur, probably due to gas hydrate dissociation. Large decreases in dissolved calcium and magnesium concentrations in interstitial waters from sediments within the uppermost 10 mbsf, and alkalinity maxima at approximately 15 mbsf, indicate precipitation of carbonate within the top 10-15 m of the sedimentary sequence. Sulfate approaches negligible concentrations near the seafloor (Holes 996C and 996D), and together with high levels of interstitial ·HS- and methane, suggests active sulfate and methane consumption near the seafloor. Active pore-water advection also is suggested by the wide difference in interstitial waters sampled immediately below the seafloor chemosynthetic community (Holes 996A, 996B, and 996C) and sampled at depth in Hole 996E.