RESULTS: ISOTOPE RATIOS OF INTERSTITIAL WATERS AT SITE 997

Chlorine Isotopes: Coupled ³⁷Cl Depletion and Downhole Chloride Decrease

In this study, ³⁷Cl values as negative as nearly -4 ³⁷Cl have been determined in pore-water samples from Site 997 drilled in the Blake Ridge gas hydrate field in West Atlantic. The Cl-isotope values decrease downward starting from a maximum near seawater values of 0 ³⁷Cl at about 30 mbsf (meters below seafloor). The minimum value (-3.68) occurs near the bottom of the site at 746.85 mbsf (Fig. 1). The steady downward depletion in the heavy chlorine isotope, ³⁷Cl, accompanies a downward decrease in chlorinity in the upper part of the hole that is followed by sections with relatively constant chlorinity punctuated, however, by pronounced low-chlorinity peaks (Fig. 1).

The chlorinity profile can be broken into four zones with different concentration levels and gradients. In Zone 1, which extends from the seafloor to the bottom of the sulfate reduction zone at 24 mbsf, Cl^- concentrations increase slightly toward the roof of the hydrate zone that is postulated to exist at this depth by Egeberg and Dickens (1999). In Zone 2 below that depth to 220 mbsf, chlorinity decreases steadily from the maximum of 561 mM in zone 1 to a plateau value of 506 mM. In Zone 3, which spans the depths from 220 mbsf to the base of the gas hydrate zone at 452 mbsf, values are below the plateau level and interrupted by pronounced intermittent low-chlorinity peaks; the gradient of the heavy-isotope depletion decreases in this zone to <0.5 per 100 m compared to that in Zone 2, which is ~1 per 100 m. In Zone 4 below the base of the hydrate zone (which corresponds to the position of the bottom-simulating reflector [BSR] in Fig. 1) down to the bottom of the site, chlorinity remains nearly constant at the plateau level. The isotopic delta values further decrease, though at the same low rate as in Zone 3.

Oxygen and Hydrogen Isotopes

Oxygen isotopes show positive delta values around 0.4 in the upper 50 m and then decrease to slightly negative values to 159 mbsf. A zone of scattered values between 220 and 452 mbsf coincides with the zone of low-chlorinity spikes in the chloride profile with a tendency of the spikes on the oxygen isotope curve to oppose those on the chloride curve (mirror-symmetrical or antithetic behavior). An overall increase in the values with depth is observed below 300 mbsf leading back to positive values of about 0.3 around 600 mbsf. The scatter in the ¹⁸O values persists below 452 mbsf, where the chloride profile is smooth (Fig. 2). Hydrogen isotopes range from nearly 11 to -12 D and show a general downhole decrease. D values remain positive from the shallowest sample at 1.5 mbsf down to 159 mbsf, showing up to 9 scatter in the upper 80 mbsf. Whereas the ¹⁸O values continue to decrease below that depth down to 159 mbsf, the D values increase in this depth interval. Below 200 mbsf down to the bottom of the hole, an overall decrease in the D values is observed, but values are scattered throughout the zone of chloride spikes (Zone 3 of the chloride profile) and Zone 4 below. The shifts are generally in the same direction downhole as those in the oxygen isotope curve, if sample points are compared for which both oxygen and hydrogen isotope measurements are available (Fig. 2, Fig. 3). In zone 3 the spikes of the D curve, like those of the ¹⁸O curve, tend to be antithetic to those of the chloride curve.