Interstitial water studies at Site 1227 define one of the most highly resolved chemical records in Ocean Drilling Program (ODP) history. An important objective with these profiles is to identify and quantify zones of prokaryotic activity based on reactive interstitial water species. A deep 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 of 3 mM/m below that) to reach twice seawater chlorinity at 120 mbsf. Downhole depletion of sulfate at a relatively shallow depth, dissolved inorganic carbon (DIC) concentrations as high as 25 mM, ammonium rising to 23,000 然 at 150 mbsf, and very high concentrations of methane all indicate that prokaryotic activity is much higher at this ocean-margin site than at open-ocean Sites 1225 and 1226. Dissolved sulfate concentrations rapidly decline in the upper 15 mbsf from a seawater value of 29 mM to 5 mM. Concentrations then decline more slowly to 0 mM at ~40 mbsf. Concentrations of dissolved hydrogen sulfide (
H2S = H2S + HS-) rise 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 bacterial sulfate reduction occurs throughout the interval. Sulfide concentrations steadily decline over the sulfate-poor remainder of the drilled section to <0.3 mM at 150 mbsf.
From 1 to 31 mbsf, dissolved barium concentrations rise slightly, from 0 to 1.9 然. Over the next several meters, barium concentrations rise at an increasingly steep rate, climbing from 9 然 at 38 mbsf to 170 然 at 43 mbsf. Concentrations then rise steadily to 350 然 at ~150 mbsf. Dissolved sulfate and barium are both present throughout the entire interval of nonzero sulfate. Throughout this interval, the concentrations of dissolved barium and dissolved sulfate appear to be related by the solubility product of BaSO4 (barite). Upward diffusion of barium 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 (XRD). At slightly greater depth (~42 mbsf), dissolved sulfate concentrations decline toward 0 mM, barite begins to dissolve, and dissolved barium concentrations rise. The narrow barium 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 methane concentrations slowly rise from 7 然 at 1 mbsf to 55 然 at 35 mbsf. From 40 to 56 mbsf, methane concentrations then rapidly rise to 2 x 103 然 at 56 mbsf and hover in the range of 103 然 for the remainder of the drilled sediment column. The disappearance of almost all methane at the depth of sulfate depletion indicates that most of the methane diffusing upward through this sediment column is ultimately destroyed by anaerobic methanotrophy. The presence of methane at a low concentrations throughout the overlying sediment column indicates, as at open-ocean Sites 1225 and 1226, that methane can be maintained at a background level of several micromolar in subseafloor sediments, despite the potential for methane oxidation by sulfate reduction.
Like methane, ethane and propane 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. Concentrations of ethane decline sharply at the 40-mbsf top of the anaerobic methanotrophy zone (from 2 to 0.7 然). Concentrations of propane decline more gradually (from 3 to 0 然) 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. The presence of these small peaks demonstrates 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 (~5 mbsf for methane and 15-25 mbsf for ethane and propane). Trace concentrations of the ethane (10-1 然) 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 sulfate reduction.
In most incubation samples from Site 1227, hydrogen concentrations are between 0.2 and 0.5 nM. These concentrations closely resemble those observed at open-ocean Site 1226. Samples from the first few meters of the sediment column exhibit significantly higher hydrogen concentrations (0.9-2.4 nM). These concentrations are consistent with observation in shallower nearshore sediments (Hoehler et al., 1998). However, similar concentrations in samples from 93 and 113 mbsf are a factor of five to ten lower than those observed in nearshore 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 hydrogen at free-energy yields lower than the generally accepted theoretical limit for actively growing cells.
The volatile fatty acids (VFAs) formate and acetate are important intermediates in the anaerobic pathways of organic matter degradation and were analyzed throughout the sediment column. Acetate concentrations range between 0 and ~10 然 and generally increase from the surface sediment down to the base of the drilled sediment column (~150 mbsf). Formate concentrations vary considerably throughout the sediment column (between 0 and ~5 然) but exhibit no mean trend over the sampled sediment column. The average VFA 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 principal prokaryotic processes, although the absolute process rates are orders of magnitude lower in the open-ocean sediments than in the coastal sediments.
Concentrations of dissolved manganese and iron are, respectively, 0-6 and 0-30 然 at Site 1227. The peak manganese concentration from Site 1227 (6 然) 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 iron concentration from Site 1227 (30 然) 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 iron and manganese 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 manganese and iron oxides, or dissolved manganese and iron are scavenged and precipitated much more quickly at this site. Stratigraphic relationships between magnetic susceptibility and dissolved sulfide concentrations suggest that these dissolved metals are scavenged by sulfide precipitation at Site 1227. A relatively steep decline in sulfide concentrations 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 value 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 (GRA) bulk density, grain density, P-wave velocity, natural gamma radiation (NGR), 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 iron, manganese, silica, and phosphate are also present in this interval. Secondary peaks in the dissolved concentrations of iron, manganese, silica, and phosphate 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, iron and manganese reduction, ethanotrophy, and propanotrophy) are pinned in a narrow stratigraphic interval by physical properties and sediment composition at this site. The mineral composition and 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 iron oxides and subsequent iron reduction. To a much lesser extent, magnetic susceptibility may also be enhanced by biologically induced precipitation of reduced iron and manganese. 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 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 prokaryotes ranging from psychrophilic to thermophilic. Specific sampling was targeted to the sulfate/barium interface to study the possible attack of sulfate-reducing bacteria and methane-oxidizing archea on sulfate bound in barite.
One Adara and two Davis-Villinger Temperature Probe (DVTP) deployments combined with the Leg 112 data define a linear gradient of 49蚓/km, with a sediment/water interface temperature of 8.6蚓 and an estimated temperature at 160 mbsf of 16.4蚓. 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 with pressure (DVTP-P), the Fugro Percussion Corer (FPC), and the APC-Methane (APC-M) tool.
