CONCLUSIONS

Measurements were made of 13C and D on CH4 and CO2 gas evolved from cores, pore-water dissolved inorganic carbon (DIC), and 15N values on sedimentary organic matter samples collected during Leg 164. Methane 13C (-68.4 ± 7.0, PDB) and D (-187 ± 20, SMOW) values for the gases collected from Blake Ridge sediments have isotopic values characteristic of microbial CH4 production via the CO2 reduction pathway.

The isotopic depth profiles from three sites (Sites 994, 995, and 997) drilled along a ~10-km-long transect across the Blake Ridge are indistinguishable and show distinct depth trends in the 13C values of DIC, gaseous CO2, and CH4. Mean values of sedimentary organic matter (13C = -22.0 ± 1.4, PDB; 15N = +5.6 ± 0.7, air; and C:N values = 8 ± 2) suggest that the ultimate source for the majority of the organic matter is marine. Because significant stratigraphic variation in the organic matter does not occur, variations in the organic carbon source are not a major factor in generating variations in the isotopic composition of DIC, gaseous CO2 and CH4. D values of CH4 also do not vary with depth.

Distinct variations in 13C profiles of CO2 gas and DIC samples with depth parallel each other with an offset of 12.5 over the interval that they both could be sampled. Detailed shallow (<50 mbsf) DIC 13C profiles (largely from Site 995) diverge from seawater-like values to a sharply defined minimum of -37.7 (PDB) at the base of the sulfate-reduction zone (~20 mbsf). Collection of gaseous CO2 requires sufficient gas evolution within the core to create gas voids. The shallowest CO2 gas sample (34.9 mbsf; Site 997) had a 13C value of -20.6. With increasing depth, the 13C values of CO2 and DIC increase to a broad maximum between 120 and 150 mbsf, where the highest values of 1.1 (CO2 gas) and 10.8 (DIC) occur. Below ~150 mbsf, the values decrease linearly with depth.

At the top of the CH4-bearing zone (>20 mbsf), 13CCH4 values are very negative (-101.3, Site 995 at ~21.45 mbsf) and increase smoothly downward to a nearly constant value of -64.0 ± 0.9 below ~300 mbsf. The difference between the CH4 and CO2 isotopic values is not constant with depth and shows a distinct maximum between 120 and 150 mbsf. Models of how the isotopic composition of the CO2 gas (or DIC), CH4, and host organic carbon in these sediments change with depth indicate that a closed system with respect to carbon has not occurred during diagenesis. The models require that the CH4 and CO2 pools are decoupled from each other. Open-system models where the majority of CH4 and CO2 (>90%) are added into the system from below, and where the extent of the microbial activity decreases with depth, fit the data well. Thus, the high concentrations of CH4 and CO2 at Sites 994, 995, and 997 are interpreted to result from migration of CH4 and CO2 from depth rather than local production.

The isotopic stratigraphies at Site 996 (located at a fault conduit over the Blake Ridge Diapir) differ from all other Leg 164 sites. DIC 13C values as light as -45 at 3.96 mbsf suggest that the carbon was derived from methane. Here, upward advection of CH4 and increased amounts of anaerobic methane oxidation near the seafloor where seawater sulfate is encountered dramatically affect the isotopic composition of the CO2 pool in the top 10 mbsf.

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