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SITE SUMMARIES (continued)

Ocean-Margin Sites (continued)
Site 1229

Background and Objectives. Site 1229 was one of three Leg 201 sites selected for drilling on the continental shelf of Peru. These shelf sites were collectively selected to provide records of microbial activities, communities, and geochemical consequences in organic-rich ocean-margin sediments.

The principal objectives at this site were

  1. To test by comparison with other sites during this expedition whether microbial communities, microbial activities, and the nature of microbe-environment interactions are different in organic-rich ocean-margin sediments than in open-ocean sediments with less organic matter;
  2. To test how the occurrence of sulfate-bearing subsurface brine affects microbial communities, microbial activities, and microbial influence on sediment chemistry in organic-rich, sulfate-depleted, methane-rich sediments; and
  3. To provide multiple opportunities for recovering and identifying the sulfate-reducing methanotrophic communities of deeply buried marine sediments.

Site 1229 is located on the Peru shelf in 150.5 m water depth. It is in the immediate vicinity of Leg 112 Site 681. As described in "Background and Objectives" in "Site 1227," geochemical studies of Leg 112 sites show that brine is present several tens of meters below the seafloor in the Trujillo and Salaverry Basins (Suess, von Huene, et al., 1988). Site 1229 provides an opportunity to study how the occurrence of sulfate-bearing brine affects subseafloor life in organic-rich, sulfate-depleted, methane-rich sediments. Consequently, it provides an excellent standard of comparison for Sites 1227 and 1228, which are, respectively, affected by the intrusion of sulfate-free brine into organic-rich, sulfate-depleted sediments and the intrusion of sulfate-rich brine into sediments with sulfate-bearing interstitial waters.

Shipboard chemical analyses from Leg 112 indicate that the concentration of methane at Site 681 increases from 102 to 105 µL/L in the first 40 m of the sediment column and declines from 105 to 102 µL/L between 73 and 100 mbsf (Shipboard Scientific Party, 1988b). In contrast, the concentration of dissolved sulfate declines to 0 mM over about the first 30 mbsf, remains at or near 0 mM until 75 mbsf, and then increases steadily with greater depths (Shipboard Scientific Party, 1988b). This downhole pattern of sulfate concentration indicates active sulfate reduction at depths <30 mbsf and at depths >~75–100 mbsf. The downhole pattern of methane concentration indicates that methane is created at depths of 60–70 mbsf and diffuses to the overlying and underlying zones of active sulfate reduction, where both sulfate and methane are destroyed.

Chloride concentration increases steadily to the base of the hole. Ammonium concentration declines slightly from the sediment/water interface to 12 mbsf, increases from 12 to 80 mbsf, and then begins to decline again. Alkalinity also declines from the sediment/water interface to 12 mbsf, increases to a subsurface maximum at 32 mbsf, and then declines again with depth. Calcium and magnesium concentrations exhibit minimum values at ~30 mbsf and then increase steadily to the base of the hole. The magnesium/calcium ratio exhibits a broad peak from ~0 to 40 mbsf and then steadily declines to the base of the hole (Shipboard Scientific Party, 1988b).

These patterns of sedimentary pore water concentration are collectively inferred to result from high levels of biological activity and biologically driven solid-phase alteration throughout the sediment column, coupled with diffusive exchange with the overlying ocean and with a sulfate-bearing brine introduced at depth. AODCs show that bacterial cells are present in samples taken from as deep as 80 mbsf at Site 681 (Cragg et al., 1990). Viable bacteria were found and potential activity rates were identified in the same samples (Cragg et al., 1990). The subsurface extent of key electron donors (hydrogen, acetate, and formate) and electron acceptors with standard free-energy yields greater than that of sulfate (oxygen, nitrate, manganese oxide, and iron oxides) was not determined for Site 681.

Principal Results. An important objective for Site 1229 is to identify and quantify zones of microbial activity based on reactive pore water species. Toward this end, we established a highly resolved chemical record throughout the drilled sediment column. Profiles of conservative ions provide evidence of diffusive mixing between seawater diffusing downward from the sediment/water interface and a hypersaline brine diffusing upward from older sediments. For example, the concentration of dissolved chloride increases linearly from 559 mM at the sediment/water interface to 1208 mM at the base of the drilled sediment column (186 mbsf). Peak concentrations of biologically affected chemical species, such as ammonium (5.8 mM) and dissolved inorganic carbon (22 mM), indicate that rates of subseafloor microbial activity are much higher at this ocean-margin site than at open-ocean Sites 1225 and 1226. These peak concentrations also indicate that the subseafloor microbial activity at Site 1229 is slightly greater than that at Site 1228 (which lies just seaward of Site 1229) and perhaps is slightly less than that at Site 1227 (which is situated 310 km to the north on the Peru shelf).

As at Site 1228, the concentration profiles of several biologically affected chemical species exhibit a pronounced anomaly just below the seafloor (at 2–3 mbsf). This anomaly consists of a brief positive excursion in alkalinity, DIC, ammonium, and sulfide, with a co-occurring negative excursion in dissolved sulfate. The same anomaly is also apparent in the ammonium and alkalinity profiles of Site 681 (Shipboard Scientific Party, 1988b). As described in "Principal Results" in "Site 1228," this near-surface pore water anomaly indicates that the steady-state diffusion of biologically active chemicals past the upper sediment column was disrupted by late Quaternary environmental change and has not yet fully recovered. There are least three possible general explanations of this anomaly. It may result from ongoing activity in a microbial "hot spot" at this shallow sediment depth, it may be a chemical relic of past microbial activity and is now relaxing back to a diffusional steady state, or it may a result of the establishment of an oxygen minimum at this water depth, causing the extinction of a bioirrigating benthos and a stimulation of sulfate reduction.

The most striking biogeochemical feature of this site is the reversal of the biogeochemical zonation at depth. This reversal is immediately apparent in the dissolved SO42– profile. The SO42– concentration declines from a seawater value of 29 mM at the sediment surface to 0 mM at ~35 mbsf. It remains at 0 mM from 35 to 88 mbsf and then steadily rises from 0 to 38 mM at 186 mbsf. The SO42– that sustains microbial reduction over the uppermost 35 mbsf of the sediment column ultimately diffuses downward from the overlying ocean. The SO42– that sustains microbial reduction below 88 mbsf is inferred to diffuse upward from the underlying brine. Both intervals of SO42– reduction are marked by local maxima in the concentration of dissolved sulfide, with a broad peak from ~20 to 40 mbsf and a sharper peak at ~90 mbsf.

The SO42– profile is mirrored by the dissolved CH4 profile. The dissolved methane concentration is <100 µM from 0 to 20 mbsf, holds steady at a few hundred micromolar from 20 to 35 mbsf, and then rises to values of ~2000 µM (exceeding 1 bar partial pressure) between 65 and 75 mbsf. It then steadily declines to <100 µM at 93 mbsf and remains in the range of 100 µM or less to the base of the sampled sediment column. As at Site 1227, the disappearance of almost all CH4 at the depths of SO42– depletion indicates that most of the CH4 in this sediment column is ultimately destroyed by anaerobic methanotrophy. As observed at all previously drilled Leg 201 sites, the Site 1229 CH4 and SO42– profiles indicate that CH4 can be maintained in subseafloor sediments at background concentration that is inversely related to the co-occurring dissolved SO42– concentration.

The dissolved iron and manganese concentration profiles demonstrate that net reduction of iron and manganese oxides occurs in the methanogenic zone at higher rates than in the overlying sulfate reduction zone. The principal foci of net manganese and iron reduction are at slightly different depths, with iron reduction peaking at 75–90 mbsf and manganese reduction just above and below that interval. These co-occurrences of manganese and iron reduction with abundant methanogenesis appear unlikely to be fully explained by standard thermodynamic models of competition between microbes using different electron-acceptor pathways. The presence of methanogenesis in iron- and manganese-reducing environments may result from a limited availability of mineral-supplied electron acceptors relative to electron donors. In these organic-rich sediments, electron donors may be supplied to the microbial community faster than mineral dissolution can supply dissolved reducible manganese and iron. Relatively high concentrations of manganese and iron in the lower sulfate zone could be due to either in situ mineral reduction or to diffusion from the underlying brine-rich sediment.

The dissolved Ba2+ profile is broadly similar to the CH4 profile. The dissolved Ba2+ concentration is <2 µM from 0 to 24 mbsf. The concentration of Ba2+ in pore water then rapidly rises to 18 µM at 40 mbsf and remains near 19 µM until almost 80 mbsf. It then declines steeply to 2 µM at ~100 mbsf and <2 µM for the remainder of the drilled sediment column. As at Site 1227, the inverse relationship between SO42– and Ba2+ is inferred to be controlled by the solubility product of BaSO4 (barite). Upward diffusion of Ba2+ past 35 mbsf and downward diffusion of Ba past 90 mbsf is suspected to sustain modern barite formation at, respectively, ~24 and 100 mbsf. Similarly, the shoulders of the Ba2+ peak at ~40 and 80 mbsf are inferred to mark the principal depths of current barite dissolution at this site.

Microbial cell counts were done at 10-m intervals throughout the upper sediment column and across both sulfate/methane interfaces. These data show that mean sedimentary cell concentrations are several-fold higher at this ocean-margin site than at the Leg 201 open-ocean sites and may be slightly higher than mean concentrations at nearby Site 1227. The most striking features of the shipboard cell counts are the thousandfold increase in cell concentrations in the lower zone of overlapping sulfate and methane concentrations and the tenfold increase in cell concentrations in the upper zone of overlapping concentrations. The maximum cell concentrations observed in the lower sulfate/methane zone are actually an order of magnitude higher than the concentrations observed at the sediment/water interface. Given the coarse spacing of these samples and their positions relative to the chemically defined sulfate/methane overlap zones, the peak cell concentrations observed in the upper sulfate/methane zone may greatly underestimate the peak concentrations in that zone.

Acetate and formate concentrations exhibit strong local maxima of ~6 µM in both of the sulfate/methane interface zones. These maxima are centered at 37 and 90 mbsf. As with the cell counts, these local maxima are higher than the local maxima exhibited by both acetate (~2 µM) and formate (3 µM) at the sediment/water interface. Throughout most of the remaining record at this site, concentrations of both species were between 1 and 2 µM. As at Site 1227, the concentrations of both species reach their highest values near the base of the drilled sediment column (~15 µM). These results are intriguing because these volatile acids are important substrates for both sulfate reducers and methanogens. H2 is another important electron donor in anaerobic communities. Almost all H2 concentrations measured at this site were <0.5 nM, and most were <0.2 nM. These concentrations resemble those observed at open-ocean Site 1225 and ocean-margin Site 1227. As noted in "Principal Results" in "Site 1225" and "Site 1227," these concentrations are much lower than expected from experiments with sulfate-reducing and methanogenic communities of surface sediments. The accurate interpretation of these acetate, formate, and hydrogen concentrations must await postcruise analyses of microbial energetics in subseafloor environments.

These cell concentration data and sulfate and methane gradients demonstrate that the subseafloor microbial population and activity are locally strongly focused at the sulfate/methane overlap zone defined by the upward-diffusing sulfate-bearing brine and the downward-diffusing seawater sulfate. The dissolved barium profiles indicate that microbial activity in this zone directly influences sediment chemistry by mediating the precipitation and dissolution of barite. In these effects on subsurface biological activities and biogeochemical cycles, this brine-caused sulfate/methane interface mirrors the effects of the overlying "normal" sulfate/methane interface. Postcruise microbiological studies will be required to demonstrate whether or not the microbial community supported by the brine-induced interface is locally unique or the same as that supported by the overlying interface.

The upper sulfate-rich zone at Site 1229 lies entirely within lithostratigraphic Subunit IA, a stratigraphic interval of primarily hemipelagic sediments (0–40 mbsf). The underlying methane-rich zone is largely limited to lithostratigraphic Subunit IB, which is the upper portion of a longer interval (40–138 mbsf) of mixed terrigenous and hemipelagic sediments. The AMO zones that separate the upper and lower sulfate-rich zones from the intervening methane-rich zone are associated with brief sedimentary intervals characterized by high grain density, high NGR, high resistivity, and low porosity. These brief low-porosity intervals are unusually rich in terrigenous sediment and are interpreted to have been deposited during the two most recent lowstands of four onlap/offlap cycles that define the 40- to 138-mbsf interval.

In short, as at Site 1228, the upper sulfate-reducing interval at Site 1229 is composed of predominantly hemipelagic sediments, the strongly methanogenic zone is rich in terrigenous sediment relative to the overlying sulfate-reducing zone, and the intervening AMO zone is present just above an interval of low-porosity, high-density lowstand sediments. The lower AMO zone at Site 1229 is present within a similar interval of high-density, low-porosity lowstand sediments. The lithologic association of AMO zones with high-density, low-porosity lowstand sediments at Sites 1229, 1228, and 1227 provides intriguing evidence that, on the Peru shelf, the position of AMO zones is pinned within the sediment column by lithologic properties and, by extension, depositional history.

As at Site 1227, stratigraphic patterns of magnetic susceptibility and dissolved Mn, Fe, and sulfide concentrations indicate similar control of other microbial processes by depositional history at Site 1229. Magnetic susceptibility is generally much higher in the methanogenic zone and in the lower sulfate-reducing zone than in the overlying sulfate-reducing zone. This circumstance suggests that mineral sources of reducible iron and manganese are much more abundant in the terrigenous-dominated sediments of the lower sulfate-reducing zone and the mixed terrigenous and hemipelagic sediments of the methanogenic zone than in the mostly hemipelagic sediments of the upper sulfate-reducing zone. The relatively high magnetic susceptibility of the intervals with more strongly terrigenous sediments is nicely consistent with our finding that dissolved Mn and Fe concentrations are generally higher in the lower methanogenic zone and the underlying sulfate-reducing zone than in the upper sulfate-reducing zone. The presence of higher Mn and Fe concentrations and lower sulfide concentration in these relatively high-susceptibility intervals in turn provides strong evidence that the current rates and stratigraphic foci of iron reduction, manganese reduction, and sulfide precipitation depend more strongly on depositional history than on competition between microbes reliant on different electron-accepting pathways.

Experiments on major bacterial processes and on enumeration of viable bacteria were initiated at selected depths ranging from near the mudline to the bottom of the drilled sediment column. The studied processes include methane and acetate formation and consumption, sulfate reduction, hydrogen oxidation, and rates of cell growth. The cultivation experiments include selective growth conditions for a wide range of autotrophic and heterotrophic microorganisms ranging from psychrophilic to thermophilic. Detailed microbiological sampling targeted the top of the sediment column and both the upper and lower sulfate/methane overlap zones.

The results from one DVTP deployment were combined with temperature data from Site 681 to define a linear gradient of 35.5°C/km for this site. The mean sediment/water interface temperature defined by this gradient is 13.4°C. The temperature defined for the base of the drilled sediment column (193 mbsf) is 20.2°C. Throughout this interval (0–193 mbsf), temperatures are in the low mesophilic range.

Trials were undertaken of four experimental tools at this hole: the PCS, the DVTP-P, the APC-Methane (APC-M) tool, and the HYACE.

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