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Open-Ocean Sites

Peru Basin Site 1231

Background and Objectives

Site 1231 was selected for drilling in order to study the microbial activities and communities of the organic-poor sediments that characterize much of the world's open ocean. Before drilling at Site 1231, the nature of subseafloor microbial communities in open-ocean clays had never been assessed.

The principal objectives at this site were

  1. To test by comparison with other sites during this expedition whether microbial activities, microbial communities, and the nature of microbe-environment interactions are different in very organic-poor open-ocean sediments than in the more organic-rich sediments of the equatorial upwelling region and the coastal upwelling region and
  2. To document the microbial activities, communities, and environmental context of an expanded Mn-reducing zone in very organic-poor, relatively deeply buried marine sediments.

Site 1231 is in the Peru Basin at 4827 m water depth. The lithologies, age, and many geochemical characteristics of the targeted sediments were characterized by Leg 34 studies at nearby Site 321 (Shipboard Scientific Party, 1976). The total sediment thickness at Site 321 is 115 m. The sediment is composed of 58 m of late Oligocene to Holocene clay and 57 m of iron-rich late Eocene to early Oligocene nannofossil ooze (Shipboard Scientific Party, 1976). The lower 50 m of sediment at Site 321 is rich in iron and manganese (Dymond et al., 1976). Iron and manganese accumulation rates estimated for the sediments present below 49 mbsf are about an order of magnitude higher than those estimated for sediments from above 49 mbsf (Boström et al., 1976). In analyses of six interstitial water samples, dissolved manganese was present at relatively higher concentration in the upper 45 m of the sediment column (3.5–7.4 ppm) than in the lower 50 m (0–1 ppm) (Brady and Gieskes, 1976). Dissolved sulfate concentration also appeared to be slightly higher in the upper 45 m (>28 mM) than in the lower 50 m (27 mM) (Brady and Gieskes, 1976). Little or no evidence for other postdepositional reactions was seen among major dissolved ions at Site 321. This finding led Brady and Gieskes (1976) to conclude that any reactions in these sediments occur at such slow rates that their chemical signature is annihilated by diffusional exchange with the top and bottom of the sediment column. Consequently, Site 1231 provided a challenging opportunity for assessing the microbial activities and communities of low-activity sediments typical of much of the open ocean.

The subsurface extent of key electron donors (hydrogen, acetate, and formate), electron acceptors with standard free-energy yields greater than that of sulfate (oxygen and nitrate), products of key metabolic reactions (dissolved iron), and other biologically important chemicals was not determined for Site 321.

Principal Results

At Site 1231, the DIC profile hovers slightly at or below 3.3 mM for most of the sediment column. It exhibits three slight exceptions to this relative constancy: it slightly increases from 2.8 mM near the sediment/water interface, it exhibits a small peak of ~3.7 mM centered at 55 mbsf, and it declines slightly to ~3.0 mM at the sediment/basement interface. These DIC concentrations are even lower than those at Site 1225 (3.0–4.0 mM). They are much lower than the DIC concentrations observed at the other Leg 201 sites. Dissolved ammonium concentration is also generally lower at Site 1231 than at the other Leg 201 sites. As at Site 1225, concentrations of DIC, ammonium, and alkalinity peak in the middle of the sediment column and decline toward both the sediment/ocean interface and the sediment/basement interface. The relatively low variability in the concentration profiles of these chemical species suggests that net microbial activity is lower at Site 1231 than at any other Leg 201 site. The midcolumn peaks in these profiles and their relatively low values near both the sediment/water and sediment/basement interfaces indicate chemical exchange from the sediment to the ocean and from the sediment to the basement.

The dissolved sulfate concentration is >28 mM at the sediment surface and decreases linearly to 27 mM near the basement. The slight total downhole decrease in sulfate concentration suggests that Site 1231 is characterized by lower sulfate-reducing activity than all of the other Leg 201 sites. Dissolved sulfide is below the detection limit (0.2 µM) throughout the entire sediment column.

Electron acceptors with higher standard free-energy yields than sulfate are present throughout most of the sediment column at Site 1231. Dissolved nitrate appears to be present in the uppermost meter and the lowermost 60 m of the sediment column (where it ranges from 15 µM at 114 mbsf to 2 µM at 77 mbsf). Dissolved oxygen similarly appears to be present in the top 0.6 m below the seafloor as well as the last 3.8 m of sediment above basaltic basement. The diffusion of oxygen and nitrate from the overlying ocean into the sediment is readily predictable from deep-ocean chemistry. However, the first Leg 201 location, Site 1225, provided the only previous precedent for upward diffusion of nitrate and oxygen into deeply buried sediment from the underlying basaltic crust. As at Site 1225, the introduction of dissolved nitrate high into the sediment column at Site 1231 indicates that nitrate-utilizing microbial activity is present but may proceed at a very low rate in the site's lowermost sediments. Also as at Site 1225, the presence of dissolved oxygen and nitrate in these deepest sediments suggests that microbial activity in the underlying basement is insufficient to strip even the scarcest preferentially utilized electron acceptors from the seawater that flows through the basement.

Dissolved Mn is present from 1 to 65 mbsf at Site 1231. Its concentration steadily rises from ~17 µM at 1.4 mbsf to a local peak of 78 µM at ~17 mbsf, declines briefly by a few micromolar, and then rises to sustain its highest concentration of 120 µM from 36 to 46 mbsf. The Mn concentration below this peak steadily declines to essentially 0 µM at 68 mbsf. A relatively broad zone of generally high but variable dissolved Fe concentration (7–36 µM) spans the interval from 1 to 30 mbsf. A very small secondary peak in dissolved Fe (5 µM) is centered near 74 mbsf. Two aspects of these broad patterns run counter to the general thermodynamically based expectation that manganese reduction should precede iron reduction in marine sediments because the former reaction yields higher free energy than the latter under standard conditions. The first aspect is the broad co-occurrence of dissolved Fe and Mn from 1 to 30 mbsf. The second aspect is the presence of maximum dissolved Fe concentration much closer than maximum dissolved Mn concentration to the sediment/water interface. It appears likely that rates of Mn reduction in these sediments are limited by the availability of manganese oxides that supply dissolved manganese. Rates of Fe reduction may be similarly limited by the presence and solubility of the minerals that supply dissolved iron.

As at other Leg 201 sites, the downhole distribution of microbial Mn and Fe reduction at Site 1231 appears to be ultimately determined by lithology and depositional history. The peak intervals of dissolved manganese production are limited to the clays that lie between 11 and 55 mbsf. The maximum Mn concentration (120 µM) is present in the yellow volcanic-rich clay of Subunit IIA (31–44 mbsf). The secondary peak (78 µM) is centered in the green diatom-rich clay of Unit I (11–30 mbsf). Dissolved iron is similarly limited to the clay-rich portions of the upper sediment column. It exhibits a sharp maximum concentration (36 µM) a few meters below the seafloor in the radiolarian and clay-rich diatom ooze of upper Unit I (0–11 mbsf). Most of the dissolved iron at Site 1231 is present in a broad maximum of 26 µM in the green clay of Unit I. Dissolved Fe concentration is consistently 5 µM in the nannofossil oozes that lie between 55 mbsf and the basaltic crust (114 mbsf). Dissolved Mn is consistently <1 µM over the same interval.

Although Site 1231 may be the microbially least active of the Leg 201 sites, its sediments still contain methane at a concentration of up to 15 µM. At this site, methane is limited to the upper clay-rich portion of the sediment column between 0 and 42 mbsf. This methane-bearing interval is completely within the interval of high dissolved manganese concentration. Interestingly, this methane was only detected after prolonged incubation of headspace samples over a couple of days, whereas short 20-min incubation according to the standard ODP safety protocol showed only a trace methane concentration throughout the sediment column. The appearance of methane over time is currently interpreted as a release of sorbed methane. From sediments below 42 mbsf, no release of sorbed methane was observed and concentration remained at trace levels of <0.2 µM. The relationship of this sorbed methane to current microbial activity remains unknown.

Acetate concentration ranges between 1 and 14 µM at Site 1231. Formate varies between 1 and 19 µM. Concentrations of both fatty acids are lowest in the top 3 m below seafloor (1–2 µM). They are slightly higher (3–6 µM) in the nitrate-reducing zone that spans the last 50 m above basement. Acetate and formate exhibit their highest concentrations (4–14 µM and 9–19 µM, respectively) at intermediate sediment depths (25–75 mbsf and 25–80 mbsf, respectively). These broad patterns suggest that at Site 1231, fatty acid concentration may be lower in the sedimentary intervals that include electron acceptors with the highest energy yields. Curiously, the acetate and formate concentrations at this site are generally an order of magnitude higher than concentrations in sediments of the equatorial Pacific sites but are similar to those found at the Peru margin sites and in other very active coastal marine sediments. Accurate understanding of the fatty acid distribution and the microbial relevance of this will require thorough postcruise analyses of microbial energetics in subseafloor environments.

Hydrogen concentration is extremely high in the uppermost 35 m of the sediment column, with a peak value of 102 nM at 15 mbsf. This is the highest H2 concentration measured at any Leg 201 site. It exceeds the theoretical H2 concentration for an iron-reducing environment by >100-fold. The zone of high H2 coincides with the zone of iron reduction but does not show any direct correlation with distributions of fatty acids or methane. The presence of an extremely high H2 concentration at the sediment site with the lowest organic carbon mineralization rates remains unexplained at this point. From 44 mbsf down to the basement, H2 concentration is, in contrast, very low (0.05–0.22 nM).

Microbial cell counts were conducted on samples from throughout the sediment column at Site 1231. These data show that mean cell concentrations are generally lower at this open-ocean site than at any previously enumerated ocean drilling site. Cell concentrations exhibit a distinct local concentration peak at 10–15 mbsf, the approximate depth of the Unit I zone of iron and manganese reduction.

Experiments on major bacterial processes and on enumeration of viable bacteria were initiated at selected depths ranging from near the seafloor 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. Cultivation experiments particularly focused on manganese- and iron-reducing bacteria throughout the column. Studies of sulfate-reducing bacteria in macrofaunal burrows were also initiated. Detailed microbiological sampling targeted sediment depths of particular biogeochemical interest, such as the midcolumn reduced manganese interface and the sediment/basalt interface.

The results from six Adara temperature tool deployments define a temperature profile composed of two distinct intervals: a linear gradient of 90°C/km from 0 to 55 mbsf and a linear gradient of 35°C/km from 55 to 115 mbsf. The sediment/water interface temperature measured by a mudline Adara tool deployment is 1.7°C. The estimated temperature at the base of the drilled sediment column (115 mbsf) is 8.6°C. Throughout the entire drilled interval (0–121 mbsf), temperatures are in the psychrophilic range.

Trials were undertaken of two experimental tools at this hole: the Davis-Villinger Temperature Pressure Probe (DVTP-P) and the catwalk infrared (IR) camera. The single DVTP-P deployment indicated minor overpressure at 108 mbsf.

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