At Site 1109, the shipboard organic geochemistry 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.
Headspace methane concentrations for Site 1109 displayed a typical downhole profile. Very low methane concentrations (~2-4 ppmv) were found between 0 and 100 mbsf. Below this, methane levels increased rapidly and generally remained between 1000 and 10,000 ppmv to 620 mbsf. Subsequently, the methane concentration gradually dropped to 3-6 ppmv below ~710 mbsf (Fig. F72). Ethane (C2) was the only other hydrocarbon consistently detected, but it remained at very low concentrations (
3 ppmv) throughout the hole (Table T12; Fig. F72). For this reason, the C1/C2 ratio remained high (>1000) throughout the Hole (Fig. F72). The increase in methane below 100 mbsf coincided with the disappearance of sulfate in the pore water (Fig. F70A;
also see "Inorganic
Geochemistry"). This, coupled with low concentrations of C2 and the high C1/C2 ratio downhole (>1000), indicates that the gas is of biogenic origin.
The abundance of CaCO3 was found to correlate well with the lithology (see "Lithostratigraphy"). The only exception to this is the outlying point at ~160 mbsf that represents a very thin carbonate layer unlike the major lithology of the unit. The sulfur abundance was very low throughout the hole, averaging ~0.16 wt% (Table T13). The organic carbon content was also low throughout the core, averaging ~0.42 wt%, although peak concentrations of 2.73 and 1.10 wt% were observed at 218 and 702 mbsf, respectively (Fig. F73). These occur in lithostratigraphic Units III and VIII, which were both found to contain wood fragments. The peaks correlate with a high C/N ratio, indicating a terrestrial source. Below 300 mbsf, a gradual transition is seen from a mixed marine and terrigenous source to a dominantly marine source by 500 mbsf. This extends to ~630 mbsf, where there is an abrupt increase in the C/N ratio to >20, indicating terrestrially derived carbon. This change is consistent with the change in the lithologies with lithostratigraphic Units VII and VIII representing successively shallower water deposition. More terrestrial input was experienced downhole (>630 mbsf) through this transition.
Two coal samples were collected for analysis from the base of litho-stratigraphic Unit VII. These (Samples 180-1109D-38R-1, 130-131 cm, and 38R-2, 82-83 cm) were found to have organic carbon contents of 43.81 and 13.95 wt% and sulfur contents of 6.73 and 7.77 wt%, respectively (Table T13). These data have not been plotted in Figure F73.
