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SITE SUMMARIES (continued)

Ocean-Margin Sites

Peru Shelf Sites

Site 1227

Background and Objectives. Site 1227 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 presence of sulfate-depleted subsurface brine affects microbial communities, microbial activities, and microbial influence on environmental properties in organic-rich, sulfate-depleted sediments.

Site 1227 (427 m water depth) is in the immediate vicinity of Leg 112 Site 684, in a small fault-bounded sediment pond in the Trujillo Basin on the Peru continental shelf. The Trujillo Basin lies within the Peru upwelling zone, and its sediments are correspondingly rich in organic carbon. The TOC content of Site 684 sediment samples ranges between 1.2% and 10.6%, (Shipboard Scientific Party, 1988c). The average TOC concentration of these samples is approximately an order of magnitude higher than the average concentration at open-ocean Site 846 (Leg 201 Site 1226) (Shipboard Scientific Party, 1988c, 1992a). It is about two orders of magnitude higher than the TOC content of open-ocean Site 851 (Leg 201 Site 1225) (Shipboard Scientific Party, 1988c, 1992b).

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). The composition of the brine differs from site to site, perhaps because of differences in its degree of dilution and the nature of its interaction with the surrounding sediments (Suess, von Huene, et al., 1988). Detailed chemical analyses indicate that this brine is of marine origin and is early Miocene in age (Kastner et al., 1990). The Leg 112 Initial Reports volume suggested that it enters the younger sediment column by diffusion from interstitial brine in underlying Miocene sediments (Suess, von Huene et al., 1988). Kastner and colleagues (1990) inferred that it is emplaced by stratigraphically bounded advection from north to south. The sulfate depletion of the brine at Site 1227 presumably results from microbial sulfate reduction closer to the brine's source (e.g., deeper in the sediment column). Whatever the brine's mode of emplacement, Site 1227 provides an opportunity to study how the presence of sulfate-depleted brine affects subseafloor life in organic-rich sediments. Consequently, it provides an excellent standard of comparison for Sites 1228 and 1229, which are affected by the intrusion of sulfate-rich brine into, respectively, sulfate-rich and sulfate-depleted sediments.

Leg 112 shipboard chemistry suggests that the concentration of methane at Site 684 increases by at least three orders of magnitude (from 102 to 105 µL/L) over the first 50 to 60 mbsf and remains between 104 and 105 µL/L to at least 100 mbsf. Ethane and butane concentrations also increase downhole to maximum concentrations at ~60 mbsf (Shipboard Scientific Party, 1988c). In contrast, the concentration of dissolved sulfate declines from a near-seawater value to zero over the uppermost 30 or 40 mbsf (Shipboard Scientific Party, 1988c). These profiles of dissolved hydrocarbons and sulfate indicate that the hydrocarbons and the sulfate are simultaneously destroyed by sulfate-reducing microbial communities at ~40 mbsf.

Concentrations of several dissolved chemical species increase steadily to the base of the hole (ammonium, chloride, calcium, and magnesium). The increases in dissolved chloride, calcium, and magnesium provide evidence of the brine diffusing upward into the sediment column. Alkalinity exhibits a maximum value at ~40 mbsf, where the rate of anaerobic methane oxidation appears to be greatest. The magnesium/calcium ratio peaks at 12 mbsf and steadily declines to the base of the hole, presumably a result of dolomitization throughout the methane-rich sedimentary interval (Shipboard Scientific Party, 1988c).

All of 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 and with the brine introduced at depth. 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) were not determined for Site 684.

Principal Results. Pore water studies at Site 1227 define one of the most highly resolved chemical records in ODP history. An important objective with these profiles is to identify and quantify zones of microbial activity based on reactive pore water species. A deep hypersaline brine dominates the profiles of conservative seawater ions at this site, including chloride, which increases (with a linear gradient of 5 mM/m down to 70 mbsf and with 3 mM/m below that) to reach twice seawater chlorinity at 120 mbsf. Downhole depletion of SO42– at a relatively shallow depth, DIC concentration as high as 25 mM, ammonium rising to 23 mM at 150 mbsf, and a very high concentration of subsurface CH4 all indicate that microbial activity is much higher at this ocean-margin site than at open-ocean Sites 1225 and 1226. The dissolved SO42– concentration rapidly declines in the upper 15 mbsf from a seawater value of 29 mM to 5 mM. It then declines more slowly to 0 mM at ~40 mbsf. The concentration of dissolved H2S rises rapidly over the same 0- to 40-mbsf interval, from 0.04 mM at 0.24 mbsf to 9 mM at 39–40 mbsf. The convex-upward shape of both the sulfate and sulfide profiles from the sediment/water interface to ~40 mbsf indicates that microbial sulfate reduction occurs throughout the interval. The sulfide concentration steadily declines over the sulfate-poor remainder of the drilled section, to <0.3 mM at 150 mbsf.

From 1 to 31 mbsf, the dissolved Ba2+ concentration rises slightly, from 0 to 1.9 µM. Over the next several meters, the Ba2+ concentration rises at an increasingly steep rate, climbing from 9 µM at 38 mbsf to 170 µM at 43 mbsf. It then rises steadily to 350 ÁM at ~150 mbsf. Dissolved SO42– and Ba2+ are both present throughout the entire interval of non-zero SO42–. Throughout this interval, the concentrations of dissolved Ba2+ and dissolved SO42– appear to be related by the solubility product of BaSO4 (barite). Upward diffusion of Ba2+ from 43 to 38 mbsf appears to sustain modern barite formation in this Peruvian shelf sediment. The barite is visible as lighter bands in the sediment column and was confirmed by X-ray diffraction. At slightly greater depth (~42 mbsf), the dissolved SO42– concentration declines toward 0 mM, barite begins to dissolve, and the dissolved Ba2+ concentration rises. The narrow Ba2+ peak centered at 43 mbsf is inferred to mark the principal depth of current barite dissolution.

A similarly well-defined sulfate/methane interface coincides with the dissolved sulfide peak at ~40 mbsf. Dissolved CH4 concentration slowly rises from 7 µM at 1 mbsf to 55 ÁM at 35 mbsf. From 40 to 56 mbsf, CH4 concentration then rapidly rises to 2 x 103 µM at 56 mbsf and hovers in the range of 103 µM for the remainder of the drilled sediment column. The disappearance of almost all CH4 at the depth of SO42– depletion indicates that most of the CH4 diffusing upward through this sediment column is ultimately destroyed by anaerobic methanotrophy. The presence of CH4 at a low concentration throughout the overlying sediment column indicates, as at open-ocean Sites 1225 and 1226, that CH4 can be maintained at a background level of several micromolar in subseafloor sediments, despite the potential for CH4 oxidation by SO42– reduction.

Like methane, ethane (C2H6) and propane (C3H8) are detected throughout most of the sediment column. Ethane is present throughout the sediment column below ~1 mbsf, and propane is present throughout the column below ~11 mbsf. The concentration of ethane declines sharply at the 40-mbsf top of the anaerobic methanotrophy zone (from 2 to 0.7 µM). The concentration of propane declines more gradually (from 3 to 0 µM) in parallel with methane across the same interval. These distributions demonstrate that ethane and propane are biologically consumed in the anaerobic methanotrophy zone at this site. Concentrations of all three hydrocarbon species exhibit small distinct peaks in the upper part of the sulfate-rich zone. These small peak occurrences demonstrate that methane, ethane, and propane are all biologically produced in sulfate-rich sediments at this site. Methanogenesis occurs at 1 mbsf, whereas ethanogenesis and propanogenesis occur at ~10 mbsf. Most of the methane, ethane, and propane produced in these sulfate-rich sediments are consumed within a few meters (at ~5 mbsf and 15–25 mbsf). Trace concentrations of the ethane (10–1 µM) persist throughout the sulfate-rich sediments at this site. This persistence indicates that ethane can be maintained at a very low background level in sulfate-rich sediments, despite its potential for oxidation by SO42– reduction.

In most samples from Site 1227, H2 concentration is between 0.2 and 0.5 nM. This concentration closely resembles that observed at open-ocean Site 1226. However, it is a factor of 2 to 10 lower than the concentration predicted for aquatic sediments where cells are actively growing and SO42– reduction is the dominant electron-accepting process (Lovley and Goodwin, 1988). It is a factor of 10 to 50 lower than predicted for actively growing aquatic sedimentary communities that rely on methanogenesis as their dominant electron-accepting process. Samples from the first few meters of the sediment column exhibit a significantly higher H2 concentration (0.9–2.4 nM). This concentration is consistent with the standard prediction for sediments where SO42– reduction is the dominant electron-accepting pathway (Lovley and Goodwin, 1988). However, similar concentrations in samples from 93 and 113 mbsf are a factor of five to ten lower than predicted for methanogenic sediments. As for Site 1226, further investigation will be needed to determine whether or not these results indicate that the Site 1227 methanogenic and sulfate-reducing communities utilize H2 at free-energy yields lower than the generally accepted theoretical limit for actively growing cells.

The volatile fatty acids, formate and acetate, are important intermediates in the anaerobic pathways of organic matter degradation and were analyzed throughout the sediment column. Acetate concentration ranges between 0 and ~10 µM and generally increases from the surface sediment down to the base of the drilled sediment column (~150 mbsf). Formate concentration varies considerably throughout the sediment column (between 0 and ~5 µM) but exhibits no mean trend over the sampled sediment column. The average acetate and formate concentrations of this site are an order of magnitude higher than concentrations in sediments of the equatorial Pacific sites and are similar to concentrations found in very active coastal marine sediments. These results suggest that relative substrate concentrations of different sites may be related to the activity levels of the main microbial processes, although the absolute process rates are orders of magnitude lower in the open-ocean sediments than in the coastal sediments.

Concentrations of dissolved Mn and Fe are, respectively, 0–6 and 0–30 µM at Site 1227. The peak Mn concentration from Site 1227 (6 µM) is a factor of 27 lower than that of equatorial Pacific Site 1225 and a factor of 7 lower than that of equatorial Pacific Site 1226. The peak in situ Fe concentration from Site 1227 (30 µM) is a factor of 1.3 greater than that at Site 1225 and a factor of 1.5 lower than that at Site 1226. There are at least two possible explanations why the dissolved Fe and Mn concentrations are low at Site 1227 relative to the open-ocean sites. Either the ferrimagnetic material at this ocean-margin site is not an effective source of bioavailable Mn and Fe oxides, or dissolved Mn and Fe are scavenged and precipitated much more quickly at this site. Stratigraphic relationships between magnetic susceptibility and dissolved sulfide concentration suggest that these dissolved metals are scavenged by sulfide precipitation at Site 1227. A relatively steep decline in sulfide concentration from 40 to 75 mbsf is associated with the prominent magnetic susceptibility peak from 40 to 50 mbsf. The ultimate sink for sulfide diffusing deeper into the column is associated with the other most prominent magnetic susceptibility peak at this site (which begins at ~140 mbsf).

A pronounced peak in the values of almost every physical property measured at this site spans the interval from 40 to 50 mbsf. These physical properties include magnetic susceptibility, gamma ray attenuation bulk density, grain density, P-wave velocity, natural gamma radiation, thermal conductivity, and axial formation factor. Smaller peaks in the values of most of these properties are present in the uppermost 20 m of the sediment column. The bulk porosity profile mirrors the variability in other physical properties at this site; its downhole record is nearly the exact inverse of the bulk density and grain density records. These variations in physical properties result from variations in the bulk lithology of the sediment column. The porosity lows and high values in other physical properties are present in sandier intervals of the sediment column.

The 40- to 50-mbsf interval is composed of sandy silt, rich in glauconite, dolomite, quartz, feldspar, pyrite, and shell fragments. It grades upward into dolomite-bearing clayey silt, rich in diatoms and nannofossils. It directly overlies clay- and nannofossil-bearing diatom ooze. Traces of bioturbation are much more abundant in the 40- to 50-mbsf interval than in the overlying and underlying sediments. The primary front of active anaerobic methanotrophy occurs at the top of this 40- to 50-mbsf sandy interval. The successive fronts of barite precipitation and barite dissolution are present in the same interval. Peak concentrations of dissolved Fe, Mn, Si4+, and PO43– are also present in this interval. Secondary peaks in the dissolved concentrations of Fe, Mn, Si4+, and PO43– are present between 0 and 20 mbsf and are similarly associated with relatively coarse-grained sediments. These relationships suggest that several principal activities of the subsurface biosphere (including anaerobic methanotrophy, Fe and Mn reduction, ethanotrophy, and propanotrophy) are pinned in a narrow stratigraphic interval by physical properties and sediment composition at this site. Its mineral composition and its traces of relatively intensive bioturbation indicate that the physical and compositional properties of this interval are primarily determined by the nature of the sediment when it was first deposited on the seafloor. However, to some extent, these properties may have been modified by the postdepositional microbial activities that still occur in them today. Density and porosity can be affected by biologically mediated precipitation and dissolution of authigenic minerals, such as barite, dolomite, and apatite. Magnetic susceptibility may be diminished by biologically mediated dissolution of solid-phase Fe oxides and subsequent Fe reduction. To a much lesser extent, magnetic susceptibility may also be enhanced by massive biologically induced precipitation of reduced Fe and Mn. More detailed determination of the extent to which physical and compositional properties control the microbial activities at this site and the extent to which those activities control the physical and compositional properties will require further investigation.

Preliminary cell counts of eight samples from Site 1227 suggest that sedimentary cell concentrations at most sediment depths are slightly higher at this ocean-margin site than at equatorial Pacific Site 1225. Based on the same few data, at most sediment depths, cell concentrations from Site 1227 may be roughly equivalent to those of open-ocean Site 1226. This data set will be expanded by postcruise analyses.

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. Specific sampling was targeted to the sulfate/barium interface to study the possible attack of sulfate-reducing bacteria on sulfate bound in barite.

One Adara and two DVTP deployments combined with the Leg 112 data define a linear gradient with a sediment/water interface temperature of 8.6°C and an estimated temperature at 160 mbsf of 16.4°C. Throughout the sediment column, temperatures are in the low mesophilic range.

Trials were undertaken of three experimental tools at this hole: the pressure-coring sampler (PCS), the DVTP-P, and the Fugro pressure-coring device (Hydrate Autoclave Coring Equipment [HYACE]).

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