At Site 1115, the shipboard organic geochemistry consisted of determinations of total organic carbon, inorganic carbon, total carbon, total nitrogen, and total sulfur, in addition to the routine hydrocarbon gas safety monitoring procedure. The analytical techniques used are outlined in "Organic Geochemistry" in the "Explanatory Notes" chapter.
Headspace methane concentrations for Site 1115 displayed a profile with two distinct highs (Fig. F46). Very low methane concentrations (~2-4 ppmv) were found between 0 and 210 mbsf. Below this depth, methane levels increased rapidly to ~1200 ppmv and generally remained between 1000 and 3200 ppmv to 448 mbsf. From 448 mbsf, the methane concentration gradually dropped to 2-3 ppmv at ~515 mbsf (Fig. F46), only to rise again to >15,000 ppmv by 584 mbsf; they remained generally between 20,000 and 45,000 ppmv until the bottom of the hole (798 mbsf). Ethane (C2) was the only other hydrocarbon consistently detected. Increased C2 concentrations coincided with the two regions of elevated methane concentration, but remained low (<10 ppmv) throughout the hole (Table T8; Fig. F46). For this reason, the C1/C2 ratio remained high (>1000) throughout the Hole (Fig. F46).
The increase in methane below 210 mbsf coincided with the disappearance of sulfate in the pore water (see Fig. F44A), indicating that the methane in the upper interval of high concentration is bacterially derived. The origin of methane in the lower interval is less certain and could be either biogenic, as suggested by the corresponding increase in NH4+ at 600 mbsf (see Fig. F44A), or thermogenic in origin.
Organic carbon and sulfur contents at Site 1115 were both found to be low, averaging 0.34 and 0.35 wt%, respectively. The peaks in both carbon (3.84 wt%) and sulfur (6.52 wt%) at 563 mbsf correspond to an organic-rich claystone that is probably partially pyritized (see "Lithostratigraphic Unit VIII"). The few available C/N ratios show that the organic carbon has a mixed marine and terrigenous source with periods of predominantly terrigenous organic material (Table T9; Fig. F47). The CaCO3 concentrations are shown in Table T9 and Figure F47.