Eighteen samples for microbiological analysis were obtained from Hole 1175A for direct microscopic enumeration 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 all 18 samples (Table T15; Fig. F22). The near-surface sample (Sample 190-1175A-1H-1, 116-117 cm) contains 6.97 × 107 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), although in this instance, the population is slightly smaller than expected.
The deepest sample is 399.99 mbsf (Sample 190-1175A-43X-2, 89-90 cm) with 3.59 × 105 cells/cm3, representing 0.5% of the near-surface population. Bacterial populations decline rapidly from the near surface, consistent with the decrease in sulfate concentrations (see "Inorganic Geochemistry"), and then increase below 14.6 mbsf, consistent with increases in methane concentrations (see "Organic Geochemistry"). The sample at 50.8 mbsf (Sample 190-1175A-6H-4, 112-113 cm) is notable in that it contains 7.28 × 107 cells/cm3 (i.e., slightly more bacteria than the near-surface sample). This is followed immediately with almost the lowest population enumerated of 3.71 × 105 cells/cm3 at 59 mbsf (Sample 190-1175A-7H-3, 134-135 cm).
The depth distribution of total bacterial numbers in sediments from Site 1175, apart from at the two depths noted in the above paragraph, conforms unusually well to the general model for bacterial populations in deep-sea sediments (Parkes et al., 1994) from the surface to 400 mbsf (Fig. F22). The unexpected uniformity of bacterial populations between 78 and 299 mbsf (Samples 190-1175A-9H-3, 134-135 cm, to 32X-5, 121-122 cm) may be due to lithostratigraphic factors, as there was significant chaotic stratification to ~220 mbsf and many turbidites between 220 and 300 mbsf (see "Lithostratigraphy"). Uniform bacterial populations with depth, such as here, have been observed in the Amazon Fan (Cragg et al., 1997) where massive turbidites were cored (Flood, Piper, Klaus, et al., 1995).
There is a strong positive correlation between bacterial population size and methane concentration (R2 = 0.68; N = 15; P = <0.005) on data below the sulfate-reduction zone (below 14.5 mbsf). However, as the data for both bacterial numbers and methane concentration vary little with depth (Fig. F23), this correlation may not be particularly valid. Nevertheless, there are two pertinent observations of this relationship: (1) the two anomalous bacterial data in the upper part of the core at 50.8 and 59 mbsf (Samples 190-1175A-6H-4, 112-113 cm, and 7H-3, 134-135 cm) exactly match the maximum observed methane concentration and a low concentration trough, respectively, and (2) below 300 mbsf, both bacterial populations and methane concentrations decrease together.
Tracer tests were conducted while coring with APC (Cores 190-1175A-5H and 6H) at this site. 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 is equivalent to 0.01 µL of drilling fluid. The tracer was detected on the outer edge of four of the six sections of core examined (Table T17). The PFT concentration in water collected on the catwalk from the top of the Core 190-1175A-6H was ~17 µg/L, confirming the delivery of the tracer to the drill bit. This value is 17-fold higher than the target concentration of 1 µg/L, possibly because of a mismatch between the tracer injection rate and the mud pump rate. The low concentration of the tracer in the samples from the outer edge of the core, despite the high concentration of PFT in the drill string, demonstrates that very little drilling fluid is entrained with the core liner when the APC is fired into the formation. Estimates of drilling fluid intrusion into the interior of the cores range from below detection to 0.02 µL/g (Table T18).
PFT content of aliquots of interstitial water collected in the chemistry laboratory from Sections 190-1175A-5H-2, 5H-5, and 6H-4 were all below the detection limit.
Fluorescent microspheres were detected on the outside of four of the six sections examined in Core 190-1175A-5H (Table T18). Interestingly, no microspheres were observed in the first section of either core. This was observed in previous cores and suggests that the bag containing the microspheres may not rupture immediately as the core enters the liner. Microspheres were observed in samples collected midway between the core liner and the center of the core in Sections 190-1175A-5H-1 and 6H-3, which is consistent with the PFT data (Table T17). No microspheres were observed in the samples taken from the center of the cores examined at Site 1175.