Thirty samples were obtained from Holes 1178A and 1178B for direct microscopic enumeration of bacteria on board ship. Seventeen whole-round cores were taken for shipboard enrichment cultures, cell viability, and shore-based microbiological analysis to measure potential bacterial activities, culture microorganisms, characterize nucleic acids, and investigate fatty acid biomarkers.
Bacteria are present in 23 of the samples (Table T15; Fig. F25), and populations are all below the general model for bacterial populations in deep-sea sediments (Parkes et al., 1994). The surface sample (Sample 190-1178A-1H-1, 0-1 cm) contains 4.75 × 108 cells/cm3, which follows a general trend observed at other Ocean Drilling Program (ODP) sites where near-surface bacterial populations decrease as overlying water depths increase (Table T16). Populations rapidly decrease in the upper few meters, and from ~72 mbsf (Sample 190-1178A-9X-5, 149-150 cm) bacterial populations deviate significantly even from the lower prediction limit. This decrease steadily continues to 272 mbsf (Sample 190-1178A-30X-4, 149-150 cm) apart from a single datum at 210 mbsf (Sample 190-1178A-24X-1, 129-130 cm) where populations showed an additional significant decrease. This was a sample from the vicinity of a small amount of methane hydrate and consisted of cold, spongy, foaming sediment. Local reductions in bacterial populations around hydrates have been previously observed at Leg 146 sites (Cragg et al., 1995).
Bacteria were not detected at 302 or 341 mbsf (Samples 190-1178A-33X-5, 122-123 cm, and 37X-5, 124-125 cm) but return to relatively high concentrations (~4 × 105 cells/cm3) between 374 and 408 mbsf (Samples 190-1178A-41X-2, 0-1 cm, and 44X-5, 124-125 cm). Although the overlap between Hole 1178A and 1178B shows differing bacterial abundance, the variability of these data indicate no significant difference. Between 396 and 498 mbsf (Samples 190-1178B-2R-2, 0-1 cm, to 190-1178A-13R-1, 112-113 cm) bacterial populations are present at, or just below, the lower prediction limit (Fig. F25). Overall, between these depths, they are present in greater numbers than between 166 and 272 mbsf (Samples 190-1178A-19X-3, 129-130 cm, to 190-1178A-30X-4, 149-150 cm). Below 500 mbsf, bacteria are either below detection or present in barely detectable numbers.
No correlation was observed with methane concentrations, which were low but relatively constant, averaging ~4100 ppmv throughout both Holes 1178A and 1178B. Between 72 and 120 mbsf, methane concentrations almost doubled to ~7500 ppmv (see "Organic Geochemistry"). Nevertheless, this increase was not reflected in any change in bacterial abundance. The reasons for the unusually rapid decline in bacterial populations remain unclear.
The chemical tracer test was conducted during APC, XCB, and RCB coring at this site (Cores 190-1178A-8H and 11X and 190-1178B-19R). The particulate tracer was used during XCB coring (Core 190-1178A-11X). In order to estimate the amount of drilling fluid intrusion into the recovered cores, chemical and particulate tracers were deployed as previously described (Smith et al., 2000).
Perfluoro(methylcyclohexane) was used as the perfluorocarbon tracer (PFT). Calibration of the gas chromatograph (HP 5890) with standard solutions yielded a slope of 9.2 × 1011 area units/gram of PFT. The detection limit for these samples was equivalent to 0.01 µL of drilling fluid. The tracer was detected on the outer edge all of sections of core examined (Table T17). Because of the hardness of the sediment in Core 190-1178B-19R, the samples were not be taken with syringes. Instead, the outer edge of the core was pared away with a hammer and chisel. Estimates of drilling fluid intrusion into the interior of the cores range from below detection to 0.2-36 µL/g for samples midway between the core liner and below detection to 0.86-0.07 µL/g in samples collected in the center of the core (Table T17).
Fluorescent microspheres were detected on the outside of all three sections examined in Core 190-1178A-11X (Table T18). Microspheres were not observed in the center samples from any of the cores.