PREVIOUS WORK

Previous interstitial chemical data from Blake Ridge sediments are available from Deep Sea Drilling Project (DSDP) Leg 11 (Sites 102, 103, and 104; Hollister, Ewing, et al., 1972), from DSDP Site 533 (Leg 76, Sheridan, Gradstein, et al., 1983), and from 87 piston cores taken over the Blake Ridge and Carolina Rise (Borowski et al., 1996, 1997) (Fig. 1).

Three sites drilled on Leg 11 form a transverse transect on the Blake Ridge (Fig. 1) over prominent bottom-simulating reflectors (BSRs). The BSRs suggest the presence of gas hydrates in the subsurface (Lancelot and Ewing, 1972; Shipley et al., 1979). Moreover, although interstitial data from this leg are sparse, chloride data show strong downward freshening (Sayles et al., 1972; Presley and Kaplan, 1972), also suggesting gas hydrates are contained within the sediments (Hesse and Harrison, 1981). Sulfate data indicate that shallow sulfate-methane interfaces occur between 6 and 45 mbsf.

Site 533 was also drilled over a strong BSR, and interstitial chemical data show downward-freshening chloride profiles and ¹⁸O-enrichments in pore waters (Jenden and Gieskes, 1983), indicating the presence of gas hydrates (Hesse and Harrison, 1981; Davidson et al., 1983). Sulfate data are insufficient to fully document the shape of the concentration profile, but sulfate is depleted between 9.8 and 14.4 mbsf (Claypool and Threlkeld, 1983). Methane and CO₂ data were used to identify large ¹³C depletions of carbon in the dissolved CO₂ (¹³CCO₄ = -31.4 at 14.4 mbsf; Claypool and Threlkeld, 1983) and methane (¹³CCH₄ = -91.3 at 33.9 mbsf; Galimov and Kvenvolden, 1983) pools, but did not provide isotopic data through the SMI.

Sulfate gradients at nondiapir sites over the CR-BR gas hydrate field are linear and vary by sixteenfold over the area as shown by piston coring (see fig. 1 in Borowski et al., 1996; fig. 2.1 in Borowski, 1998). However, the sulfate-methane interface was penetrated by only one piston core (PC 11-8; Fig. 1), where the depth to the sulfate-methane interface is 10.3 mbsf (Fig. 2C), the shallowest SMI depth observed or inferred in the CR-BR area.

The inference that linear sulfate gradients indicate a focused sink for pore-water sulfate over the breadth of the Blake Ridge, as well as the occurrence of large ¹³C depletions in interstitial CO₂ and methane (Borowski et al., 1996, 1997), prompted us to sample Leg 164 sediments in an unusually detailed manner. Our objective was to gather additional supportive evidence documenting the occurrence of AMO, and to quantify its role in sulfate depletion in methane-rich, gas-hydrate settings.