Sediment samples from Holes 1179B and 1179C were collected to characterize both the chemistry and the microbial activities in this environment. Microbial activities will be inferred from incubation experiments and from shore-based lipid analyses. As indicated in "Microbiology and Inorganic Geochemistry" in the "Explanatory Notes" chapter, whole-round cores (WRCs) were collected from four different depths (5, 30, 100, and 200 mbsf) for incubation experiments. WRCs were collected from all of the depths indicated in Table T4 for analyses of bacterial lipids. Because certain cell membrane lipids are diagnostic of particular groups of bacteria, the lipid analyses may reveal the presence of different types of bacteria within the sediment column. Interstitial pore waters were also collected for chemical analyses from the same approximate depths as samples taken for lipids in order to relate the microbial communities with the geochemistry of the sedimentary environment. Whole rounds cut on the catwalk were also sampled on two occasions for contamination tests using perfluorocarbon tracers. A list of all of the samples taken at Site 1179 for geochemical and microbiological analyses are shown in Table T4.
Compounds whose concentrations in pore waters are strongly influenced by bacterial activities include sulfate, methane, dissolved iron(II), dissolved manganese(II), nitrate, ammonia, and phosphate. Of these, the first five are generally associated with successive changes in redox chemistry that accompany the anaerobic degradation of organic carbon by bacteria. Measured changes in the redox chemistry can help define the environments in which particular groups of anaerobic bacteria reside. For example, once oxygen becomes consumed with depth in the sediments during oxidation of organic matter, the subsequent disappearance of nitrate usually reflects a zone of denitrification. Beneath this zone, the appearance of dissolved Mn(II) followed by Fe(II) in pore waters generally identifies the suboxic zone where Mn(III, IV)-reducing and Fe(III)-reducing bacteria exist, respectively. Deeper yet, in more reduced sediments, the appearance of sulfide and then methane identifies increasingly anaerobic regions where dissimilatory sulfate reduction and methanogenesis occurs. Therefore, measured changes in the pore-water concentrations of these various compounds provide a useful diagnostic tool for identifying the habitats of these anaerobic bacteria. Most of the pore-water measurements, down to a depth of 221 mbsf, were made in lithostratigraphic Unit I (see "Sedimentology") and correspond to a similar sequence sampled from 0 to 115 mbsf during Leg 185 at Site 1149 (Plank, Ludden, Escutia, et al., 2000). The bottom two samples of Hole 1179C, at 250 and 278 mbsf, were composed of the red pelagic clays of Unit III, which at Site 1149 corresponds to a depth interval of ~115 to 175 mbsf. For Leg 191, shipboard measurements of pore-water phosphate and nitrate concentrations were completed (Table T4) but they await shore-based analysis.
Dissolved NH4+ concentration gradually increases from 0 µM at the sediment/water interface to a maximum of ~300 µM between 90 and 100 mbsf, reflecting its release during the degradation of organic matter (Fig. F26). A second maximum of ~285 µM may exist at 62.8 mbsf, producing a bimodal peak, with a small drop in concentration at 72.2 mbsf. This drop may be real, as a bimodal peak in NH4+ also occurred at Site 1149 within lithostratigraphic Unit I (Plank, Ludden, Escutia, et al., 2000). At Site 1149, the corresponding drop in NH4+ occurred at a depth of ~45 mbsf. This drop could be due to uptake during clay diagenesis.
The gradual decline in NH4+ concentration from its maximum at ~100 mbsf to a concentration of 114 µM at 278 mbsf may be due to uptake during clay diagensis, as was previously suggested for Site 1149 (Plank, Ludden, Escutia, et al., 2000). However, at this time, one cannot rule out possible microbial consumption throughout the sedimentary column, even if the rate is very low.
The dissolved Mn(II) profile at Site 1179 revealed a very sharp and steady increase in concentration from the sediment/water interface to a depth of 6.5 mbsf, where a maximum concentration of 260 µM was measured (Fig. F27). Below 6.5 mbsf to a depth of 30 mbsf, concentrations dropped to ~100 µM. Fe(II) concentrations were much lower than Mn(II) concentrations and were more scattered (Fig. F27). However, several profile trends were similar to those of Mn(II). Fe(II) also showed a distinct maximum of ~30 µM at 6.5 mbsf, with a general decline to 31 mbsf. It is apparent from these profiles that the zones of iron and manganese reduction occurred near 6.5 mbsf and reflect the anaerobic degradation of organic carbon in surface sediments. Beneath 31 mbsf, concentrations of Mn(II) and Fe(II) both rise and fall, with a smaller subsurface maxima in Mn(II) of 128 µM at a depth of 40 mbsf. Both Mn(II) and Fe(II) show minima of 74 and 2.6 µM, respectively, at 100 mbsf. The covariance of these changes in Mn(II) and Fe(II) concentrations between 30 and 100 mbsf suggest that they may reflect dynamic redox processes rather than simple analytical artifacts. Although the Mn(II) concentrations are relatively consistent from 100 to 196 mbsf, showing only a slight increase to 109 µM at 167 mbsf, Fe(II) showed a pronounced subsurface maximum of 25 µM at 167 mbsf and a distinct minimum of 1.4 µM at 196 mbsf. Although carbon, nitrogen, and sulfur (CNS) analyses of the acidified and nonacidified sediments have not been completed, results might indicate whether the minima in Fe(II) at 196 mbsf and some of the other depths might be due to formation of Fe(x)S(y). Below 200 mbsf, Fe(II) concentrations show a moderate rise, whereas the dissolved Mn(II) shows a distinct subsurface maximum of 191 µM at 252 mbsf in the upper portions of lithostratigraphic Unit III. Near the bottom of the red clays at 278 mbsf, Mn(II) shows a distinct minimum of 7.4 µM, which agrees with results from Site 1149 (Plank, Ludden, Escutia, et al., 2000).
In general, the pore-water sulfate showed a relatively gradual decline in concentration from 29 mM near the sediment/water interface to 21 mM in the deepest sample collected at 278 mbsf within the pelagic red clays. The profile suggests a slow and gradual removal of sulfate by dissimilatory sulfate reduction throughout the sedimentary column (Fig. F28). The largest drop in sulfate appears to occur between 0 and 40 mbsf. From 40 to 196 mbsf, the decrease in sulfate concentration is small, suggesting low rates of sulfate reduction. In combination with the pore-water profiles observed for dissolved Mn(II) and NH4+, it appears that to a depth of ~200 mbsf the sedimentary column is suboxic. Concentrations of sulfate again drop more steeply between 196 and 278 mbsf. These changes in sulfate gradients with depth may reflect different zones where rates of sulfate reduction change. Confirmation of the bacterial activity associated with sulfate reduction will come from postcruise stable isotopic analyses of the sulfate collected from the pore waters and from analysis of the microbial incubation experiments.
Standard shipboard measurements of sedimentary carbonate, organic carbon, C, N, S, and H content are presented in Table T2.
Sediments collected from 4.5, 31, 101, and 196 mbsf and incubated with gas headspace were analyzed by gas chromatography (GC) after 12-14 days (t = 1) for changes in CH4 and CO2 concentrations. Vials sampled at t = 0 showed a small background concentration of methane between 10 and 13 ppmv, which was consistent for replicates prepared from each sample depth. Apparently, the methane is a trace contaminant of the anaerobic gas mixture. None of the incubation vials showed detectable changes in methane, indicating that neither methane production nor consumption had occurred. In contrast, concentrations of CO2 were higher in some of the t = 1 vials relative to the t = 0 controls. The increase was most apparent in the 196-mbsf samples, which were incubated at 20°-22°C. These samples showed an approximate 15% increase in CO2 headspace concentration during the first 12-day period. Whether this increase represents microbial respiration or simply CO2 degassing will be confirmed with continued time-course measurements.
PFTs were used during the drilling of Cores 191-1179B-5H and 191-1179C-21H to test for contamination of collected sediments by drilling fluids. Sediments collected were immediately stored in a 3°C refrigerator and were analyzed by GC/electron capture detector (ECD), although the data awaits further analysis. However, several PFT-methanol standards were prepared with varying PFT concentrations as indicated in previous studies (Smith et al., 2000). As control blanks, we added 10 µL of fresh methanol to the crimp-sealed 20-mL headspace vials without any PFT. For 0.5-mL injection volumes of the prepared PFT-methanol standards that had been stored at 70°C, a linear fit of the data was observed for PFT concentrations >4.4 × 10-11 g when plotted on a log-log plot (Fig F29), as previously reported (Smith et al., 2000). However, we observed that at PFT concentrations <4.4 × 10-11 g, the GC response was not linear. By subtracting the average peak height measured for the methanol control blanks from the PFT standards in this lower concentration range, a linear fit was again obtained (Fig. F29). However, the slope of this line was different from that obtained at PFT concentrations >4.4 × 10-11 g. This blank correction also improved the sensitivity of the method by approximately two orders of magnitude relative to that previously reported (Smith et al., 2000). However, it is questionable whether the curve in this lower concentration range is relevant to samples measured (which lack the methanol addition).
Using GC injections of 5 mL volume, PFT standards also provided a linear fit on a log-log plot, although the regression was consistently shifted downward relative to the curve generated from 0.5-mL injections (Fig. F29). This indicates a slight decrease in peak height and GC sensitivity for a given concentration (as total grams of PFT injected) when using the 5-mL injection volume. This might be explained by sample loss that results from withdrawing a 5-mL gas sample from a 20-mL headspace vial. Heating the vials from ~25° to 70°C effectively increases the headspace volume by ~3 mL (using the ideal gas law, PV = nRT) and creates an overpressure of 3 mL inside the vial. However, withdrawing 5 mL will still deplete ~2 mL of headspace gas from a total of 20 mL, resulting in a ~10% loss. The shift downward of the standard curve using the 5-mL injections rather than the 0.5-mL injections might be explained by this sample loss.