RESULTS

Tables T1 and T2 contain concentration and carbon isotopic data for the Carolina Rise (Sites 1054 and 1055), the Blake Ridge (Sites 1058-1061), the Bahama Ridge (Site 1062), and the northeast Bermuda Rise (Site 1063). Figure F2 shows representative concentration-depth profiles of sulfate, alkalinity, and CO₂, and ¹³C of CO₂ and methane at Sites 1058 and 1059, which were plotted as a single hole location and are referred to as Site 1058/1059. Figure F3 shows the carbon isotopic composition of CO₂ and methane, with respect to the SMI, for all sites. Between the sites, the depth of the SMI varies between 10 and 67 mbsf (13 and 75 mcd); the order of increasing depth to SMI at 172 sites is 1058/1059, 1060, 1061, 1055, 1063, 1054, and 1062.

Dissolved CO₂

The carbon isotopic composition of the CO₂ pool is most depleted in ¹³C at the SMI at all sites, but the intensity of ¹³C-depletion varies from site to site (Table T1). Starting at the sediment-water interface, CO₂ becomes progressively enriched in ¹²C with increasing depth into the sulfate reduction zone, reaching maximum ¹³C depletion (¹²C enrichment) at the SMI. Site 1058/1059 (-31.8 at 8.25 mbsf) exhibits the most depleted ¹³C pool (Figs. F2, F3A), whereas Site 1062 (-20.2 at 66.60 mbsf) is least depleted in ¹³C at the SMI (Fig. F3A). The mean value for ¹³C CO₂ at the SMI is -26.2 ± 5; the median value is -28.5. Sites in order of increasing ¹³C depletion at the SMI are 1062, 1054, 1055, 1063, 1060, 1061, and 1058/1059.

With increasing depth into the methanogenic zone, CO₂ generally becomes progressively enriched in ¹³C. Maximum ¹³C enrichment occurs at Site 1054 (+12.2 at 195.70 mbsf) and minimum ¹³C enrichment occurs at Site 1063 (-2.6 at 234.10 mbsf). The mean value of maximum ¹³C enrichment for ¹³C CO₂ is +4.3 ± 4.8; the median value is +3.3. The sites in order of increasing ¹³C enrichment are 1063, 1062, 1060, 1061, 1055, 1058/1059, and 1054. The ¹³C values of CO₂ at Sites 1061 (+3.3, 226 mbsf) and 1063 (-2.6, 234 mbsf) reach maximum values at ~171 and 215 m into the methanogenic zone, respectively, and then move toward increasing ¹³C depletion with depth.

Methane

Methane ¹³C values of samples collected from sediment voids range from -65.8 (Site 1054; 197.16 mbsf) to -91.4 (Site 1063; 100.66 mbsf) (Table T2; Fig. F3B). The mean and median ¹³C values are -78.1 ± 6.2 and -79, respectively.

The carbon isotopic composition of methane is most depleted in ¹³C nearest the SMI. Most methane samples are from sediment voids within core liners created by methane degassing during core recovery. Only when methane concentrations reach threshold levels are such gas-filled sediment voids available for sampling. However, at Site 1058/1059, six gas samples were collected in the uppermost methanogenic zone using Reeburgh squeezers. The topmost sample has a ¹³C value of -100.6, almost 15 more depleted than the first sediment void sample at Site 1058/1059, and ~9 more depleted than the most negative ¹³C value of any other gas sample from Leg 172.

Methane becomes progressively enriched in ¹³C with increasing depth below the SMI (Fig. F3B). Maximum ¹³C enrichment occurs at Site 1054 (-65.8, 197 mbsf), whereas ¹³C enrichment (-82.8, 416 mbsf) is at a minimum at Site 1063.