Site 1226 provides an excellent series of samples from the sediment/water interface down to basement, including good cores from the contact zone between sediment and basalt. The geochemical gradients that span the 420-m-thick sediment column are bounded at the sediment/seawater interface and the sediment/basement interface by comparable but opposite reduction-oxidation (redox) zonations. Sulfate reduction appears to be the predominant electron-accepting pathway at this site. A broad maximum of dissolved inorganic carbon (DIC) and ammonium in the interstitial water demonstrates the mineralization of organic material throughout the sediment column at several-fold higher rates than at Site 1225. Concentrations drop steeply to near-seawater values at the sediment/water interface and less steeply toward seawater values at the contact with basement. Water flow through the basement thus provides an effective sink for sedimentary metabolic DIC.
As at Site 1225, the overall chemical zonations are consistent with thermodynamic control of electron acceptor use by subsurface prokaryotes. The data show that water flow through the underlying basaltic basement introduces electron acceptors with high free-energy yields to sediment hundreds of meters below the seafloor. Relative to Site 1225, however, these zonations are more compressed because of higher rates of prokaryotic activities. Oxygen was not detected at any sediment depth within the column at Site 1226, and any oxic surface layer of sediment must thus have been closer to the ocean interface than the depth of our first dissolved oxygen measurement in Hole 1226B, in Section 201-1226B-1H-1, 10 cm. Nitrate, however, was detected in core sections nearest to the sediment/water and sediment/basement interfaces. As at Site 1225, nitrate diffuses upward into the overlying sediment from water flowing through the basement. However, at Site 1226 the diffusing nitrate barely penetrates into the sediment column before being reduced.
As the next electron acceptor in the classical redox sequence, the interstitial water distribution of manganese shows a more complex pattern. Dissolved manganese peaks just at the sediment/water interface and again 9 m below, followed by a steep drop to a zone of near-zero concentration between 100 and 250 meters below seafloor (mbsf). Yet another distinct peak in manganese is present at 300 mbsf. At the bottom of the sediment column, manganese peaks again between 400 mbsf and the sediment/basement interface. The near-basement peak and the 300-mbsf peak together define a broad 160-m interval of unusually high dissolved manganese concentrations.
Comparison with the Leg 138 Initial Reports data indicates that the most deeply buried interval of high dissolved manganese is composed of hydrothermally influenced sediments immediately above the basement. The 300-mbsf peak is present in sediments that were deposited at low rates during the Miocene "carbonate crash." In contrast, the sediments that define the overlying interval of near-zero dissolved manganese concentrations were deposited at high rates during the 7- to 4.5-Ma "biogenic bloom" that occurred throughout much of the world ocean (Farrell et al., 1995). These results suggest that the availability of electron-accepting pathways to current subseafloor activity directly depends on broad-scale patterns of past oceanographic change. More detailed interpretation of these multiple zones of apparent manganese reduction and oxidation must await further solid-phase and interstitial water chemical analyses.
The zone of dissolved sulfide extends from just 5 m below the sediment/water interface to a depth of 280 mbsf, and the broad peak reaches 0.7 mM at ~100 mbsf. Throughout this sulfidic sediment column, iron is <10 然 but displays narrow peaks of ~40 然 just above and below the sulfide zone. This pattern reflects both the low equilibrium concentration of ferrous iron in sulfidic interstitial water and the presence of reducible iron near the sediment surface and in the deep sediment column, including a third iron peak at 380 mbsf. The interstitial water data identify a sink for both sulfide and manganese within an interface at 250-280 mbsf, where manganese may precipitate with sulfide.
Methane exhibits a broad peak at 100-250 mbsf with concentrations of 2-3 然. Although this is still a trace level of biogenic methane, it is more than tenfold higher than that at Site 1225. Sulfate is present at >68% of seawater concentration throughout the sediment column and indicates active sulfate reduction over the entire methane peak. The coexistence of methane and sulfate at these levels demonstrates the ability of methanogens to maintain an active metabolism in a high-sulfate environment where competition for energy substrates must be strong and where the methanogens may be limited to noncompetitive substrates (Oremland and Polcin, 1982; Oremland et al., 1982). The results also show that sulfate-reducing bacteria in this environment are apparently unable to exploit methane beyond the existing low concentrations.
Acetate and formate concentrations are low in the upper 0-100 m of sediment (<0.5 然). At greater depths, their concentrations increase to 1-3 然. This shift in concentrations appears to result from regulation mechanisms that are not yet understood for any sedimentary environment. The volatile fatty acids (VFAs), acetate and formate, are known to be important substrates for most anaerobic respiring bacteria and for methanogens (Winfrey and Ward, 1983; Wellsbury and Parkes, 1995). The interstitial water concentrations of these intermediate fermentation products are regulated by a balance between production and consumption. Concentrations of both acetate and formate are usually found to be relatively higher in organic-rich marine sediments, where they appear to be a function of the rate of fatty acid production and of the energy-yielding metabolism of the consumers. For example, sulfate reducers are able to outcompete methanogens in their efficiency of substrate uptake and thereby drive acetate and formate concentrations to lower levels. However, control of VFA concentrations by such competition is difficult to reconcile with their increased concentrations in the deeply buried Site 1226 sediments that exhibit high dissolved manganese and iron concentrations.
Hydrogen was very low in incubated sediment samples from Site 1226, ranging from 0.1 to 0.8 nM. This is below the equilibrium concentration of a few nanomolar measured in the sulfate reduction zone of more active nearshore sediments (Hoehler et al., 1998) and is even below the concentrations measured at Site 1225, where prokaryotic activity is significantly lower than at Site 1226. According to theoretical calculations of the minimum energy yield required for prokaryotic respiration (Thauer et al., 1977; Schink, 1997) and also according to hydrogen data from a range of sedimentary environments, equilibrium concentration of hydrogen is maintained at the lowest limit that provides the lowest required energy yield of the hydrogen-metabolizing prokaryotes (Hoehler et al., 2001). Based on the dissolved sulfate, manganese, and iron data, sulfate reduction is the predominant respiration process throughout most of the sediment, with the other electron acceptors gaining relative significance near the top and bottom of the sediment column. However, the Site 1226 hydrogen incubation concentrations are lower than those in surface sediments, where sulfate reduction is the predominant process. This finding suggests that either the Site 1226 sulfate-reducing communities derive the canonical minimum energy yield at lower hydrogen concentrations than surface sulfate-reducing communities or they utilize hydrogen at energy yields below the previously accepted theoretical limit.
Experiments on samples from selected sediment depths were conducted on the major microbial processes, including methanogenesis, acetogenesis, sulfate reduction, hydrogen oxidation, and prokaryotic growth. Although most of these data will be available only postcruise, initial results show a time constant of hydrogen turnover on the order of a few days. Other substrates for microbial activities will have much longer turnover of months to years, and only the postcruise radiotracer results will demonstrate these rates. Total prokaryotic cell counts show 106-107 cells in the upper 100 m of the sediment column. This is an order of magnitude higher than at Site 1225, in accordance with the higher availability of organic material at Site 1226. Below 100 mbsf, the cell concentrations are rather similar at the two sites. A broad spectrum of prokaryotic most probable number (MPN) counts and enrichments was initiated at this site, ranging from heterotrophs to autotrophs and from psychrophiles to thermophiles. Samples were also taken for cultivation from pieces of basaltic rock recovered at the bottom of Hole 1226B. Because of the slow growth rate of the indigenous microorganisms, successful counts and cultures are expected to require many months to years for growth and development.
Contamination tests are very important for all the microbiological work and were conducted continuously throughout drilling by injecting perfluorocarbon tracer (PFT) into the drilling water. In all cores that were used for microbiological experiments, counts, or isolations, a contamination test was also conducted with prokaryote-sized fluorescent microbeads released within the core catcher upon impact with the sediment (five tests at Site 1226). The detection limit of the PFT method is 0.02 無 drilling fluid (seawater)/g sediment. The results show low to nondetectable contamination in most advanced hydraulic piston corer (APC) cores (<0.1 無 drilling fluid/g sediment) but significant potential contamination in extended core barrel (XCB) cores where the sediment was also visibly disturbed. Subsampling of XCB cores was done here with a reduced sampling program from intact biscuits of sediment. Slurry samples used for an extensive program of microbiology and process studies all (apart from one) have nondetectable contamination when using the PFT method and nondetectable or extremely low contamination using the bead method.
Eight Adara tool deployments and four deployments of the Davis-Villinger Temperature Probe (DVTP) define a sediment/water interface temperature of 1.7蚓 and an estimated sediment/basement interface temperature of 24.4蚓. An accurate linear temperature gradient of 54蚓/km was determined through the 420-m-thick deposit. As the sediment depth increases, temperatures thus shift from the psychrophilic microorganism range to the mesophilic range. Deployment of the corresponding pressure tool (Davis-Villinger Temperature-Pressure Probe [DVTP-P]) showed ambient hydrostatic pressure.
As at Site 1225, most cores from the first deep hole (Hole 1226B) were logged on the catwalk with an infrared (IR) camera for postcruise analysis of the IR logging utility. In order to continue building a temperature database suitable for assessing the microbiological effectiveness of catwalk core handling strategies and for determining microbial cultivation strategies, the IR camera was also used to immediately log temperature gradients across cut section ends.