RESULTS

Sulfate Gradients

Sulfate gradients at Sites 994, 995, and 997 are linear through most of the sulfate reduction zone (Site 994, Fig. 2A; Site 995, Fig. 2B; Site 997, not shown, see Paull, Matsumoto, Wallace et al., 1996; Table 2). Sulfate gradients calculated by least-square linear regression are 1.41, 1.30, and 1.23 mM m^-1 respectively for the three sites; the sulfate-methane interface lies at 20.5, 21.0, and 22.7 mbsf. The sulfate gradient in piston core (PC) 11-8 is 2.88 mM m^-1 with a SMI depth of 10.3 mbsf (Fig. 2C).

Methane

Methane concentrations (Table 3) are low through the sulfate reduction zone and then increase rapidly once low (<1 mM) pore-water sulfate concentrations occur (Fig. 2). These data are consistent with the placement of the sulfate-methane interfaces as shown at sites 994, 995, and PC 11-8. Methane concentration data from sites 994 and PC 11-8 are much noisier relative to that of Site 995 because of the crude headspace methods used. The methane data from Site 995 shown in Fig. 2B were obtained from measurements of methane concentration in two subcores, and these duplicate measurements deviate by <5% (Hoehler et al., Chap. 8, this volume).

Interstitial CO₂ and Alkalinity

The CO₂ concentrations increase monotonically as sulfate concentrations decrease (Table 2; Fig. 2). Within the sulfate reduction zone of PC 11-8, CO₂ concentrations are generally linear, but concave-down curvature occurs in the profiles near the sulfate-methane interface. The CO₂ concentrations show little variation immediately below the interface.

The CO₂ concentration data for ODP Sites 994 and 995 are stratigraphically sparse near the sulfate-methane interface (Table 2), so we have also included alkalinity values. Because CO₂ is the predominant control on alkalinity in this system (Fig. 2), the shape of alkalinity profiles mimics CO₂ profiles, with alkalinity also increasing with increasing sulfate consumption downcore. The values are linear in the upper sulfate reduction zone, but alkalinity shows an inflection point near the SMI, where alkalinity is higher than expected based on linear extrapolation from above. Alkalinity values show little variation for about 10 m below the interface, but begin to increase below 30 mbsf (Paull, Matsumoto, Wallace et al., 1996).

¹³C-CO₂

The carbon isotopic composition of dissolved CO₂ of the overlying seawater is near 0 per mil (, PDB), but decreases rapidly within interstitial waters of the sulfate reduction zone (Sites 994 and 995, Fig. 2B, C; Table 2). Maximum enrichments of light carbon (¹²C) occur at the sulfate-methane interface, where ¹³CCO₂ values are -37.3 and -37.7, respectively for sites 994 and 995. Further downcore, light carbon composes less of the CO₂ pool. No ¹³CCO₂ values are available for PC 11-8. At Site 995, ¹³C values more depleted in ¹³C than -30 lie between 13 and 24 mbsf.

³⁴S-SO₄

The sulfur isotopic composition of interstitial sulfate changes progressively downcore as sulfate is depleted (Table 2; Fig. 2). Sulfur in modern seawater sulfate has a ³⁴S value of +20.0 ± 0.1, CDT (Rees et al., 1978), and each site shows that interstitial sulfate becomes progressively enriched in heavy sulfur (³⁴S) with increasing depth into the sulfate reduction zone. The fractionation observed in sulfate of PC 11-8 is significantly less than that at Sites 994 and 995, with the respective maximum ³⁴S values of +29.1, +49.6, and +51.6.