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SITE SUMMARIES (continued)

Ocean-Margin Sites (continued)
Site 1228

Background and Objectives. Site 1228 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 and
  2. To test how the occurrence of sulfate-bearing subsurface brine affects microbial activities, microbial communities, and microbial influence on environmental properties in organic-rich, sulfate-rich sediments.

Site 1228 is in the immediate vicinity of Leg 112 Site 680. 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). Interestingly, at Site 680 the deep brine source of sulfate prevents the pore water concentration of sulfate from becoming depleted at any depth. Site 1228 therefore provides an opportunity to study how the introduction of sulfate-bearing brine affects subseafloor life in organic-rich, sulfate-rich sediments. Consequently, it provides an excellent standard of comparison for Sites 1227 and 1229, which are, respectively, affected by the intrusion of sulfate-free brine and sulfate-rich brine into organic-rich, sulfate-depleted sediments.

Site 1228 is located at 252 m water depth on the outer shelf edge in the middle of the modern oxygen minimum zone of the Peruvian high-productivity upwelling system. At this depth on the Peru shelf, an oxidized sediment zone is practically absent at the sediment/water interface and sulfate reduction is the predominant mineralization process to the very surface (Rowe and Howarth, 1985; Fossing, 1990; Parkes et al., 1993). The organic content is high at Site 680 (3%–10% TOC), and sulfate reduction rates are still detectable with radiolabeled sulfate in samples taken from as deep as 80 mbsf (Parkes et al., 1990).

The lithologic and physical properties at Site 680 change strongly through the 200-m-deep interval drilled during Leg 112 (Shipboard Scientific Party, 1988a). The sediment consists of mainly diatom mud in the upper 50 m of the Pleistocene deposit. Below 50 mbsf, the terrestrial component of the mud is higher but the sediment is primarily biogenic. The lower part of the sediment column consists of a coarse-grained phosphate and feldspar gravel interpreted as drilling artifacts overlying coarse-grained sand cemented by dolomite. Dolomite is the primary authigenic phase, but calcite and apatite are also common.

Shipboard chemical analyses from Leg 112 indicate that the concentration of methane at Site 680 is in the range of 10–100 µL/L (0.4–4 µM) in the upper 100 m of the sediment column. Methane was not analyzed at greater depths at Site 680. The concentration of dissolved sulfate declines from a near-seawater value to a minimum of 6 mM over about the uppermost 50 mbsf and then rises toward higher values in the underlying sediment because of diffusion from the underlying sulfate-rich brine (Shipboard Scientific Party, 1988a). A peak sulfide concentration is present between 20 and 40 mbsf (Mossman et al., 1990). Sulfide concentration was not measured in deeper portions of the underlying brine-affected interval.

Chloride concentration increases steadily to the base of the hole, and ammonium steadily increases to at least 80 mbsf. Alkalinity exhibits a maximum value at 20 mbsf. Concentrations of calcium and magnesium exhibit minimum values at 5 and 20 mbsf, respectively, and then increase steadily to the base of the hole. The magnesium/calcium ratio peaks at ~5 mbsf and also steadily declines to the base of the hole (Shipboard Scientific Party, 1988a; Kastner et al., 1990).

These patterns of sedimentary pore water concentration are inferred to result from relatively high levels of biological activity throughout the sediment column, coupled with diffusive exchange with the overlying ocean water and with a sulfate-bearing brine introduced at depth. Microbial cell counts and activities were studied to a depth of 9.1 mbsf at Site 680. Nearly 109 cells/mL were present in all samples analyzed. In MPN cultivation studies, 101 to 105 cells/mL were shown to be viable (Cragg et al., 1990; Parkes 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 680.

Principal Results. Continuous APC coring from the seafloor to 200 mbsf enabled high-quality sampling for geochemistry and microbiology throughout the drilled sediment column of Site 1228. Because of the overall predominance of sulfate reduction in the highly sulfidic sediment and the presence of sulfate throughout the sediment column, there were no distinct chemical interfaces to target in the sampling scheme for Site 1228. The concentration of chloride ranges linearly from a typical seawater concentration at 0 mbsf to twice the seawater concentration at 200 mbsf. This linear profile demonstrates the long-term stability of brine diffusion and provides a reference for all other pore water constituents. Analyzed nonconservative species that are affected by microbial activity in the subsurface included sulfate, DIC, and ammonium. Pore water analyses at high depth resolution show unexpected details with implications for both the long-term process rates and for more recent changes.

Sulfate reduction in the upper 50 m of the sediment column is not sufficient to deplete sulfate at depth. The overall sulfate distribution shows a steep drop in concentration over the first few meters below the sediment/water interface, a sigmoidal curve in dissolved sulfate concentration over the following 10 m, a decrease to 2.5 mM at 38 mbsf, and then a continuous increase to 30 mM at 200 mbsf. The sigmoidal curve of the first 10 m indicates that the near-surface distribution of sulfate reduction and/or transport processes changed strongly in geologically recent time and diffusion through the sediment column has not yet fully adjusted to a new steady state. The continuous increase in sulfate concentration from 40 to 200 mbsf results from upward diffusion of the underlying sulfate-rich brine.

The depth profiles of DIC and ammonium nicely match the described sulfate distribution. The overall DIC profile reveals a distinct DIC maximum of 19 mM at 2 mbsf, a decline to 15 mM, a rise to a second, broader maximum of 20 mM at 25 mbsf, and then a gradual downhole decrease to 4 mM. Ammonium similarly increases from ~2 mM near the sediment surface to a local maximum of 2.6 mM at 2 mbsf, declines slightly, and then increases gradually to 5 mM downhole. Comparison to Site 680 biochronostratigraphic data (Shipboard Scientific Party, 1988a) suggests that the sediment that contains the DIC and ammonium maxima may have been deposited a few tens of thousands of years ago. These near-surface pore water anomalies indicate that 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. The exact nature of these changes will be analyzed when a more complete data set becomes available.

Concentrations of manganese and iron in the pore water are extremely low (<0.1 µM) down to ~60–80 mbsf. Below this depth, they increase gradually to ~10 µM (Mn) and 50 µM (Fe) at 200 mbsf. The source of these dissolved metals at depth may be either diffusion from below or in situ manganese or iron reduction in the lower sediment column.

In contrast to most other ocean-margin sites, including Site 1227, a sulfate/methane interface is absent from the sediment of Site 1228. Methane concentration remains low throughout the 200-m sediment column, reaching a maximum of only 8 µM. Yet, the distribution of methane clearly reflects the sulfate distribution, with a maximum coinciding with the sulfate minimum and a general inverse correlation between sulfate and methane concentrations throughout the sediment column. These results indicate that even at a concentration above 9% of its seawater level (minimum = 2.5 mM; seawater = 28.9 mM), sulfate regulates the ability of methane-oxidizing consortia to take up methane and maintain a low background concentration. In this respect, Site 1228 provides a unique opportunity to analyze the energetics of anaerobic methane oxidation and to test current theories of the limiting parameters for this microbial key process.

Acetate and formate are important fermentation products as well as substrates for sulfate-reducing bacteria. Their concentrations in this organic-rich shelf sediment are tenfold higher than in deep-sea sediments of the tropical Pacific (Sites 1225 and 1226) but only about half their concentrations at Peru shelf Site 1227. The Site 1228 data show considerable scatter with depth. Acetate concentration falls mostly in the range of 1–4 µM and formate concentration in the range of 0.5–3 µM. The higher concentrations of both fatty acids are present below 100 mbsf. These concentrations are regulated by uptake mechanisms that are not yet fully understood.

Interestingly, the depth of the distinct sulfate minimum at ~40 mbsf is present in an interval of strong lithologic and physical change. At this depth, the sediment shifts from a diatomaceous silt of predominantly hemipelagic origin to older quartz- and feldspar-bearing silt with a more abundant terrestrial component. At 43 mbsf, there is a distinct minimum in porosity and maxima in density, thermal conductivity, and magnetic susceptibility. It is intriguing to speculate that such a physical boundary may lock the position of biogeochemical zonations in the sediment column.

The temperature gradient in the Site 1228 sediment column was defined from two discrete temperature measurements taken with the DVTP. The results were combined with Leg 112 data to define a linear temperature gradient of 34°C/km and a heat flow of 32 mW/m2. This heat flow estimate is lower than the 46-mW/m2 estimate for Site 680 by the Leg 112 Shipboard Scientific Party (1988a) and confines the previous broad estimate of 20–70 mW/m2 for this site (Yamano and Uyeda, 1990). The temperature increases down through the sediment column from an estimated annual mean of 12.5°C at the seafloor to an extrapolated 19.3°C at 200 mbsf. These temperatures are all within the low mesophilic range for microorganisms.

Samples were taken for total counts, viable (MPN) counts, and isolations of bacteria from selected depths throughout the sediment column. Because of the short transfer time between Sites 1227, 1228, and 1229, the AODCs of total bacterial numbers at Site 1228 will be conducted postcruise. A large number of MPN samples and isolation incubations target a broad physiological spectrum of heterotrophic and autotrophic microorganisms that utilize diverse electron acceptors and donors in their energy metabolism. The selective influence of increased salinity and brine composition is also targeted in some incubations. The expected slow growth of deep subsurface bacteria will require long postcruise incubation of samples before definite results are obtained from these experiments. This is also the case for the many experiments on bacterial processes measured by radiotracer techniques on samples taken from throughout the entire sediment column.

Because the absence of bacterial contamination from drilling and sampling operations is critical for the isolation of indigenous bacteria and measurement of their activities, a PFT tracer was continuously added to the drill water. Tracer samples were taken on the catwalk or in the laboratory from all core sections and subsamples used for microbiology. It was demonstrated that PFT concentration is invariably higher at the periphery than at the sampled center of whole-round core segments and that microbiology subsamples have a PFT concentration below or just at the detection limit. This limit corresponds to the potential introduction of 0.04 µL seawater/g sediment. Such seawater introduction could maximally introduce 50 bacteria/g, based on the mean bacterial density in seawater. An additional contamination test uses fluorescent microbeads dispersed on impact at the head of the core barrel. At Site 1228, this test consistently indicates that contamination is most unlikely. This method releases nearly 1012 bacteria-sized beads at the most sensitive position during drilling, and the tests on microbiological samples showed no beads or, at most, one bead in the >60 microscopic fields of view routinely scanned. The extensive contamination tests applied at this site thus confirm the high quality of microbiology samples that are now routinely taken by careful aseptic techniques from APC cores without visible disturbance.

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