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.

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