At Site 1108, the shipboard organic geochemistry analyses consisted of determinations of total organic carbon (TOC), inorganic carbon, total carbon, total nitrogen, and total sulfur in sediments in addition to the routine hydrocarbon gas safety monitoring procedure. The analytical techniques used are outlined in "Organic Geochemistry" in the "Explanatory Notes" chapter.
Because of poor sediment recovery in the top section of Hole 1108B, the headspace hydrocarbon gas profiles were poorly resolved between 0 and 100 mbsf (Table T8; Fig. F37). Very low methane concentrations (~3 ppmv) were found between 0 and 72 mbsf. Below 120 mbsf, the methane levels increased to ~1000 to 60,000 ppmv and remained relatively constant throughout the rest of the hole to the last sample measured at a depth of 476 mbsf. Small quantities of ethane and propane were also detected below 120 mbsf and increased with depth (Fig. F37). Below ~400 mbsf, traces of branched and straight chain C4 and C5 components were detected. The increase in methane below 120 mbsf coincided with the disappearance of sulfate in the pore water (Fig. F35). This is indicative of biogenic methane. Traces of higher chain volatile hydrocarbons, together with the decrease in the C1/C2 headspace ratio below ~300 mbsf, indicated that a thermogenically derived gas input was present, either being generated in situ or by migration from source.
Concentration of CaCO3 was found to be generally below 10 wt% throughout the core, with the exception of the surface and peaks at ~150, 250, and 475 mbsf (Fig. F38). These correlate with the observed lithology. Sulfur was very low throughout the hole, averaging ~0.2 wt% (Table T4). Organic carbon was also low throughout the core, averaging ~0.5 wt%, although a peak (1.1 wt%) was seen at 131 mbsf (Fig. F38). The C/N ratio ranged mainly between 8 and 20 throughout the hole, indicating a mixed terrigenous and marine origin for the organic matter. The peak of 34 at 131 mbsf indicates a terrigenous source for the organic material. This, accompanied by the peak in organic carbon (1.1 wt%), correlates with the occurrence of fibrous material thought to be wood recovered in Section 180-1108B-15R-CC (see "Lithostratigraphy"), which could explain these excursions.