Monitoring of volatile hydrocarbons was conducted for safety reasons, and organic, petrological, and geochemical studies were carried out to assess the types of organic matter and the molecular compositions of the hydrocarbons found in the sediments.
A total of 51 sediment samples were collected at Site 1177 (at ~10-m intervals) from 304.7 to 830.3 mbsf (Table T14). All sediments were analyzed for methane concentrations and light hydrocarbon compositions during headspace analyses (Fig. F19A). In addition, molecular gas compositions, TOC, inorganic carbon (carbonate), nitrogen, and sulfur analyses were performed (Table T15) (see "Organic Geochemistry" in the "Explanatory Notes" chapter for analytical procedures).
TOC contents for the sediment samples examined at Site 1177 ranged from 0.03 to 1.62 wt% at 695.19 mbsf, with an average value of 0.50 wt% (Fig. F20). The high average TOC value is due to the presence of terrestrial organic matter in the form of plant detritus (wood pieces), especially in the 650-750 mbsf section of the lower Shikoku turbidite facies. The distribution of sulfur was similar to that of TOC, ranging from 0 to 0.81 wt% (average = ~0.21 wt%), with the highest concentrations found at 400-520 mbsf and 650-770 mbsf (Fig. F20). The nitrogen content corresponds to a mixture of both marine and terriginous sources contributing to the composition of the Site 1177 sediments. The two-component mixture for Site 1177 sediments is evident in the carbon/nitrogen (C/N) ratio (Fig. F19B), which clearly shows a strong component for both marine and terrigenous input, especially in the Shikoku turbidite facies (Unit III).
In contrast to Sites 1175 and 1176, inorganic carbon values were generally low at Site 1177 (0.02-2.64 wt%) with some rare high values in thin layers that ranged between 4.32 and 8.75 wt%. Accordingly, the bulk-carbonate values were also low (0-22 wt%) with the exception of a few samples that varied from 36 to 72 wt%; however, the overall average concentration of carbonate in the sediments was only 2.7 wt%. In general, carbonate contents were confined to very thin layers throughout the hole with higher carbonate concentrations scattered over an interval between 530 and 670 mbsf, which is characterized as a rare carbonate-cemented claystone unit within the Shikoku turbidite facies (Unit III).
Headspace gas concentrations of methane differed from the previous sites drilled during Leg 190, with low concentrations between 304 and 363 mbsf followed by an abrupt drop in methane concentrations to 1.8-5.3 ppm over a 270-m interval to 734 mbsf. The low concentrations of methane below 400 mbsf are the result of an unusually high sulfate content (see "Inorganic Geochemistry"), which inhibits methanogenesis or microbial mediation of methane production throughout the upper and lower Shikoku Basin facies (Units I and II) and the Shikoku turbidite facies (Unit III). A sharp increase from low levels to ~1600 ppm was measured in the last 90 m to total depth at 830.3 mbsf, coincident with the transition from the Shikoku turbidite facies (Unit III) to the volcaniclastic facies (Unit IV). Interestingly, the Bernard C1/(C2+C3) ratio for the hydrocarbons at 350-400 mbsf clearly plot in the mixing zone, indicating that some hydrocarbons in these sediments were produced in situ from organic matter present in the sediments or that some hydrocarbons migrated in from a deeper source. Methane also increases through this section coincident with a sharp drop in sulfate, suggesting that methanogenesis is also occurring in these sediments. The thermal gradient (6.0°C/100 m, corresponding to ~50°C at 830.3 mbsf) in Hole 1177A, coupled with the presence of high methane concentrations, indicates that both biogenic and thermally produced hydrocarbons are present in the volcanoclastic facies sediments (Unit IV). Isotopic analyses of TOC in these samples will allow for a more detailed assessment of the nature and distributions of the hydrocarbons at Site 1177.
Organic geochemical analyses at this site leads to the following conclusions: