The biogeochemical zonation of Site 1230 is more typical of an upper-slope sediment than a typical deep-sea sediment; its uppermost sediment contains a narrow suboxic zone, and sulfate depletion occurs at <9 mbsf. Oxygen and nitrate are not detectable at the top of the mudline core. Dissolved manganese is present in the uppermost 0.5 m of sediment but is near the detection limit (<1 然) throughout the remaining sediment column. Dissolved iron is likewise low (mostly 1-3 然) in the upper 25 m of the sediment. Below the narrow suboxic zone, sulfate reduction is the dominant mineralization process down to the bottom of the sulfate zone at 8-9 mbsf. The sulfate gradient is nearly linear and indicates that most of the net sulfate reduction takes place at the sulfate/methane interface (Iversen and J鷨gensen, 1985; Niew鐬ner et al., 1998; Borowski et al., 1996, 2000).
Methane builds up steeply beginning at the sulfate boundary, and it reaches >1 bar partial pressure by 11 mbsf. Below that depth, methane concentrations in recovered cores fluctuate around a few millimolar, which is the usual pattern in supersaturated cores with gas escape upon depressurization. At Site 1230, however, nine successful deployments of the pressure coring sampler (PCS) at depths ranging from 22 to 277 mbsf allowed the methane concentration profile from the entire sediment column to be accurately determined. The PCS recovered a full 1-m core in most deployments. Its highest internal pressure was 8086 psi in a core recovered from 254.6 mbsf. At 254.6 mbsf, 8086 psi is equivalent to 105% of hydrostatic pressure. The overpressure is caused by dissolving gas hydrate resulting from warming during the wireline trip (Dickens et al., 2000). The total amount of methane retrieved by the PCS reached 400,000 然 methane at 157 mbsf. This greatly exceeds methane solubility at the ambient temperature and hydrostatic pressure but is consistent with the presence of several percent gas hydrate in the sediment pore space.
The presence of gas hydrate was also monitored by rapid infrared (IR) scanning of the recovered cores. Immediately after retrieval, each core was brought to the catwalk and scanned along the core liner surface with a digital IR camera. Our purpose was to detect the cooling effect caused by rapid gas hydrate dissolution. This approach was successful, as core segments with negative temperature anomalies of about -5蚓 proved to harbor gas hydrate. Hydrate was recovered from ~82 and ~148 mbsf as small pieces mixed with sediment. The recovered hydrate probably represented only a small fraction of the in situ hydrate because of rapid dissolution and loss in the expanding cores. Samples from four additional horizons (123, 142, 150, and 200 mbsf) probably contained disseminated gas hydrate, based on observed fizzing and scanned temperature anomalies as low as -3.2蚓. Downhole sonic and resistivity logs suggest broad intervals of possible hydrate presence. Preliminary comparison of inferred hydrate distributions and PCS methane data suggests that the interstitial concentrations of dissolved methane build up to reach the phase boundary of hydrate formation at ~50 mbsf. The dissolved concentrations may remain at this phase boundary at depth, with intervals of hydrate formation determined by the lithology and physical properties of the sediment.
The depth distribution of chloride in the interstitial water also provided evidence of hydrates, which release freshwater by dissolution during the wireline trip of the sediment core. Chloride shows a distinct gradient with a peak at 18 mbsf. This subsurface peak is presumably a remnant of the last glacial salinity excursion. It is accentuated by a drop in chlorinity below 18 mbsf that is probably due to freshening by hydrate dissolution. Within the methane zone, the drop in chlorinity is 10-27 mM and the concentrations show strong depth fluctuations with minimum values that appear to coincide with depths of hydrate occurrences (e.g., at 82 mbsf).
Ethane and propane are present at 1-2 ppm concentrations throughout the methane-rich zone down to ~140 mbsf. Their concentrations increase three- to fivefold over the next 70 m. Their distribution profiles suggest that ethane and propane are products of organic carbon degradation in the methanogenic zone.
Interstitial water analyses at Site 1230 provide clear evidence of very high microbial activity with extreme accumulations of products from organic degradation processes. Alkalinity and dissolved inorganic carbon (DIC) increase steeply with depth from near seawater values at the sediment/water interface to a broad maxima of 155 mM at 100-150 mbsf, deep in the methanogenic zone. These concentrations are among the highest ever measured in marine sediments. Below this maximum, the concentrations drop again with depth. Ammonium likewise builds up extreme concentrations of 35,000 to 40,000 然 from 100 to 150 mbsf.
Below the interface of counter-diffusing sulfate and methane, there is a second diffusive interface between hydrogen sulfide and iron at 25 mbsf. The hydrogen sulfide produced from sulfate reduction reaches a peak concentration of 9.4 mM at the bottom of the sulfate zone. From there it decreases steeply both upward and downward, reaching zero near the sediment/water interface and at 25 mbsf. Iron is abundant (5-57 然) in the interstitial water of the methane zone from 200 to 25 mbsf, where it meets the hydrogen sulfide and is inferred to precipitate as ferrous sulfide and pyrite.
A diffusive interface between sulfate and barium is encountered at 8-9 mbsf. Barium concentrations are only a few micromolar in the sulfate zone but increase steeply below that zone to plateau at 400 然 between 50 and 150 mbsf. At 250 mbsf barium concentrations approach 1000 然, which may be the highest interstitial water concentration of barium ever recorded in deep-sea sediments. The narrow depth interval of coexisting barium and sulfate appears to be a zone of barite precipitation. We infer their concentrations to be determined by the solubility product of barite in that zone. Consequently, the shallow sulfate zone is an effective barrier against upward diffusion of dissolved barium. Barium fronts associated with the sulfate boundary have also been observed in sediments of the Gulf of California and the South Atlantic Ocean (Brumsack, 1986; Kasten et al., 2001). Based on data from Leg 112, von Breymann et al. (1990) concluded that the deepest sites have the highest dissolved and solid-phase barium concentrations because detritus sedimenting through a deepwater column scavenges barium from seawater and enriches the sediment in barium.
Acetate and formate are generated as fermentation products and are used as substrates by sulfate-reducing or methanogenic prokaryotes. These volatile fatty acids (VFAs) are present at much higher concentrations at Site 1230 than at any other site studied during Leg 201. The acetate level is 5-20 然 in the sulfate reduction zone and reaches 230 然 in the methane zone at 145 mbsf. This acetate concentration is fivefold higher than at the most active sites on the Peru shelf and is even 10- to 100-fold higher than at the other deep-sea sites. Formate remains mostly at 5-10 然 throughout the sediment column. Hydrogen concentrations are low, in the 0.1- to 1.5-nM range.
The interstitial water at Site 1230 has a distinct yellow color that is not present at any other Leg 201 site. We presume this color is probably due to dissolved organic matter. The intensity of the color, which was measured spectrophotometrically, increases steeply from zero at the sediment/water interface to a broad maximum between 25 and 150 mbsf. Below that depth, it drops again to reach 15%-20% of the maximum value at 250 mbsf.
Prokaryotic cell concentrations in the organic-rich Pleistocene to Holocene sediments are near the average of previously studied subseafloor sediments in the upper 60 m of the sediment column. They are about threefold above average in the next 150 m. However, in the older accretionary wedge sediments below 216 mbsf, the cell density abruptly drops fourfold, from 7.9 x 106 to 1.9 x 106 cells/cm3. This shows that the concentration of subseafloor prokaryotic cells at Site 1230 is closely related to sediment age rather than sediment depth. The factor that ultimately regulates cell concentrations may be the availability of energy substrates for prokaryotic metabolism.
Samples were taken at regular depth intervals through the entire sediment column for deoxyribonucleic acid (DNA) and fluorescent in situ hybridization-secondary ion mass spectrometry (FISH-SIMS) analysis, measurements of sulfate reduction rates, hydrogen turnover, methanogenesis rates, acetate turnover, thymidine incorporation, and prokaryote lipid biomarkers. Samples for cultivations and viable counts (most probable number [MPN]) target specific depths and geochemical zones, including the sulfate/methane interface and the hydrate-rich methane zone. Contamination tests with perfluorocarbon tracer (PFT) and fluorescent beads show that the potential seawater contamination of Site 1230 microbiological samples is low or undetectable. The only case of detectable bead contamination in a slurry used for Site 1230 microbial cultivations is based on two beads counted in 100 microscopic fields of view scanned. By the experience accumulated during this leg, our confidence has strengthened that, with rigorous contamination controls and aseptic sampling techniques, deep subsurface samples can routinely be obtained without the introduction of microorganisms from the surface environment.
Four successful temperature measurements (two Adara tool deployments and two Davis-Villinger Temperature Probe [DVTP] deployments) over a depth interval of 0-255 mbsf defined a geothermal gradient of 34.3蚓/km at Site 1230, with a mudline temperature of 1.7蚓 and an estimated temperature of 11.2蚓 at 278 mbsf. The estimated local heat flux is 28 mW/m2. This is similar to the heat flux calculated by Yamano and Uyeda (1990) at Site 685 from wireline logging data over 75-150 mbsf. Based on a downhole measurement of overpressure, upward interstitial water advection of ~1 cm/yr may occur at this site.