As part of the routine geochemical analyses, monitoring of volatile hydrocarbons was conducted for safety considerations, and some organic, petrological, and geochemical studies were carried out to obtain additional information about the sources and formation mechanisms for the organic matter and hydrocarbons observed in the sediments at Site 1175.
At Site 1175, 44 sediment samples were collected at ~10-m intervals from 5.0 to 406.5 mbsf. Sediments were analyzed for methane concentrations and light hydrocarbon compositions during headspace analyses (Fig. F21A; Table T13) (see "Organic Geochemistry" in the "Explanatory Notes" chapter for analytical procedures). In addition, inorganic carbon (carbonate), TOC, N, S, and H analyses were performed (Fig. F21B; Table T14).
In general, the sediments from Hole 1175A have low inorganic carbon values (0.11-4.59 wt%; average = 2.10 wt%) and carbonate concentrations up to 40 wt%. The very young age of the sediments (late Pliocene; ~2 Ma) and low temperatures encountered in Hole 1175A are responsible for the immature organic matter and low production of hydrocarbons observed at this site. The highest carbonate concentrations are found between 120 and 280 mbsf (~10%-40%) followed by a gradual decrease to <1 wt% at ~440 mbsf, the base of the hole (Fig. F21B). The high methane concentrations and weight percent carbonate profiles correlate between 100 and 300 mbsf, and both gradually decrease with depth to the bottom of the hole.
Headspace gas concentrations of methane through the first 11.7 mbsf in the sulfate reduction zone are low (2.1-4.3 ppm) (Fig. F21), increase sharply in concentration just below this interval at 18.2 mbsf (6018-558,612 ppm), and remain high through 307 mbsf. Methane concentrations decline over the remaining 99 m of depth, ranging from 3785 to 10.7 ppm near the bottom of the hole. Only trace amounts of ethane (0.5-2 ppm) were detected in a few samples, indicating that the sediments encountered at Hole 1175A are highly immature and that little, if any, generation of hydrocarbons can be attributed to thermal maturation processes. Thus, all of the methane throughout Hole 1175A sediments is bacterial in origin (Claypool and Kvenvolden, 1983).
Organic geochemical analyses at Site 1175 lead to the following conclusions: