Multiple sources (biogenic, thermogenic, and catagenic) and production mechanisms for the hydrocarbons encountered during Leg 190 were identified by plotting the methane, Bernard ratios (C1/[C2+C3]), and sulfate profiles for each Site (Fig. F41). At Reference Site 1173, the hydrocarbon profile is dominated by biogenic methane down to 260 mbsf, followed by a shift to a mixture of thermally produced methane and lighter hydrocarbons around 300 mbsf that dominates the composition of gases down to 724 mbsf. The marked shift in hydrocarbon type is likely due to an increase in sulfate concentration below ~400 mbsf that inhibits biogenic production of methane. The hydrocarbons detected in sediments at Site 1174 are indicative of mixing between biogenic and thermogenic sources down to ~850 mbsf, with the thermogenic component increasing in concentration with temperature and depth. Like Site 1173, as sulfate increases with depth, methane production decreases (low parts per million). The hydrocarbons below ~850 mbsf are the products of both rapid thermal maturation of immature organic matter present in the sediments (thermogenic) and the thermal cracking of more mature organic matter or "kerogen" (catagenic).
Site 1175 is characterized by very young sediments and immature organic matter as observed by the production of biogenic methane throughout the hole. As at Site 1175, biogenic methane dominates the first 300 mbsf of Hole 1176A; however, over the last 100 m there is an abrupt shift into the thermogenic zone. Interestingly, the C1/(C2+C3) ratio indicates that no mixing has occurred; therefore, the hydrocarbons detected below 300 mbsf are a separate component. The very low thermal gradient at Site 1176 (~25°C at total depth; 401.6 mbsf) coupled with high sulfate concentrations and the predominance of C2 over C1 indicate that thermogenic hydrocarbons migrated into the lower 100 m of Hole 1176 from a deeper source rather than forming in situ by diagenesis at low temperature. The hydrocarbon profile for Site 1177, drilled on the Ashizuri Transect, differs significantly from those sites drilled on the Muroto Transect, with low concentrations of methane between 304 and 363 mbsf followed by an abrupt drop in methane concentration (a few parts per million) over the next 270 m of hole. The low abundance of methane below 400 mbsf is the result of an unusually high sulfate content that is controlling methanogenesis or microbial mediation of methane production. The relatively low concentrations of organic matter available in these sediments may be preventing sulfate reducers from consuming the excess sulfate, keeping concentrations at high levels. A sharp increase from the low parts per million level to ~1600 ppm was measured in the last 90 m to total depth at 830.3 m, coincident with the transition from the Shikoku turbidite facies (Unit III) to the volcaniclastic facies (Unit IV). As at Site 1176, there is an abrupt shift of the C1/(C2+C3) ratio to the mixing zone with both ethane and biogenic methane occurring between 750 and 830 mbsf. The temperatures within the mixing zone at Site 1177 are in the range of thermogenic in situ production of the C2 lighter hydrocarbons.
Through their metabolic activity, bacteria are responsible for shaping many of the chemical profiles within deep marine sediments. The distribution of the microbial community was characterized and their impact on deep marine sediments was investigated in cores from the Nankai Trough (Fig. F42). The apparent environmental controls on bacterial distribution varied among sites from physical (temperature) to geochemical. Overall, the total numbers of bacteria at these sites was either at the low end or below the range predicted by a general model constructed from bacterial distributions in deep marine sediments at previous ODP sites (Parkes et al., 1994). This is consistent with observations made at a previous accretionary prism site—Cascadia margin.
The abundance of bacteria at Site 1173 appears to be primarily controlled by a steep thermal gradient. The profile agrees with the model for the upper 250 m, with a significant increase in numbers between 43 and 80 mbsf associated with high concentrations of methane and TOC. At 250 and 460 mbsf the temperature boundaries for mesophiles/thermophiles and thermophiles/hyperthermophiles, respectively, were crossed. At the upper boundary populations decreased overall, and at the second boundary there was a single significant increase at ~85°C. Bacteria were not observed in the five deeper, and warmer, samples at this site (536-673 mbsf). Overprinting this temperature control is a significant correlation between bacterial numbers and in situ methane concentration between the near surface below the sulfate reduction zone and ~400 mbsf. At 400 mbsf (~76°C), rising concentrations of both methane and sulfate suggest that bacteria are no longer controlling sediment geochemistry and that temperature effects dominate.
Bacterial abundance at Site 1174 falls within predicted values from the surface to ~370 mbsf, apart from two significant excursions in the upper layers associated with sandy cores. This site is also characterized by a steep temperature gradient, and at 370 mbsf estimated temperature is ~50°C. From this depth, bacterial populations rapidly decline to not detectable at 575 mbsf. Bacteria were not observed in the 15 samples collected from 623 to 1091 mbsf with the exception of two samples (779 and 796 mbsf) located just above the décollement zone. At these depths, with an estimated temperature of 90°C, bacterial abundance reappears within the envelope predicted by the model. Bacterial populations were not correlated with any of the in situ chemistry investigated and appear to be regulated solely by temperature at this site.
At Site 1177 bacterial abundance appears to be determined by sulfate. Coring began at 300 mbsf, where bacterial abundance was relatively low and remained so until 380 mbsf. Abundance in deeper samples (380-675 mbsf) steadily increased and was within predicted values from below 400 mbsf. This increase in bacterial numbers strongly correlated with increases in IW sulfate concentration that were unexpectedly present at near-seawater concentrations between 400 and 700 mbsf. Bacteria were also present at lower and decreasing concentrations between 725 and 811 mbsf despite methane concentrations increasing significantly below 740 mbsf. Overall low bacterial populations and high sulfate concentrations are probably attributable to very low TOC concentrations restricting bacterial sulfate reduction. Why populations do not react to increasing concentrations of methane below 740 mbsf remains unclear as the temperature gradient at this site was shallow with the hole estimated at <40°C throughout.