RESULTS

Methane Isotopes

The 251 ¹³C analyses of CH₄ samples collected on Leg 164 have a mean of -68.4 ± 7.0 and range between -101.3 and -61.1 (Table 1). The stratigraphic profiles of ¹³CCH₄ values are indistinguishable between Sites 994, 995, and 997 (Fig. 2). At these three sites, ¹³CCH₄  values become more positive with depth from a minimum of -101.3 (21.45 mbsf, Site 995) near the top of the methane-bearing zone to a stable value of -64.0 ± 0.9 (n = 65) between 300 mbsf and the bottom of the holes (at 700-750 mbsf). The rate of change in the CH₄ profile is highest near the top of the CH₄-bearing zone.

Forty-one analyses of D of the selected CH₄ samples have a mean value of -187 ± 20 (Table 5). No stratigraphic variation in the D values is discernible. No variation in CH₄ isotopic values occurs near the base of gas hydrate-bearing sediments (~450 mbsf) at these sites (Paull, Matsumoto, Wallace, et al., 1996).

Five gas samples were measured that came directly from decomposing gas hydrate (Table 1). The isotopic composition of this CH₄ is indistinguishable from nearby gas samples collected by other methods. The isotopic composition of the CH₄ samples obtained from the PCS is separately considered in a companion paper (Wallace et al., Chap. 10, this volume).

The two ¹³CCH₄ measurements available from Sites 991 (40.6 mbsf) and 993 (27.5 mbsf; Fig. 2) plot near the trend outlined by gases from Sites 994, 995, and 997. In contrast, the 21 measurements of ¹³CCH₄ from Site 996 range from -72.1 to -62.5 with a mean value of -68.38 ± 1.9. These samples do not follow the same depth trend seen at Sites 994, 995, and 997 (Fig. 2), but rather are similar to the values that occur below 200 mbsf at Sites 994, 995, and 997.

Ethane Isotopes

The 12 measurements of ¹³CC₂H₆ values (Table 3) range from -69.4 at 216 mbsf to -63.3 at 727 mbsf, with a mean value of -65.7 ± 2. The values change linearly (R = 0.95) with depth.

¹³C of CO₂ Gas and DIC

At Sites 994, 995, and 997 the ¹³C values of gaseous CO₂ samples ranged from -20.6 to +1.24, whereas the ¹³CDIC values ranged from -37.7 to +10.8. The ¹³C values of the CO₂ gas and the DIC samples are generally parallel but offset by ~12.5 (Fig. 2).

The stratigraphic variations in carbon isotopic composition of both the CO₂ in the gas that evolved from the cores (and the parallel values of the measured DIC) reveal distinct depth zones (Fig. 3). DIC ¹³C values in the upper 40 mbsf show a sharply defined minimum at ~20 mbsf (Fig. 2, Fig. 3). Above this level there is an approximately linear change in the DIC ¹³C values that roughly corresponds with a mixing line between overlying seawater DIC (¹³C = ~0) and the very ¹³C depleted values at 20 mbsf (e.g., 37.7 at Site 995). Below ~20 mbsf the DIC ¹³C values become more positive. The increase over the interval between 20 and 40 mbsf is the most rapid and is almost symmetrical with the decrease occurring over the first 20 mbsf (Fig. 3). Below 20 mbsf the ¹³C values increase slowly before reaching a maximum between 120 and 150 mbsf. Below 150 mbsf, the ¹³C trend of both the DIC and CO₂ gas reverses and linearly (or perhaps asymptotically) converges to within ~3 of the organic carbon values at the base of the holes.

Site 996 has isotopically distinct CO₂ gas and DIC profiles from those at the deeper sites. The most negative value of ¹³CDIC (-45.0) occurred at a shallow depth of 3.72 mbsf at Site 996 (Fig. 3). The mean of the eight samples with sub-bottom depths less than 10 mbsf is -38.65 ± 4.5, whereas the samples below 28 mbsf have a mean value of 5.5 ± 6.7. The isotopic composition of the carbonates from Site 996 is considered in Naehr et al. (Chap. 29, this volume).

Organic Carbon Characteristics

Values of ¹³C of bulk organic carbon at Sites 994, 995, and 997 range between -19.7 and -25.4, with a mean value of -22.0 ± 1.4 (Fig. 2; Table 6). ¹⁵N values of the same sedimentary organic matter samples range from 3.2 to 6.8 with a mean value of 5.6 ± 0.7 (Table 6). The mean value for the C:N value in all these samples is 8 ± 2 (Table 6). Although the values in the upper hundred meters show more scatter, no significant stratigraphic trends are revealed by these data.