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The principal objective of Leg 201 was to document the nature, extent, and biogeochemical consequences of microbial activity and microbial communities in several different deeply buried marine sedimentary environments. To maximize the scientific utility of Leg 201 results, biogeochemical, microbiological, physical property, and sedimentological sampling and analyses and downhole tool deployment and downhole logging were all closely integrated. Leg 201 physical property, sedimentology, and downhole studies provided detailed evidence of the environmental factors that influence subseafloor microbial life and are in turn influenced by it. To build comprehensive records of net microbial activities and their consequences, the Leg 201 shipboard party undertook a depth and range of biogeochemical studies that were unprecedented in ODP history. To develop comprehensive records of subseafloor microbial communities, the shipboard party also undertook and initiated a range of microbiological studies that was unprecedented in ODP history.

Subseafloor Microbial Activities of Different
Sedimentary Geochemical Regimes

During Leg 201, a variety of sediments (Fig. F2) were cored in both open-ocean and ocean-margin provinces in the eastern tropical Pacific Ocean. Neogene deep-sea clays and Paleogene nannofossil ooze were cored at Peru Basin Site 1231. Miocene to Holocene carbonate and siliceous oozes and chalk were cored at Sites 1225 and 1226. Miocene to Holocene biogenic oozes and terrigenous sediments of the shallow Peru shelf were cored at Sites 1227, 1228, and 1229. Organic-rich Miocene to Holocene sediments were cored at Site 1230 on the Peru slope, in the accretionary wedge just landward of the Peru Trench.

In situ data from Leg 201 demonstrate that comparable temperature ranges are found at all the sites, allowing direct comparisons of microbial activities and communities in very different environments under similar thermal conditions (Fig. F3). In situ temperatures of the organic-poor sediments of Sites 1225, 1231, and the organic-rich Peru shelf Site 1230 are in the range preferred by psychrophilic bacteria (0°–10°C). In situ temperatures of the organic-rich Peru shelf sediments and throughout most of the sediment column at eastern equatorial Pacific Site 1226 are in the low end of the range preferred by mesophilic bacteria (which inhabit 10° to 35°C environments).

AODCs showed that cell concentrations of the Leg 201 sediments generally followed the well-established trend of exponential declines in cell concentration with subsurface depth. Cell counts are generally higher at the ocean-margin sites than at the open-ocean sites (Fig. F4). Cell concentrations progressively increase from Peru Basin Site 1231 to equatorial Pacific Site 1225 to eastern equatorial Site 1226 to Peru shelf Site 1227 to Peru shelf Site 1229 and Peru slope hydrate Site 1230. A similar difference between open-ocean and ocean-margin sites was noted in a recent survey of previously studied ODP sites (Parkes et al., 2000).

The geometric mean concentrations for all of the Leg 201 sites are close to, or even slightly below, the mean concentrations for all previously studied ODP sites (Fig. F5). This finding is somewhat surprising because so many of the Leg 201 sites contained organic-rich sediments, including the Peru shelf sites, Peru slope hydrate Site 1230, and eastern equatorial Site 1226. Because previously enumerated sites targeted a representative range of ODP sampled environments, the similarity of our results to the average for previously studied sites may reflect a collective ODP bias toward drilling at the ocean margins and away from drilling in such regions as the oceans' central gyres.

Interstitial water studies of the Leg 201 sites showed that net subseafloor microbial activity is much higher at the ocean-margin sites than at the open-ocean sites. For example, subseafloor concentrations of dissolved inorganic carbon (DIC = HCO3 + CO2 + CO32–) and ammonium (NH4+) are much higher at ocean-margin Sites 1227, 1228, 1229, and 1230 than at open-ocean Sites 1231, 1225, and 1226 (Fig. F6). DIC and ammonium are generic products of microbial activity, regardless of the principal electron-accepting pathway. Consequently, the very high concentrations of those chemical species at the ocean-margin sites and the generally low concentrations at the open-ocean sites indicate that net respiration of organic carbon to CO2 and net mineralization of organic nitrogen to NH4+ is much higher in subseafloor sediments of the ocean margin than subseafloor sediments of the open ocean.

On a more precise level of comparison, the profiles of DIC and ammonium show that the Leg 201 sites span a wide range of subsurface activity levels (Figs. F6, F7). The lowest net activity occurs at clay-rich open-ocean Site 1231. Rates of net activity are visibly higher at equatorial Site 1225 and still higher at eastern equatorial Site 1226. Still higher rates occur at the Peru shelf sites (1227, 1228, and 1229). The highest rates occur at Peru slope hydrate Site 1230, downslope of the shelf sites and directly beneath the Peru upwelling zone. This site-to-site progression of increasing DIC and ammonium concentrations closely matches the progression of increasing cell concentrations seen in Figure F4. The similarity of these progressions suggests that subsurface cell concentrations are related to net subsurface microbial activity.

Leg 201 profiles of dissolved chemicals in interstitial waters also document the subseafloor occurrence of specific microbial processes. Despite the large apparent differences between net microbial activities of the ocean-margin sites and those of the open-ocean sites, all of the microbial processes that we interpret to occur observed in subsurface ocean-margin sediments also occur in the subseafloor open-ocean sediments.

Downhole depletion of dissolved sulfate occurs at all of the Leg 201 sites (Fig. F8). The lowest magnitude of subseafloor depletion occurs at open-ocean Site 1231, where dissolved SO42– declines by <2 mM. The highest magnitude of subseafloor SO42– depletion occurs at the Peru slope hydrate Site 1230, where all of the SO42– diffusing downward from the seafloor disappears by 9 mbsf.

Concentration profiles of dissolved Mn and Fe (Fig. F8) suggest that Mn and Fe reduction also occur at all of the sites sampled during this cruise. Reduced Mn and Fe are, respectively, products of Mn(IV) and Fe(III) reduction. Consequently, concentration profiles of dissolved Mn and Fe can be used to estimate net rates of Mn and Fe reduction in subseafloor environments. Concentrations of dissolved Mn (Fig. F8) and Fe are generally higher in the deeply buried open-ocean sediments than in the deeply buried ocean-margin sediments. By inference, Mn and Fe reduction are overall more important for diagenesis in subseafloor open-ocean sediments. This finding is consistent with the general perception that manganese oxides and iron oxides are largely depleted at shallow sediment depths in shelf environments (Canfield et al., 1993).

Methane is a stable product of some microbial activities. It is a source of carbon and energy for other microbial activities. The occurrence of distinct minima and maxima in the subsurface profiles of dissolved methane at all Leg 201 sites indicates that methane is biologically created and destroyed in the subsurface realm of both the ocean-margin and open-ocean provinces (Fig. F9). Documentation of methane profiles at these open-ocean sites required Leg 201 scientists to develop new sample handling protocols and to accurately measure CH4 at extremely low concentration. The discovery of methane at all Leg 201 sites strongly suggests that methanogenesis occurs in deeply buried sediments throughout the world ocean. This discovery builds on the recent demonstration that methane is commonly present in subseafloor sediments of the open ocean, despite the presence of high SO42– concentration (D'Hondt et al., 2002). Unexpectedly, the Leg 201 studies also demonstrate that methanogenesis occurs in subseafloor realms of active Mn and Fe reduction. This discovery echoes the recent demonstration that methanogenesis and iron reduction co-occur in the continental subsuface environment of a freshwater aquifer (Bekins et al., 2001). Even more unexpectedly, Leg 201 studies also showed that both ethane and propane are biologically created and destroyed in parallel with methane at both open open-ocean and ocean-margin sites (example from Site 1227 in Fig. F10).

Acetate and H2 are generally understood to be the most common products of microbial fermentation. They are also generally understood to be the most common energy sources in microbial respiration reactions. Consequently, they are expected to be key intermediates in subsurface microbial activities. Leg 201 studies showed that acetate, formate, and H2 are present throughout the subsurface environment at all sites. The properties that control subsurface concentrations of acetate, formate, and hydrogen, whether kinetic or thermodynamic, remain to be determined.

Influences of Subseafloor Flow and Brines on Microbial Activities
and Sediment Chemistry Deep in the Sediment Column

Classic models of microbial activity in marine sediments assume that the oxidants (electron acceptors) that are used to support respiration by sedimentary microbes are introduced to the sediment through the sediment/water interface. Sulfate is the most common electron acceptor in marine sediments. Oxygen and nitrate are the electron acceptors that yield the highest free energies. Sulfate, O2, and NO3 are all introduced to the subseafloor realm by diffusion from the overlying ocean. Leg 201 studies of open-ocean Sites 1225 and 1231 showed that nitrate and traces of oxygen also enter at least some deep-sea sediments by diffusing or advecting upward from waters flowing through the underlying basaltic crust (Fig. F11). In this manner, electron acceptors with high free-energy yields enter deep in open-ocean sediment columns. Conversely, downward diffusion from the sediment into the water flowing through the basement strips away products of microbial activity, such as methane, ammonium, and DIC. In both of these ways, chemical exchange between the sediment column and the underlying basement mirrors diffusive exchange across the sediment/ocean interface.

The discovery that nitrate and oxygen enter the sediment column from the formation waters of the underlying basalt has an additional interesting implication. It indicates that microbial activity within the basalt is insufficient to strip even the scarcest preferentially utilized electron acceptors from the water that flows through it.

At the Peru shelf sites, chemical species also enter the sediment sequences from below. However, at these sites, the chemicals diffuse upward from underlying Miocene brine. At Site 1229, dissolved sulfate diffuses upward into methane-rich Pleistocene sediment in this manner. Downhole profiles of dissolved sulfate, methane, acetate, and barium show that chemical distributions at this brine incursion interface mirror those of the overlying "normal" sulfate/methane interface (Fig. F12). Cell counts and the pronounced inflections in chemical profiles at this depth demonstrate that the subseafloor microbial cell populations and activity are locally strongly focused at this interface. Dissolved barium profiles indicate that microbial activity at this interface directly influences sediment chemistry by causing the precipitation of barite immediately below the interface and the dissolution of barite immediately above it. Perhaps the most striking feature of this interface is its thousandfold increase in cell concentration relative to the sediments that lie immediately above and below it (Fig. F12). The cell concentration observed at this interface is actually an order of magnitude higher than that observed at the seafloor 90 m above it.

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