Site 1178 is an upper slope location drilled to evaluate the chemical, physical, and structural characteristics of an older portion of the accretionary prism and possibly to sample gas hydrates. 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 75 sediment samples were collected at Site 1178 from Holes 1178A and 1178B (at ~10-m intervals) from 1.5 to 669.0 mbsf. All sediments were analyzed for their methane concentrations and light hydrocarbon compositions during headspace analyses (Table T13). In addition, molecular gas compositions, total organic carbon (TOC), inorganic carbon (carbonate), and sulfur analyses were performed (Table T14) (see "Organic Geochemistry" in the "Explanatory Notes" chapter for analytical procedures).
The TOC contents of the sediment samples examined at Site 1178 ranged from 0.57 to 1.03 wt% over the first 383.6 mbsf, with an average value of 0.73 wt% (Fig. F23). The high TOC values reflect a more mature sedimentary package and a mixture of mostly marine and some terrestrial matter (wood pieces) in this interval and range from 0.24 to 1.45 wt% with the highest values found at 200 and 350 mbsf, coincident with the highest TOC values.
The inorganic carbon values were generally low at Site 1178, with a few scattered values that ranged between 0.04 and 2.74 wt%. Thus, the bulk carbonate values were also low (1.46-5.00 wt%), except for a few samples that varied from 8.08 to 22.81 wt%, with an overall average concentration of carbonate in the sediments of ~5 wt%. In general, carbonate contents were confined to very thin layers throughout the hole with the higher carbonate concentrations found over the first 90 mbsf in the upper slope-apron facies (Subunit 1A). Because Site 1178 was the last site drilled during Leg 190, TOC, sulfur, and bulk carbonate analyses of the lower 300 mbsf were not performed.
Headspace gas concentrations of methane were moderate to low (thousands of parts per million) throughout Hole 1178A with low concentrations in the first 20 mbsf (the sulfate-reducing zone) followed by an abrupt increase in methane concentration (1152 to ~16,000 ppm) over the next 100 mbsf. Below, concentrations decrease sharply to ~1000 ppm and remain in that range to total depth. Some higher concentrations (~10,000 ppm range) were measured throughout Hole 1178A; however, the average concentration was a few thousand parts per million and remained in this range into Hole 1178B to the base of the hole at 669 mbsf.
The moderate to low concentrations of methane throughout Holes 1178A and 1178B are attributed to the elevated concentrations of light hydrocarbons from ethane to hexane, indicative of older, more mature organic matter within the sediments (diagenesis) or migration (thermogenesis) of hydrocarbons from greater depths. Ethane and propane are present in most of the sediment samples measured, increasing in concentration with depth (C2 = 0-118 ppm; C3 = 0-121.5 ppm), also consistent with an increase in maturity. Propane concentrations are higher than ethane concentrations from 250 to 408 mbsf in Hole 1178A, which may be attributed to diffusive migration of ethane from the sediments or to preferential removal of ethane from sediments by microbial processes. In Hole 1178B, the ethane to propane ratios decrease with depth and ethane exceeds propane in sediment samples from 566 to total depth at 669 mbsf, consistent with production of these hydrocarbons in more mature sediments.
The same low ethane/high propane concentrations were also observed in the shallower sediments at Sites 808 during Leg 131 (Taira et al., 1991) and at Site 1174 (see "Organic Geochemistry" in the "Site 1174" chapter). Carbon isotopic studies of ethane in sediments from Site 808 did not display significant isotopic changes in sediments from shallower and deeper (below the décollement) depths, indicating that low concentrations of ethane can not be explained by diffusion (Berner and Faber, 1993). It was concluded that ethane and propane in these sediments were likely generated from isotopically different precursors within the kerogen (i.e., terrigenous vs. marine) in the sediments. A more complete assessment of the organic matter in both Sites 1178 and 1174 sediments is needed to address these unusual concentration ratios for ethane and propane that appear to be consistent throughout the sites drilled in the Nankai accretionary system.
Site 1178 is the only location drilled during Leg 190 that had significant levels of light hydrocarbons heavier than propane. Normal butane and pentane concentrations are low in Hole 1178A sediments (0.5-7 ppm) but increase with depth in Hole 1178B sediments with concentrations up to 18.3 ppm for butane and 13.7 ppm for pentane. Isobutane and isopentane concentrations display a similar trend with values up to 13.7 and 16.7 ppm, respectively, at 566 mbsf, whereas normal hexane and isohexane concentrations are generally low (0-2 ppm). Overall, concentrations of C2-C6 hydrocarbons reflect the thermal evolution of the sedimentary organic matter at Site 1178. Since the formation reactions for lighter hydrocarbons are strongly temperature dependent, concentration and isotopic variations of these gases are sensitive indicators of the thermal maturation of organic matter. Although diagenetic processes at low temperature could explain the presence of C2-C6 hydrocarbons, the unusual C2/C3 ratios, especially in the upper 400 mbsf, coupled with increasing concentrations of butane and pentane with depth, suggest that some thermally produced hydrocarbons have migrated in and mixed with in situ biogenic hydrocarbons (mostly methane).
The Bernard ratios (C1/[C2+C3]) (Fig. F24) for hydrocarbons at Site 1178 also indicate that some contribution of the lighter hydrocarbons (C2-C6) in these sediments was produced from more mature organic matter present in situ mixed with themogenic hydrocarbons that migrated from a more mature source at depth. Methane concentrations are constant throughout and are likely derived from biogenic or microbially mediated processes. C4-C6 plot exclusively in the thermogenic zone, again suggesting that these hydrocarbons formed in more mature sediments at higher temperature. The relatively low temperatures at Site 1178 (25°-40°C), coupled with the young age (early Miocene) of the sediments drilled, also support these conclusions. Moreover, unlike at any of the other sites drilled during Leg 190, significant faulting has been observed throughout the lower 300 m of Hole 1178B, which would facilitate fluid migration of more mature hydrocarbons from greater depths to shallower sediments.
Organic geochemical analyses at Site 1178 lead to the following conclusions: