At Site 1174, real-time monitoring of volatile hydrocarbons was carried out for safety reasons and organic, petrological, and geochemical studies were conducted to (1) determine the amount and type of organic matter preserved in the sediments; (2) characterize the light hydrocarbons generated by biogenic, thermogenic, and categenic processes; and (3) further relate hydrocarbon distributions to the thermal evolution and structural properties of the Nankai accretionary prism.
Eight sediment samples and two vacutainer samples were collected from Hole 1174A from 1.35 to 65.13 mbsf, and 102 sediment samples were collected from Hole 1174B (~10-m intervals) from 148.63 to 1110.25 mbsf. All sediments were analyzed for methane concentration and light hydrocarbon composition during headspace analyses (Fig. F34; Table T15). In addition, molecular gas composition, total carbon, and inorganic carbon (carbonate) analyses were performed, and carbon/nitrogen (C/N) ratios were determined (Fig. F35; Table T16).
The total carbon content for the sediment samples examined is relatively low, ranging from 0.03 to 1.6 wt% and averaging 0.74 wt% (Fig. F35A). The highest carbon values are found in the upper 171 mbsf (0.8-1.6 wt% at 171.33 mbsf), followed by a decrease in carbon content (0.8-4 wt%) between 350 mbsf and just below the décollement at 807.60 mbsf. Carbon contents slightly increase between 1000 and 1100 mbsf and then drop abruptly to 0.2 wt% at greater depth. Variations in the carbon content (Fig. F35A) are correlated with changes in lithostratigraphy. For example, a steady decrease in the amount of carbon was observed in the transition from the upper to lower Shikoku Basin facies and continues to the décollement. Below the décollement, higher carbon values are observed (0.50-0.85 wt%) in some thin layers that also coincide with an increase in ethane and propane hydrocarbons (Fig. F34B, F34C). The inorganic carbon (carbonate) concentrations are generally low (average = ~5 wt%) with several maxima of >40 wt% below ~900 mbsf (Fig. F35C). High concentrations of carbonate nodules were seen in some calcareous claystones (~700 mbsf) in the lower Shikoku Basin section (also observed in sediments of Hole 1173A).
Nitrogen contents of the sediments are also low (~0.1 wt%) with one minimum dropping to zero at ~1020 mbsf (Fig. F35B). The C/N ratios are consistent with a marine origin. At Site 808 (Leg 131), however, the upper Shikoku Basin sediments likely contain a significant terrigenous component as a result of the flux of terrestrial organic matter to these sediments (Berner and Faber, 1993). At greater depths the ratios drop abruptly (<5), consistent with carbon loss due to thermogenic or catagenic C2 and C3 hydrocarbon generation. As at Site 808, the generation of light hydrocarbons is relatively low (10-70 ppm) and is attributed to the low TOC and very high temperatures (up to 120°C) (see "In Situ Temperature and Pressure Measurements"), which is in the range of categenic or "thermal cracking" of organic matter (Tissot and Welte, 1984).
Headspace gas concentrations of methane in Hole 1174A are low (~2.5 ppm) in the first core (1.5 mbsf) within the sulfate reduction zone (Fig. F34A). However, a significant increase in methane concentrations is observed just below this zone (~20,000 ppm in sediment samples and 940,000 ppm in vacutainer samples) in the first 65 mbsf of Hole 1174A. Methane concentrations are also high from 300 to 550 mbsf in Hole 1174B (up to ~40,000 ppm). As was observed at Site 808 and in other previous legs, an increase in methane in sediments below the sulfate reduction zone is indicative of bacterial origin (Claypool and Kvenvolden, 1983). The presence of small concentrations of methane in the sulfate reduction zone, however, may be due to migration from below.
Ethane and propane are also present in sediments below 300 mbsf, with a sharp increase in propane from 550 to 700 mbsf (10-30 ppm). The atypical predominance of propane over ethane suggests that fermenting bacteria are preferentially utilizing ethane to produce acetate and H2 (King and Blackburn, 1998), whereas propane remains unaffected (see "Microbiology" for microbial counts in these sediments). Interestingly, the ratios of ethane to propane in sediments at Site 808 show the same trend. The concentrations of ethane and propane in both Sites 1174 and 808 are proportional at depth and consistent with sediments of higher maturity.
The Bernard gas ratio C1/(C2+C3) shows that the hydrocarbons above ~900 mbsf are mixtures of biogenic and thermogenic components (Fig. F36). At depths >900 mbsf, the activity of methane-producing archae bacteria has ceased and only thermogenic hydrocarbons are present. Two hydrocarbon mixing zones (biogenic and thermogenic) are observed between 400 and 900 mbsf (Fig. F36). The increase in C1/(C2+C3) ratios below 900 mbsf may suggest that at this depth cracking of longer chain and less thermally stable hydrocarbons occurs rather than in situ thermal generation of these components. The higher concentrations of propane observed in the upper zone, where maturities of organic matter and temperatures are lower, implicitly suggests that these hydrocarbons were migrated from deeper sections of the accretionary prism. A previously proposed interpretation is that hot fluids moving along the décollement caused extensive heating and cracking of kerogenous organic matter, generating light hydrocarbons that may slowly leak out of the décollement, (Taira et al., 1991). The shipboard geochemical data for Site 1174, however, indicate that there is presently no active advection of light hydrocarbons occurring along the protothrust or the décollement.
One of the more interesting results from the organic geochemistry shipboard measurements for Site 1174 is the distribution of hydrocarbons within discrete sedimentary zones, suggestive of migration along the frontal and protothrusts, to stratigraphically or structurally controlled areas within the prism. Although it is difficult to reconcile the containment of light hydrocarbons such as methane, the presence of propane and higher hydrocarbons in the trench to basin transition facies and the upper Shikoku Basin strongly suggests that the physical properties of the accretionary prism are influencing the distributions of these components.
A reassessment of the organic geochemical data for Site 808 also revealed some striking similarities to Site 1174 in the distributions of hydrocarbons. As stated earlier in the hydrocarbon section, both propane and ethane show identical profiles in sediments above and below the décollement (Berner and Faber, 1993). A predominance of propane over ethane (Fig. F34) is unusual and was first interpreted by Berner and Faber (1993) as evidence for fluid migration along the frontal thrust from below the décollement. The lack of supportive data for vertical fluid movement in the pore-water chemistry, however, led to a reassessment of these findings (Kastner et al., 1993) and the suggestion that both ethane and propane formed from isotopically different precursor kerogens (terrigenous vs. marine) in situ at low temperature (Taira et al., 1991).
In a more detailed study of the origin of light hydrocarbons at Site 808, carbon isotopic ratios for propane in gas samples collected from the frontal thrust showed similar values to those measured below the décollement (Berner and Faber, 1993). These data are the first indication that hydrocarbons from beneath the décollement are indeed related to those at shallower depth and that migration of fluids along the frontal thrust has occurred (Berner and Faber, 1993). The observation of the same high ratios of propane over ethane in the sediments at Site 1174, coupled with the isotope data for Site 808, strengthens the evidence for migration. Future investigations of the nature of the organic matter in sediments collected at Site 1174 will allow for a more detailed assessment of the organic matter and the biological and physical properties that are found within the Nankai accretionary prism.
Organic geochemical analysis at Site 1174 leads to the following conclusions: