At Site 1225, concentrations of methane, ammonium, dissolved inorganic carbon (DIC), and alkalinity peak in the middle of the sediment column and decline toward both the sediment/ocean interface and the sediment/basement interface. In contrast, sulfate concentration is lowest in the middle and lower part of the sediment column and nitrate and dissolved oxygen are present only at the ocean and basement interfaces. These profiles result from the balance between net subsurface microbial activities and small net fluxes of biologically utilized chemicals across the ocean/sediment and sediment/basement interfaces.
Interstitial water data also document dissolved oxygen penetration into the top 2 m of the sediment column, an interval containing nitrate in the top 1.5 m of the sediment, a peak concentration of dissolved manganese at 3.6 meters below seafloor (mbsf), a broad zone of relatively high dissolved iron centered at ~25 mbsf, and sinks for reduced manganese and dissolved iron at 100 mbsf. Sulfate concentrations decrease downhole by only ~7% from local bottom-water values; most of this decrease occurs in the upper ~60 m. This vertically extended sequence of successive interstitial water chemical zones closely resembles the centimeter- to decimeter-scale sequence seen in nearshore sediments (with depth-dependent transitions from a zone of oxygen reduction to successive zones of nitrate, manganese oxide, iron oxide, and sulfate reduction). These data are consistent with the hypothesis that subseafloor microbial communities preferentially utilize the available electron acceptor that yields the highest free energy of reaction.
In the lower portion of the sediment column, this vertical sequence of successive reduction zones is reversed as a result of water flow through the underlying basaltic basement. Diffusion of solutes from this basement water to the overlying sediment delivers nitrate to the lowermost 20 m of the sediment column (300 mbsf to basement) and possibly also dissolved oxygen to the lowermost meter of the column (319.3 mbsf to basement). This short interval of dissolved oxygen and nitrate is overlain by a broad zone of dissolved manganese centered near 250 mbsf and a broad peak of dissolved iron centered at ~230 mbsf. These dissolved nitrate and oxygen profiles show that electron acceptors yielding high free energies of reaction are introduced to at least some portions of the deep subseafloor biosphere by hydrologic processes. They also indicate that microbial activity in the underlying basalt is insufficient to strip even the scarcest preferentially utilized electron acceptors from the seawater that flows through the basalt at this site.
Dissolved hydrogen concentrations in incubations of Site 1225 sediments are generally in the range of 1-2 nM. Lovley and Goodwin (1988) and Hoehler et al. (2001) observed similar concentrations in experiments with near-surface aquatic sediments, where sulfate reduction is the primary electron-accepting reaction. On the basis of their observations, Lovley and Goodwin (1988) hypothesized that hydrogen concentration in aquatic environments is controlled by competition between different metabolic pathways. According to this hypothesis, prokaryotes using electron acceptors that yield higher free energies of reaction are able to operate at lower electron donor concentrations and thereby out-compete prokaryotes limited to electron acceptors that yield lower free energies of reaction. Documentation of these concentrations at Site 1225 suggests that even in low-activity subseafloor sediments, hydrogen concentrations may be controlled by the same thermodynamic competition between electron-accepting pathways as in high-activity sediments and can be predicted from the dominant pathway.
Methane is present at trace concentrations of <0.25 µM throughout the sediment column. This finding demonstrates the presence of methane in subseafloor sediments with sulfate concentrations that are very close to seawater values. The generation of methane in these sediments challenges models of microbial competition that are based on standard free energies. There are a number of possible reasons for the occurrence of methanogenesis in sulfate-rich sediments. For example, the methanogens and sulfate reducers may rely on different electron donors (e.g., the methanogens may utilize methylated amines and the sulfate reducers may rely on hydrogen and/or acetate) (Oremland and Polcin, 1982; Oremland et al., 1982; King, 1984).
The steady-state maintenance of methane in the subseafloor sediments of Site 1225 indicates that if anaerobic methanotrophy occurs here, it does not drive methane concentrations below a few hundredths to tenths of micromolar. Concentrations are lowest near the sediment/ocean and sediment/basement interfaces, where methane may be oxidized by prokaryotes using electron acceptors that yield relatively high energies of reaction (such as nitrate or dissolved oxygen). The highest methane concentration is present in the middle of the sediment column, where sulfate appears to be the principal terminal electron acceptor available. We hypothesize that the peak methane concentration is held at the observed level (~0.15-0.25 µM) because sulfate-reducing methanotrophs cannot oxidize methane at lower concentrations under in situ conditions.
Concentrations of acetate and formate were <1 and <0.5 µM, respectively, throughout the sediment column. These concentrations are an order of magnitude lower than those measured in continental shelf sediments (Sørensen et al., 1981; Wellsbury and Parkes, 1995) and are also lower than those in other deep sediment sites (Wellsbury et al., in press). These very low concentrations may be regulated by limiting energy yields or limited by the kinetics of active uptake by the anaerobic respiring prokaryotes. Since these results are among the first to demonstrate very low concentrations of short-chain fatty acids in cold, low-activity subsurface sediments, there is no database for comparison.
Comparison of Site 1225 physical property, sedimentology, and chemical records suggests that broad-scale patterns of past oceanographic change exert strong influence on present subseafloor metabolic activity. Concentrations of dissolved iron closely follow downhole variations in magnetic susceptibility and split-core reflectance, with peak concentrations of dissolved iron and solid-phase iron compounds (inferred from magnetic susceptibility) in the intervals from ~0 to 70 and 200 to 270 mbsf. The intact magnetic reversal record suggests that the magnetic phases were created during or shortly after sediment deposition. These intervening sediments are low in dissolved iron, are low in magnetic susceptibility, are characterized by the most intensely bioturbated intervals, and were deposited during a late Miocene-early Pliocene "biogenic bloom" that occurred throughout much of the global ocean (van Andel et al., 1975; Farrell et al., 1995; Dickens and Owen, 1999).
Four Adara tool deployments plus two deployments of the Davis-Villinger Temperature Probe (DVTP) defined a sediment/water interface temperature of 1.4°C and an estimated sediment/basement interface temperature of 7.0°C. The downhole temperature gradient curved slightly downward. The slight curvature appears to be best explained by a geologically recent decrease in basement temperature, perhaps a result of an increased rate of seawater flow through the basement. Throughout the sediment column, in situ temperatures were well within the range inhabited by psychrophilic prokaryotes.
Experiments on major microbial processes and experiments for enumeration of viable prokaryotes were initiated at selected depths ranging from near the mudline to near the basement, where samples were obtained within centimeters of the basalt. Subsamples for postcruise biomolecular assays and microbiological experiments were routinely taken from all of the distinct geochemical zones and lithologic subunits. Total cell numbers were enumerated on board. These cell counts are very close to data obtained from nearby Site 851 and consequently demonstrate the high reproducibility of acridine orange direct counts (AODCs) in subseafloor microbial studies.
At this site, novel experiments with core temperatures and contamination tracers were undertaken to determine how handling of cores and samples for microbiological studies might be improved. Catwalk experiments with an infrared (IR) camera were used to assess the effects of different core handling procedures on transient warming of the core and, consequently, on the survival of temperature-sensitive prokaryotes.