Twenty-three samples were obtained from Hole 1177A for direct microscopic enumeration of bacteria aboard ship. Twenty-one 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 but 2 of the 23 samples (Table T16; Fig. F21), and populations are all below the general model for bacterial populations in deep-sea sediments (Parkes et al., 1994). Between 303 and 381 mbsf (Samples 190-1177A-1R-2, 106-107 cm, to 9R-3, 109-110 cm), bacterial populations slowly decline by ~60% from 1.14 × 106 to 4.28 × 105 cells/cm3. There is a general trend of population increase (by a factor of 4.5) to 1.92 × 106 cells/cm3 at 675 mbsf (Sample 190-1177A-40R-1, 30-31 cm). A persistent and statistically significant (t = 4.54; N = 6; P = <0.02) increase at this depth is unexpected and presumed to relate to in situ conditions.
At 687 mbsf (Sample 190-1177A-41R-2, 99-100 cm), no cells were detected. This particular core also proved anomalous with respect to some aspects of geochemistry (see "Organic Geochemistry" and "Inorganic Geochemistry") for reasons that are currently unclear. At the next depth sampled (725 mbsf; Sample 190-1177A-45R-2, 98-99 cm) bacteria are again present, although population sizes are low and remain low to 774 mbsf (Sample 190-1177A-50R-2, 147-148 cm). Between 791 and 811 mbsf (Samples 190-1177A-52R-1, 33-34 cm, to 54R-2, 45-46 cm), bacterial populations are barely detectable, and they were not detected at all in the deepest sample at 830 mbsf (Sample 190-1177A-56R-1, 149-150 cm).
At previous sites that have been sampled for microbiology during Leg 190, a relationship is sometimes observed between bacterial populations and in situ methane concentrations (see "Microbiology" in the "Site 1173" chapter and "Microbiology" in the "Site 1175" chapter). This is not true at Site 1177. Between ~400 and 740 mbsf, high concentrations of sulfate are present in the interstitial water (IW), reaching almost seawater concentrations between 500 and 675 mbsf (see "Inorganic Geochemistry"). Methane is only present in significant quantities either above 340 or below 740 mbsf (see "Organic Geochemistry"). However, there is a strong correlation (R2 = 0.737; N = 14; P = <0.005) between bacterial population size and IW sulfate concentration. Bacterial numbers increase from ~380 mbsf, slightly above but paralleling the increase in sulfate concentrations that initiates from ~400 mbsf (Fig. F22). This may be the result of bacterial sulfate reduction partially depleting sulfate above 500 mbsf. Bacterial populations remain high to ~740 mbsf, where they decrease at a depth slightly below that of the decline in sulfate concentration. It is notable that the anomalous zero bacterial enumeration is mirrored in a very low anomalous sulfate concentration at 687 mbsf.
The presence of such high concentrations of sulfate so deep in the sediment is puzzling when bacteria are present at ~106 cells/cm3 at the same depths. Because the age of the sediments that contain this zone of elevated sulfate ranges from ~3 Ma at the top to 13 Ma at the base (see "Biostratigraphy" and "Paleomagnetism"), there has been sufficient time for considerable amounts of sulfate removal by bacteria. The continued presence at high concentration implies that there is either replenishment of sulfate by fluid flow or that TOC concentrations are too low to support rapid bacterial sulfate reduction. Sulfate reduction rates will be addressed by shore-based work; however, low TOC concentrations are the likely cause of high sulfate concentrations. TOC is low for much of Hole 1177A, particularly within the elevated sulfate zone, not exceeding 0.4 wt% (and generally <0.3 wt%) between 381 and 590 mbsf (see "Organic Geochemistry"). High TOC is only present within the elevated sulfate zone at 608 mbsf (0.81 wt%) and 695 mbsf (1.62 wt%), and the latter, curiously, is coincident with the anomalous bacterial population and sulfate concentration lows. However, visual inspection of the cores indicates that much of this TOC is present in the form of pieces (up to 3 cm) of wood. This is a recalcitrant material for bacterial degradation and additionally implies that the TOC is heterogeneously distributed in the sediment with the vast majority of the sediment being very TOC poor. Under these conditions it is not surprising that the IW sulfate concentrations remain elevated.
The chemical tracer test was conducted during RCB coring of Cores 190-1177A-24R and 25R. The particulate tracer was used during RCB coring of Core 190-1177A-6R. 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 concentrations in water collected on the catwalk from the top of Cores 190-1177A-24R and 25R were ~23 and 330 µg/L and confirmed delivery of the tracer to the drill bit. These values are higher than the target concentration of 1 µg/L, and the measured values were used to calculate the estimated drilling fluid intrusion. Estimates of drilling fluid intrusion into the interior of the cores range from below detection to 0.07 µL/g (Table T17).
Fluorescent microspheres were detected on the outside of all three sections examined in Core 190-1177A-6R (Table T18). Sections 190-1177A-6R-2 and 6R-3 contained microspheres throughout the core.