Thirty-seven samples for microbiological analysis were obtained from Holes 1174A and 1174B for direct microscopic enumeration on board ship. Fourteen whole-round cores were taken for shipboard enrichment cultures at in situ temperature and pressure, 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 24 of the 37 samples examined (Table T17; Fig. F37). The near-surface sample (Sample 190-1174A-1H-2, 99-100 cm) contains 1.47 × 108 cells/cm3, which follows a trend observed at other ODP sites where near-surface bacterial populations decrease as overlying water depths increase (Table T18).
The deepest sample in which bacteria were observed is at 796.5 mbsf (Sample 190-1174B-69R-3, 105-106 cm) with 7.95 × 105 cells/cm3, some 0.5% of the near-surface population. Prior to this depth, however, cells are not present at 578 mbsf (Sample 190-1174B-46R-4, 119-120 cm) or in five samples between 623.5 and 743.6 mbsf (Samples 190-1174B-51R-2, 149-150 cm, to 63R-5, 149-150 cm). Below 796.5 mbsf to the deepest sample at 1091.3 mbsf (Sample 190-1174B-100R-1, 11-12 cm), no bacterial cells were detected (detection limit = 6 × 104 cells/cm3).
The depth distribution of total bacterial numbers in sediments from Site 1174 conforms to the general model for bacterial populations in deep-sea sediments (Parkes et al., 1994) from the surface to ~290 mbsf (Fig. F37). Two significantly low populations are found at 26.2 and 66.5 mbsf (Samples 190-1174A-4H-2, 139-140 cm, and 8H-4, 134-135 cm), and these are both samples with substantial proportions of sand. Below 370 mbsf, rising temperatures affect interpretation of the data. The temperature at 370 mbsf was ~45°-50°C (see "In Situ Temperature and Pressure Measurements"), which is the boundary between mesophilic (medium temperature) and thermophilic (high temperature) bacteria. From this depth downward, bacterial population sizes decrease to zero by 575 mbsf. There is one further occurrence of bacteria (4.76 × 104 cells/cm3) at 598.5 mbsf (Sample 190-1174B-48R-5, 104-105 cm) before 690 mbsf. At this depth the temperature is estimated to be 80°C, which represents the microbiological boundary where hyperthermophilic bacteria are found. There are two occurrences of bacterial populations within this zone, at 778.6 and 796.5 mbsf (Samples 190-1174B-67R-4, 120-121 cm, and 190-1174A-69R-3, 105-106 cm). The deepest of these is estimated to be at 90°C. The possibility that these were contaminated samples was examined; however, the large populations enumerated meant that they could not be the result of contamination from either seawater or drilling muds. Additionally, zero counts were obtained from both above and below these intervals, and the counting procedure was checked with blanks to confirm absence of counting procedure errors. No bacteria were detected within the décollement zone (~810-840 mbsf) or at any greater depths, despite temperature not being a limiting factor until ~875 mbsf.
Tracer tests were conducted while coring with APC (Core 190-1174A-4H) and RCB (Cores 190-1174B-4R, 5R, 31R, and 32R) 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 each core, indicating a successful delivery (Table T19). Estimates of drilling fluid intrusion in these samples range from below detection to 57.9 µL/g. As expected, the intrusion of drilling fluid into the centers of Cores 190-1174B-4R and 5R (57.9 and 14.3 µL/g, respectively) was substantial. This was due to the use of the RCB in the relatively soft sediment. Visual inspection of the split cores confirmed the disturbance. No PFT was detected in the center samples from Sections 190-1174B-31R-1 and 31R-2. The chemical tracer was found throughout the Core 190-1174B-32R. It is believed that the high values are due to sample handling on the catwalk. Instead of breaking the sections after the core liner is cut, as recommended (Smith et al., 2000), the sections were separated with a knife. It is likely that the tracer was dragged through the core with the blade.
Fluorescent microspheres were detected on the outside of four cores (Table T20); the outside sample of one of the eight samples was lost before analysis. The absence of microspheres in the samples from the outside edge of the core suggests problems with the delivery of the microspheres. No microspheres were detected in the interiors of Sections 190-1174B-4H-2, 5R-1, 31R-1, or 32R-1. As with the PFT, microspheres were found throughout Section 190-1174B-4R-1, again confirming the disturbance while coring. Microspheres were not observed in the interior of the other disturbed core (Section 190-1174B-5R-1), but the very low abundance on the outside of this core (13 microspheres/g) may explain this observation. Microspheres in the interiors of Sections 190-1174B-32R-2 and 32R-3 are consistent with the PFT data from these sections.