Monitoring of volatile hydrocarbons was conducted for safety considerations, 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 accreted trench-slope sediments.
Forty-four sediment samples and two vacutainer samples were collected at Site 1176 (~10-m intervals) from 4.5 to 401.6 mbsf (Table T13). All sediments were analyzed for methane concentrations and light hydrocarbon compositions during headspace analyses (Fig. F17A). In addition, molecular gas compositions, TOC, inorganic carbon (carbonate), and sulfur analyses were performed (Fig. F18; Table T14) (see "Organic Geochemistry" in the "Explanatory Notes" chapter for analytical procedures).
TOC contents for the sediment samples examined ranged from 0.05 to 2.25 wt% (Fig. F18). The highest TOC value was found in a sample collected at 340 mbsf (2.25 wt%) that was characterized by a high content of terrestrial material observed in the form of plant detritus (fan debris flow) in the sediment. The average TOC content for sediments examined between 4.5 and 393 mbsf was ~0.50 wt%. Nitrogen contents for the same sample interval were extremely low (average = 0.03 wt%) with one spike at 200 mbsf (~0.12 wt%). Sulfur contents had a similar trend to TOC values, ranging between 0 and 2.07 wt%, with the highest S values (1.05 and 2.07 wt%) coinciding with the highest TOC values (0.86 and 2.25 wt%).
Inorganic carbon values (0.05-2.8 wt%) are similar in range to those at Site 1175 and correspond to bulk carbonate values of 0.42-23.32 wt%, with some values up to 36.78 wt%; however, the average inorganic carbon values were higher in the upper 65 mbsf at Site 1176, ranging between 0.69 and 2.09 wt%. The only exception was in a sample collected from near the bottom of the hole that corresponded to the highest values for inorganic carbon and carbonate (4.4 and 36.65 wt%, respectively). In general, carbonate contents were highest in the upper ~200 mbsf and then decreased sharply over the remaining depth to just over 0.5 wt%, a trend also observed in Hole 1175A (Fig. F17A, F17B).
Headspace gas concentrations of methane were very low in the first 11.9 mbsf within the sulfate reduction zone (7.3-11.7 ppm) (Fig. F17A, F17B), also similar to Site 1175. An increase in concentration above this section, from 3377 to 36,920 ppm, was observed over the next 135 m of the hole. Methane concentrations remained moderately high (~8000-20,000 ppm) down to ~285 mbsf before dropping steadily over the remainder of the hole, with values dipping to 5.6-30.1 ppm at ~401.6 mbsf. The very low concentrations of methane in the lowest 30 m of the hole may be partially due to low core recovery and the lithologies encountered (mostly poorly consolidated sand and cobble-sized gravel) in the last few cores. As at Site 1175, the hydrocarbons encountered at Site 1176 are dominated by biogenic methane, with only trace levels of ethane (0.3-1.5 ppm) detected in a few samples.
Organic geochemistry analyses performed on samples from Site 1176 lead to the following conclusions: