Samples for microbiological analysis were obtained from Hole 1173A. Twenty-six samples were obtained for direct microscopic examination on board ship. Eight whole-round samples were taken for shore-based microbiological analysis to measure potential bacterial activities, culture microorganisms, extract DNA, and investigate fatty acid biomarkers.
Bacteria were present in 21 samples (Table T18; Fig. F30) to 500 mbsf. The near-surface sample (Sample 190-1173A-1H-1, 139-140 cm) contained 7.23 × 107 cells/cm3. A population of this size at Site 1173 follows a trend observed at other ODP sites where near-surface bacterial populations decrease as overlying water depth increases (Table T19).
The deepest sample containing bacteria was at 499.19 mbsf, with a local maximum of 1.82 × 106 cells/cm3, some 2.5% of the near-surface population. Below this, the five further samples to 672.84 mbsf contained no detectable bacterial cells (detection limit = 4.75 × 105 cells/cm3).
The depth distribution of total bacterial numbers in sediments from Site 1173 conforms to the general model for bacterial populations in deep-sea sediments (Parkes et al., 1994) only in the upper section of Hole 1173A from the surface to ~250 mbsf (Fig. F30). The bacterial profile departs significantly from the average line in samples at 43, 60.5, and 79.5 mbsf. Below 250 mbsf, rising temperatures affect interpretation of this data. The temperature at 250 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 overall by a factor of 7. At ~460 mbsf, the temperature is estimated to be 80°C, which represents another microbiological boundary where the hyperthermophilic (very high temperature) bacteria are found. In the single datum in the hyperthermophilic zone with bacteria present (500 mbsf), population size surprisingly increased by a factor of 13. Procedural contamination was investigated and eliminated, and bacteria in a sample of drill fluid (surface seawater) were enumerated and also eliminated as a cause of contamination, with only 1.4 × 105 cells/cm3. Thus, the high bacterial population at 499.19 mbsf appears to be real.
Interpretation of the microbiological data is complicated by the sediment geochemistry (see "Organic Geochemistry" and "Inorganic Geochemistry") and a lithofacies change at 343 mbsf (see "Lithostratigraphy"). Sulfate is rapidly removed in the upper few meters and is completely depleted at 6.8 mbsf, remaining at, or near, zero until 240 mbsf (Fig. F31). Thereafter, it steadily increases to a maximum of 10 mM at 634 mbsf. This reappearance of sulfate begins at the same depth as the mesophile/thermophile transition. Between 6.8 and 240 mbsf, significant quantities of methane are present with a maximum of ~30,000 ppmv between 79 and 85 mbsf, with a broad band generally exceeding 12,000 ppmv between 8 and 108 mbsf. This coincides with the observed increase in bacterial numbers in the upper sediment layers. Evidence of bacterial activity and organic carbon degradation within this zone is provided by significant increases in both alkalinity and ammonium over the same depth range. This activity is fueled by TOC, which has a maximum concentration of 0.85 wt% at 14.53 mbsf and averages 0.54 wt% between the surface and 107 mbsf. Below 107 mbsf, TOC decreases to an average of 0.31 wt% to 343 mbsf and thereafter to an average of 0.23 wt% to the base of the hole.
The lithofacies change at 343 mbsf represents a transition to a sediment with a different geochemistry consisting of low TOC, low but increasing sulfate, negligible methane, low alkalinity and ammonium (indicative of low bacterial activity), and reduced chloride. Here, unsurprisingly, bacterial numbers are lower than average (Fig. F30) except for the population at 500 mbsf. This depth coincides with the highest TOC concentration below 343 mbsf (0.32 wt% at 489 mbsf). It is likely that the significant increase in bacterial numbers at this depth is related to the locally high TOC, rising sulfate concentrations (<5 mM), rising (possibly thermogenic) methane concentrations, and increasing hydrogen concentrations. It is also possible that acetate released by the heating of sediments (Wellsbury et al., 1997, 2000) may be fueling these bacteria. This may represent an isolated hyperthermophilic sulfate-reducing bacterial population. Bacterial sulfate reduction and methanogenesis, plus determination of interstitial water acetate concentrations, form part of the shore-based work that will test this hypothesis.
Apart from the upper 7 m, the microbiology of Hole 1173A appears to be dominated by methane. A correlation (from below the sulfate-reduction zone; 0-7 mbsf) between total bacterial numbers and methane concentration was extremely significant (r = 0.887; N = 19; P = <0.001) (Fig. F32). It is interesting to note that the small but persistent increase in methane from 10 to 180 ppmv between 460 and 557 mbsf occurs below the last record of bacterial presence, providing circumstantial evidence for a thermogenic source.
Tracer tests were conducted while coring with APC (Cores 190-1173A-4H and 5H) and XCB (Cores 190-1173A-28X and 29X) 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., 2000a).
Perfluoro(methylcyclohexane) was used as the 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 of each core, indicating successful delivery (Table T20). The PFT was found in the centers of nine of twelve sections of the APC cores examined (Fig. F33). Estimates of drilling fluid intrusion in these samples ranges from below detection to 0.86 µL/g. Eight of the twelve samples taken midway between the center and the core liner tested positive for the presence of PFT and yielded estimates from below detection limit to 1.51 µL/g. These values are very similar to previous estimates of APC-cored sediments (Smith et al., 2000b). Drilling fluid intrusion into the center of XCB cores were all below the detection limit (Fig. F34). The tracer was detected in three of ten samples collected midway from the center to the core liner. The intrusion of drilling fluid in the three positive samples ranged from 0.03 to 0.14 µL/g. This is the first time PFTs have been used while collecting cores with the XCB.
Fluorescent microspheres were detected on the outside of all cores, indicating successful delivery (Table T21). Twelve sections of APC-cored sediment were examined for the presence of the fluorescent microspheres. They were observed in one sample collected from the center of a core (Section 190-1173A-4H-3) and in two samples collected midway between the center and the core liner (Sections 190-1173A-4H-3 and 5H-2). Microspheres were shown not to penetrate into APC-cored material in previous experiments (Smith et al., 2000b). Ten sections of XCB-cored sediment were examined for the presence of microspheres. The center samples from six of ten sections examined from both the centers of the cores and the midway samples contained microspheres. This is the first time fluorescent microsphere tracers have been used during the collection of cores with the XCB.
The results from the two tracers in the XCB cores are inconsistent. Whereas the PFT was not detected in the centers of the cores, microspheres were present. Because the microspheres are relatively large compared to the chemical tracer, they should not penetrate into the interior of the core as readily as the chemical tracer. Because both tracers were analyzed on the exact same samples, this discrepancy can not be explained by differences in sample handling. Examination of XCB cores at future sites will determine whether this discrepancy persists.