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SITE SUMMARIES (continued)

Equatorial Upwelling Sites (continued)

Site 1226

Background and Objectives

Site 1226 was selected as a drilling target because its microbial activities were expected to be intermediate between those in ocean-margin settings and those in the lowest-activity open-ocean environments.

The principal objectives at this site were

  1. To test by comparison with other sites during this expedition whether microbial communities and activities are different in this deeply buried environment than in open-ocean sediments with less organic matter and shallower burial;
  2. To document the environmental and microbial circumstances under which methanogenesis occurs in sulfate-rich open-ocean sediments; and
  3. To test how basement hydrologic flow affects microbial communities, microbial activities, and microbial influence on environmental properties in the sediments that overlie the basement.

Site 1226 (3297 m water depth) is located in the eastern equatorial Pacific, 300 km south of the Galapagos Islands, near the present-day boundary between the South Equatorial Current and the Peru Current. Near the sea surface in this region, the advection of water from the Peru Current results in relatively high nutrient levels and biological productivity (Chavez and Barber, 1987). According to the calculated backtrack path for this site, it has drifted eastward but remained near its present latitude for most of its history (Pisias et al., 1995; Farrell et al., 1995). Sediment thickness at Site 1226 is 420 m. The oldest sediments immediately overlie basaltic basement and have a biostratigraphic age of 16.5 Ma (Shipboard Scientific Party, 1992a). As described in "Background and Objectives" in "Site 1225," geochemical studies of DSDP and ODP sites throughout this region have shown that seawater flows through the underlying basaltic basement (Baker et al., 1991).

The lithology, sediment age, and many geochemical and geophysical characteristics of the target site were well characterized by earlier studies of Site 846. The gross lithologic and physical properties of the carbonate and siliceous oozes and chalk at Site 846 are characteristic of sediments throughout the region (Shipboard Scientific Party, 1992a; Pisias, Mayer, Janecek, Palmer-Julson, and van Andel, 1995). Leg 138 studies showed that the region has undergone large variations in sediment accumulation over the course of its history. Accumulation of calcium carbonate and opal was unusually low at Site 846 during the Miocene carbonate crash of 11–7.5 Ma and was unusually high during the widespread Indo-Pacific biogenic bloom that occurred from ~7 to 4.5 Ma (Farrell et al., 1995). The organic accumulation rate is presently high and appears to have gradually increased throughout the Pleistocene (Shipboard Scientific Party, 1992a; Emeis et al., 1995).

Leg 138 shipboard chemical studies of Site 846 show that concentrations of several dissolved chemical species (methane, ammonium, strontium, and silica) and alkalinity peak part way down the sediment column. In contrast, dissolved sulfate, lithium, and calcium exhibit maximum values near the sediment/water interface and the basement/sediment interface (Shipboard Scientific Party, 1992a).

As at Sites 851 and 1225, these patterns of sedimentary pore water concentration are inferred to result from modest levels of biological activity throughout the sediment column, coupled with diffusive exchange with the overlying ocean and with seawater flowing through the underlying basaltic basement. The sediments of Site 846 have a higher organic content than the sediments of Sites 851 and 1225. Organic content at Site 846 ranges from 0.2% to 1.0% and is highest in the Pleistocene and upper Pliocene deposits. Accordingly, Site 846 exhibits steeper gradients than Sites 851 and 1225 in pore water chemical species that respond to microbial mineralization processes, such as sulfate, ammonium, and methane. The distinctly higher concentration of methane at Site 846 than at Site 851 is particularly intriguing because methanogenesis is generally understood to be suppressed by sulfate-reducing bacteria and methane may be oxidized in the presence of sulfate.

The subsurface distribution of key electron donors (hydrogen, acetate, and formate) and of electron acceptors with higher standard free-energy yields (oxygen, nitrate, manganese oxide, and iron oxides) was not determined for Site 846.

Principal Results

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 is the predominant electron-accepting pathway at this site. A broad maximum of DIC and NH4+ in the pore 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. Seawater flow through the basement thus provides an effective sink for DIC.

As at Site 1225, the overall chemical zonations are consistent with thermodynamic control of electron acceptor use by subsurface microbes. The data show that seawater flow through the underlying 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 microbial 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 O2 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 pore 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 mbsf. Yet another distinct peak in Mn concentration 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 Mn concentration.

Comparison with the Leg 138 Initial Reports data indicates that the most deeply buried interval of high dissolved Mn concentration 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 Mn concentration 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 Mn reduction and oxidation must await further solid-phase and pore water chemical analyses.

The zone of dissolved sulfide (H2S) extends from just 5 m below the sediment/water interface to a depth of 280 mbsf, and the broad peak reaches 700 µM around 100 mbsf. Throughout this sulfidic sediment column, iron appears in the pore water at <10 µM concentration but displays narrow peaks of ~40 µM just above and below the H2S zone. This pattern reflects both the low equilibrium concentration of ferrous iron in sulfidic pore water and the presence of reducible iron only near the sediment surface and in the deep sediment column, including a third iron peak at 380 mbsf. The pore water data identify a sink for both H2S and manganese within an interface at 250–280 mbsf, where manganese may precipitate with sulfide.

CH4 exhibits a broad peak at 100–250 mbsf with a concentration of 2–3 µM. Although this is still a trace level of biogenic methane, it is more than tenfold higher than at Site 1225. Sulfate is present at >80% 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., 1982b). The results also show that sulfate-reducing bacteria in this environment are apparently unable to exploit methane beyond the existing low concentration.

Acetate and formate concentrations are both low in the upper 0–100 m of sediment (<0.5 µM). At greater depths, their concentrations increase to 1–3 µM. This shift in concentration appears to result from mechanisms of regulation 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 pore 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.

H2 concentration is very low throughout the sediment column, 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 shelf sediments (Hoehler et al., 1998) and is even below concentration measured at Site 1225, where microbial activity is significantly lower than at Site 1226. According to theoretical calculations of the minimum energy yield required for bacterial 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 bacteria (Lovley and Goodwin, 1988; Hoehler et al., 2001). Based on the dissolved sulfate, Mn, and Fe 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 concentration is lower than expected for subsurface sediments where sulfate reduction is the predominant process. This finding suggests that the Site 1226 sulfate-reducing communities may 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 bacterial 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 bacteria will have much longer turnover of months to years, and only the postcruise radiotracer results will demonstrate these rates. Total cell counts of bacteria show 106–107 cells in the upper 100 m of the sediment column, in accordance with the mean trend from all other deep sediments analyzed (Parkes et al., 2000). 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 bacterial populations are rather similar at the two sites. A broad spectrum of bacterial 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 will expectedly require many months for growth and development.

Contamination tests are very important for all the microbiological work and were done continuously throughout drilling by injecting PFT into the drilling water. In all cores that were used for microbiological experiments, counts, or isolations, a contamination test was also conducted with bacterial-sized fluorescent microbeads released within the core catcher upon impact with the sediment (13 tests in total). The detectability using the PFT method is equivalent to the potential contamination from 0.02 µL drilling fluid (seawater)/g sediment. This corresponds to 2 x 10–5 of the sediment volume. The detectability of the bead method may be 105-fold more sensitive, corresponding to the detection of 1 bead out of the 5 x 1011 beads released. The results show low to nondetectable contamination in most piston (APC) cores (<0.1 µM drilling fluid/g sediment) but significant potential contamination in XCB cores where the sediment was also visibly disturbed. Subsampling 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 DVTP define a sediment/water interface temperature of 1.7°C and an estimated sediment/basement interface temperature of 24.4°C. An accurate linear temperature gradient of 54°C/km was determined through the 420-m-thick deposit. As the sediment depth increases, temperatures thus shift from the psychrophilic bacterial range to the mesophilic range. Deployment of the corresponding pressure tool (DVTP-P) showed ambient hydrostatic pressure without overpressure, possibly because a good seal was not established because of cracking of the surrounding sediment.

As at Site 1225, most cores from the first deep hole (Hole 1226B) were logged on the catwalk with an 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.

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