Studies of organic matter in Leg 180 sediments show that formation temperatures have been insufficient to cause an increase in vitrinite reflectance levels; the variations in reflectance relate mainly to tissue type (Cook and Karner, this volume). This suggests that formation temperatures, even in the deeper samples, have not exceeded ~50°C. This is consistent with the results of Katz (this volume), who studied the organic geochemistry of samples from Sites 1108 and 1109. Organic carbon and pyrolysis gas chromatographic data indicate that there is no significant source rock potential at Site 1108, although sufficient organic matter (0.6 ± 0.5 wt%) is present to explain the limited gas present within the recovered cores. The decrease in methane/ethane ratios with depth is not associated with an increase in gas abundance (Shipboard Scientific Party, 1999) but can be attributed to a continuum of microbial (and possibly low-temperature [<75°C] nonbiological) processes (Katz, this volume). Katz concludes that the hydrocarbons encountered during drilling at Site 1108 appear to be indigenous and not a migrated product or contaminant, suggesting that the location can be revisited safely.
In contrast, the organic-rich (1.5-33 wt%) lagoonal facies near the base of Site 1109 contains zones with significant hydrocarbon potential. However, several independent lines of evidence indicate that it is thermally immature. This evidence includes pyrolysis thermal maturation indexes, the relative abundance and overall composition of the bitumen (or total organic extract), the nature of the saturated hydrocarbon fraction gas chromatograms, and the vitrinite reflectance levels (Katz; Cook and Karner, both this volume). Likewise, Mather et al. (this volume), using purge-trap adsorption gas analysis, detected increased concentrations of hexane and 2-methyl- and 3-methylpentane in this facies at both Sites 1109 and 1115. They ascribe this formation of branched low molecular weight hydrocarbons to low-temperature bacterial processes, possibly enhanced at these levels by the more terrigenous nature of the organic matter (with potentially more isoprene polymers than marine-derived organic matter).
New sterile, anoxic sampling techniques for handling indurated (rotary cored) sediments allowed Leg 180 scientists to set the current subseafloor depth record for the known biosphere. Culturable anaerobic bacteria and realistic rates of anaerobic bacterial activity (sulfate reduction, methanogenesis, and thymidine incorporation) are present in the deepest samples from the subseafloor biosphere analyzed to date. Microbial populations occur at 842 mbsf at Site 1118, and measurable activities were detected at 800 mbsf at Site 1115 (Fig. F9) (Shipboard Scientific Party, 1999; Wellsbury et al., this volume, submitted [N1]).
Depth profiles of sulfate, ammonia, methane, and alkalinity indicate that bacterially mediated oxidation of organic matter affects the chemistry of pore fluids from Sites 1109, 1115, and 1118 nearly to the bottom of each drill hole (Fig. F9) (Shipboard Scientific Party, 1999). In addition, the presence of sulfate deep within Site 1118 as well as reversals in the 18O and Sr isotope depth profiles indicate that fluid flow is significant there (DeCarlo et al., this volume). The other dominant chemical reactions common to the Woodlark Rise sites are diagenesis of metastable biogenic carbonates and alteration of volcanic matter (in the sediments and basement) to authigenic clays (i.e., smectite).