Only six samples were obtained from Hole 1108B for microscopic determination of total bacterial populations. One sample was taken from the surface core, Core 180-1108B-1R, at 0.99 mbsf; poor core recovery (<4%) then precluded further sampling until Core 10R at a depth of 83.16 mbsf. Below this depth an increasing degree of induration meant that further samples were obtained from only 132.04, 150.07, 164.54, and 189.06 mbsf before microbiological sampling ceased.
Bacteria were present in all samples examined (Table T9; Fig. F39); their numbers decreased rapidly with increasing depth. In near-surface sediment, total bacterial numbers were 2.67·108 cells/cm3 at 0.99 mbsf. Total organic carbon at 1.14 mbsf was 0.23 wt% (Table T4). Near-surface bacterial populations in Hole 1108B are similar to those at other sites with similar overlying water depths and near-surface organic carbon concentrations (Table T10).
Geochemical data provide evidence for bacterial activity in Site 1108 sediments. Pore-water sulfate was 75% depleted by 83 mbsf in Hole 1108B (Fig. F35), whereas methane concentrations increased rapidly below 121 mbsf (Fig. F37). Depletion of sulfate allows methanogenic bacteria to outcompete sulfate-reducing bacteria for available carbon sources. Furthermore, C1/C2 ratios indicate that gas production in at least the upper 350 mbsf of sediments at Site 1108 is biogenic (Table T8).
A peak in total bacterial populations, and in the number of dividing and divided cells, was observed at 132 mbsf (Fig. F39) coincident with a peak in TOC at 131.39 mbsf (1.1%; Fig. F38). The frequency of dividing and divided cells peaking at 17.6% in this depth horizon is indicative of an active bacterial community. Below this depth, organic carbon concentrations were very low (<0.02 wt% between 150 and 165 mbsf; 0.38 wt% at 189 mbsf) in a zone of calcium carbonate sandstones (17-48 wt% calcium carbonate; Table T4). Total bacterial numbers in Hole 1108B sediments were lower than the general model for bacterial populations in deep-sea sediments (Parkes et al., 1994), although still within the 95% prediction limits. Low bacterial numbers were also characteristic of low organic carbon carbonate deposits in sediments recovered from the Japan Sea during Leg 128 (Cragg et al., 1992) and the Lau Basin, Leg 135 (Cragg, 1994).
Only one whole-round core was obtained for microbiological analysis in Hole 1108B; further shore-based processing is planned to measure potential bacterial activities and cultural microorganisms.
Samples taken at Site 1108 represent the first RCB core samples analyzed for bacterial populations, and thus concerns about contamination of samples during the RCB coring process, for example by drilling mud, need to be addressed. Although acridine orange staining and subsequent epifluorescent microscopic examination of a sepiolite drilling mud sample showed that particles of sepiolite fluoresce green, as do bacterial cells, sepiolite particles and cells are easily distinguished visually. The results of microscopic enumeration of both total and dividing and divided bacteria correlate well with previously published results, which suggests that cores obtained by RCB are comparable to APC and XCB cores for microbiological studies.