An important objective for Site 1229 is to identify and quantify zones of microbial activity based on reactive interstitial water species. Toward this end, we established a highly resolved chemical record throughout the drilled sediment column. Profiles of conservative ions provide evidence of diffusive mixing between seawater diffusing downward from the sediment/water interface and a brine diffusing upward from older sediments. For example, concentrations of dissolved chloride increase linearly from 559 mM at the sediment/water interface to 1208 mM at the base of the drilled sediment column (186 mbsf). Peak concentrations of biologically affected chemical species, such as ammonium (5800 然) and dissolved inorganic carbon (DIC) (22 mM), indicate that rates of subseafloor microbial activity are much higher at this ocean-margin site than at open-ocean Sites 1225 and 1226. These peak concentrations also suggest that the subseafloor microbial activity at Site 1229 is slightly greater than that at Site 1228 (which lies just seaward of Site 1229) and perhaps is slightly less than that at Site 1227 (which is situated 310 km to the north on the Peru shelf).
As at Site 1228, the concentration profiles of several biologically affected chemical species exhibit a pronounced anomaly just below the seafloor (at 2-3 mbsf). This anomaly at Site 1229 consists of a brief positive excursion in alkalinity, DIC, ammonium, and sulfide, with a co-occurring negative excursion in dissolved sulfate. The same anomaly is also apparent in the ammonium and alkalinity profiles of Site 681 (Shipboard Scientific Party, 1988). As described in "Principal Results" in the "Site 1228" chapter, this near-surface interstitial water anomaly indicates that the steady-state diffusion of biologically active chemicals past the upper sediment column was disrupted by late Pleistocene environmental change and has not yet fully recovered. There are least three possible general explanations of this anomaly. It may result from ongoing activity in a microbial "hotspot" at this shallow sediment depth, it may be a chemical relic of past microbial activity (now relaxing back to a diffusional steady state), or it may be a result of the recent establishment of an oxygen minimum at this water depth, causing the extinction of a bioirrigating benthos and a stimulation of sulfate reduction in the uppermost 2 m of sediment.
The most striking biogeochemical feature of this site is the reversal of the biogeochemical zonation at depth. This reversal is immediately apparent in the dissolved sulfate profile. The sulfate concentrations decline from a seawater value of 29 mM at the sediment surface to 0 mM at ~35 mbsf. They remain at 0 mM from 35 to 88 mbsf and then steadily rise from 0 to 38 mM at 186 mbsf. The sulfate that sustains microbial reduction over the uppermost 35 mbsf of the sediment column ultimately diffuses downward from the overlying ocean. The sulfate that sustains microbial reduction below 88 mbsf is inferred to diffuse upward from the underlying brine. Both intervals of sulfate reduction are marked by local maxima in the concentrations of dissolved sulfide, with a broad peak from ~20 to 40 mbsf and a sharper peak at ~90 mbsf.
The sulfate profile is mirrored by the dissolved methane profile. Dissolved methane concentrations are <100 然 from 0 to 20 mbsf, hold steady at a few hundred micromolar from 20 to 35 mbsf, and then rise to values of ~2000 然 (exceeding 1 bar partial pressure) between 65 and 75 mbsf. Methane then steadily declines to <100 然 at 93 mbsf and remains in the range of 100 然 or less to the base of the sampled sediment column. As at Site 1227, the disappearance of almost all methane at the depths of sulfate depletion indicates that most of the methane in this sediment column is ultimately destroyed by anaerobic methanotrophy. As observed at all previously drilled Leg 201 sites, the Site 1229 methane and sulfate profiles indicate that methane can be maintained in subseafloor sediments at background concentrations that are inversely related to the co-occurring dissolved sulfate concentrations.
The dissolved iron and manganese concentration profiles demonstrate that net reduction of iron and manganese oxides occurs in the methanogenic zone. The principal foci of net manganese and iron reduction are at slightly different depths, with iron reduction peaking at 75-90 mbsf and manganese reduction just above and below that interval. The presence of methanogenesis in iron- and manganese-reducing environments may result from a limited availability of mineral-supplied electron acceptors relative to electron donors. In these organic-rich sediments, electron donors may be supplied to the microbial community faster than mineral dissolution can supply dissolved reducible manganese and iron.
The dissolved barium profile is broadly similar to the methane profile. Dissolved barium concentrations are <2 然 from 0 to 24 mbsf. Concentrations of barium in interstitial water then rapidly rise to 18 然 at 40 mbsf and remain near 19 然 until almost 80 mbsf. They then decline steeply to 2 然 at ~100 mbsf and are <2 然 for the remainder of the drilled sediment column. As at Site 1227, the inverse relationship between sulfate and barium is inferred to be controlled by the solubility product of BaSO4 (barite). Upward diffusion of barium past 35 mbsf and downward diffusion of barium past 90 mbsf is suspected to sustain modern barite formation at, respectively, ~24 and 100 mbsf. Similarly, the shoulders of the barium peak at ~40 and 80 mbsf are inferred to mark the principal depths of current barite dissolution at this site.
Prokaryotic cell counts were done at 10-m intervals throughout the upper sediment column and across both sulfate/methane interfaces. These data show that mean sedimentary cell concentrations are several-fold higher at this ocean-margin site than at the Leg 201 open-ocean sites and may be slightly higher than mean concentrations at nearby Site 1227. The most striking features of the shipboard cell counts are the thousandfold increase in cell concentrations in the lower zone of overlapping sulfate and methane concentrations and the tenfold increase in cell concentrations in the upper zone of overlapping concentrations. The maximum cell concentrations observed in the lower sulfate/methane zone are actually an order of magnitude higher than the concentrations observed at the sediment/water interface. Given the coarse spacing of these samples and their positions relative to the chemically defined sulfate/methane overlap zones, the peak cell concentrations observed in the upper sulfate/methane zone may greatly underestimate the peak concentrations in that zone.
Acetate and formate concentrations exhibit strong local maxima of ~6 然 in both of the sulfate/methane interface zones. These maxima are centered at 37 and 90 mbsf. As with the cell counts, these local maxima are higher than the local maxima exhibited by both acetate (~2 然) and formate (3 然) at the sediment/water interface. Throughout most of the remaining record at this site, concentrations of both species were between 1 and 2 然. As at Site 1227, the concentrations of both species reach their highest values near the base of the drilled sediment column (~15 然). These results are intriguing because these volatile acids are important substrates for both sulfate reducers and methanogens. Hydrogen is another important electron donor in anaerobic communities. Almost all hydrogen concentrations measured at this site were <0.5 nM, and most were <0.2 nM. These concentrations resemble those observed at open-ocean Site 1225 and ocean-margin Site 1227. As noted in "Principal Results" in the "Site 1225" chapter and "Principal Results" in the "Site 1227" chapter, these concentrations are much lower than expected from experiments with sulfate-reducing and methanogenic communities of surface sediments. The accurate interpretation of these acetate, formate, and hydrogen concentrations must await postcruise analyses of prokaryotic energetics in subseafloor environments.
The cell concentration data and sulfate and methane gradients demonstrate that the subseafloor prokaryotic population and activity are locally strongly focused at the sulfate/methane overlap zone defined by the upward-diffusing sulfate-bearing brine and the downward-diffusing methane. The dissolved barium profile indicates that microbial activity in this zone directly influences sediment chemistry by mediating the precipitation and dissolution of barite. In these effects on subsurface biological activities and biogeochemical cycles, this brine-caused sulfate/methane interface mirrors the effects of the overlying "normal" sulfate/methane interface. Postcruise microbiological studies will be required to demonstrate whether or not the microbial community supported by the brine-induced interface is locally unique or the same as that supported by the overlying interface.
The upper sulfate-rich zone at Site 1229 lies entirely within lithostratigraphic Subunit IA, a stratigraphic interval of primarily hemipelagic sediments (0-40 mbsf). The underlying methane-rich zone is largely limited to lithostratigraphic Subunit IB, which is the upper portion of a longer interval (40-138 mbsf) of mixed terrigenous and hemipelagic sediments. The anaerobic oxidation of methane (AOM) zones that separate the upper and lower sulfate-rich zones from the intervening methane-rich zone are associated with brief sedimentary intervals characterized by high grain density, high natural gamma radiation (NGR), high resistivity, and low porosity. These brief low-porosity intervals are unusually rich in terrigenous sediment and are interpreted to have been deposited during the two most recent lowstands of four onlap/offlap cycles that define the 40- to 138-mbsf interval.
In short, as at Site 1228, the upper sulfate-reducing interval at Site 1229 is composed of predominantly hemipelagic sediments, the strongly methanogenic zone is rich in terrigenous sediment relative to the overlying sulfate-reducing zone, and the intervening AOM zone is present just above an interval of low-porosity, high-density lowstand sediments. The lower AOM zone at Site 1229 is present within a similar interval of high-density, low-porosity lowstand sediments. The lithologic association of AOM zones with high-density, low-porosity lowstand sediments at Sites 1229, 1228, and 1227 provides intriguing evidence that, on the Peru shelf, the position of AOM zones is currently pinned within the sediment column by lithologic properties and, by extension, depositional history.
As at Site 1227, stratigraphic patterns of magnetic susceptibility and dissolved manganese, iron, and sulfide concentrations indicate similar control of other microbial processes by depositional history at Site 1229. Magnetic susceptibility is generally much higher in the methanogenic zone and in the lower sulfate-reducing zone than in the overlying sulfate-reducing zone. This circumstance suggests that mineral sources of reducible iron and manganese are more abundant in the terrigenous-dominated sediments of the lower sulfate-reducing zone and the mixed terrigenous and hemipelagic sediments of the methanogenic zone than in the mostly hemipelagic sediments of the upper sulfate-reducing zone. The relatively high magnetic susceptibility of the intervals with more strongly terrigenous sediments is consistent with our finding that dissolved manganese and iron concentrations are generally higher in the lower methanogenic zone and the underlying sulfate-reducing zone than in the upper sulfate-reducing zone. The presence of higher manganese and iron concentrations and lower sulfide concentrations in these relatively high susceptibility intervals in turn provides strong evidence that the current rates and stratigraphic foci of iron reduction, manganese reduction, and sulfide precipitation depend strongly on depositional history.
Experiments on major microbial processes and on enumeration of viable prokaryotes 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. Detailed microbiological sampling targeted the top of the sediment column and both the upper and lower sulfate/methane overlap zones.
The results from one Davis-Villinger Temperature Probe (DVTP) deployment were combined with temperature data from Site 681 to define a linear gradient of 35.5蚓/km for this site. The mean sediment/water interface temperature defined by this gradient is 13.4蚓. The temperature defined for the base of the drilled sediment column (193 mbsf) is 20.2蚓. Throughout this interval (0-193 mbsf), temperatures are in the low mesophilic range.
Trials were undertaken of four experimental tools at this hole: the pressure coring sampler (PCS), the Davis-Villinger Temperature-Pressure Probe (DVTP-P), the Advanced Hydraulic Piston Corer-Methane (APC-M) tool, and the Fugro percussion corer (FPC).