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SITE SUMMARIES (continued)

Ocean-Margin Sites (continued)

Peru Slope Hydrate Site

Site 1230

Background and Objectives. Site 1230 was the single hydrate-bearing site selected for drilling during Leg 201. The principal objectives at this site were

  1. To determine if and how hydrate-bearing sequences differ in microbial activities, microbial communities, and the nature of microbe-environment interactions from nearby methane-rich sequences that lack hydrates (Sites 1227 and 1229) and nearby sulfate-rich sequences with low methane concentration (Site 1228) and
  2. To provide a Peru margin microbial and biogeochemical counterpoint to hydrate-rich sites in other regions of the world ocean (such as Leg 164's northwest Atlantic Blake Ridge and Leg 204's northeast Pacific Hydrate Ridge).

Site 1230 is located on the lower slope of the Peru Trench in 5086 m water depth. Sediments of this area are part of the accretionary wedge just landward of the Peru Trench (Suess, von Huene, et al., 1988). The lithologies, sediment age, and many geochemical and geophysical characteristics of the target site were well characterized by Leg 112 studies of nearby Site 685 (Shipboard Scientific Party, 1988d). The upper 200 m of Pleistocene to Holocene sediment is a clay-rich diatomaceous mud, partly accreted by downslope transport from the shelf. At ~ 200 mbsf, a stratigraphic hiatus of ~4.5 m.y. separates the slope deposit from upper Miocene diatom ooze (Shipboard Scientific Party, 1988d). Authigenic carbonates and phosphates are sparse, whereas pyrite framboids are abundant throughout the section and constitute 5%–10% of the sediment (Shipboard Scientific Party, 1988d). Calculated sedimentation rates are high; they average 250 m/m.y. for the Miocene sequence and 100 m/m.y. for the Pleistocene section (Shipboard Scientific Party, 1988d). These high rates are consistent with sedimentation in a lower-slope basin or trench axis.

The surface waters over Site 1230 are part of the Peru upwelling system and are biologically highly productive. The organic carbon content of the sediment is high at Site 685 (Shipboard Science Party, 1988d). Methane concentration was observed to rise above 1 bar already at 11.6 mbsf and remain in the range of 104–105 µL/L throughout the cored sediment column down to 432 mbsf (Kvenvolden et al., 1990). Concentrations of ethane and butane generally increase downhole from 1 to 100 µL/L, and the methane/ethane ratio decreased from 105 to 103. The Leg 112 scientific party found visual evidence of methane hydrate at 99 and 164 mbsf in the form of small pieces of dark gray hydrate (Shipboard Scientific Party, 1988d; Kvenvolden and Kastner, 1990). The samples looked like rounded pieces of mudstone but felt cold and showed bubbling foam. Based on this information, Site 1230 provides an excellent opportunity for assessing the nature of microbial communities and their activities in hydrate-bearing sediments rich in organic material and under high hydrostatic pressure.

The concentration of dissolved sulfate declines to 0 mM between the first and second core analyzed at Site 685 (between 3 and 18.1 mbsf) (Shipboard Scientific Party, 1988d). Chloride concentration ranges between 525 and 555 mM. The maximum concentration is associated with the most shallow sulfate-free sample (18.1 mbsf) and was suggested by the Leg 112 shipboard science party to lie just above hydrate at the top of the hydrate stability field. Salinity, alkalinity, dissolved ammonium, phosphate, and magnesium concentrations rise to maximum values in the interval of 107–134 mbsf, decline sharply between 165 and 235 mbsf, and then decrease gradually to the base of the hole at ~450 mbsf. The maxima in alkalinity (156 mM), ammonium (31.76 mM), and phosphate (0.826 mM) were the highest then known from deep-ocean drilling (Shipboard Scientific Party, 1988d). Downhole variation in chloride and calcium concentrations is generally opposite to the variation in these other chemical species. The pH drops to below 7 at 133 mbsf and remains below 7 to the base of the hole (Shipboard Scientific Party, 1988d).

These patterns of interstitial water chemistry are inferred to result from high levels of biological activity throughout the sediment column, coupled with hydrate formation and diffusive exchange with the overlying ocean. The subsurface extent of key electron donors (hydrogen, acetate, and formate) and electron acceptors with standard free-energy yields greater than that of sulfate (oxygen, nitrate, manganese oxide, and iron oxides) was not determined for Site 685.

Principal Results. 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 µM) throughout the remaining sediment column. Dissolved iron is likewise low (mostly 1–3 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 Jørgensen, 1985; Niewöhner 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 concentration in recovered cores fluctuates 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 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 would constitute 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 mM CH4 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 occurrence of gas hydrate was also monitored by rapid 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°C proved to harbor gas hydrate. Hydrate was visually observed in several cores between 80 and 200 mbsf. The hydrate was only recovered 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. 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 concentration of dissolved methane builds up to reach the phase boundary of hydrate formation at ~50 mbsf. The dissolved concentration 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 pore 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 10 mbsf. This subsurface peak is presumably a remnant of the last glacial salinity excursion. It is accentuated by a drop in chlorinity below 10 mbsf that is probably due to freshening by hydrate dissolution. Within the methane zone, the drop in chlorinity is 10–27 mM and the concentration shows 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 concentration 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.

Pore water analyses at Site 1230 provides clear evidence of very high microbial activity with extreme accumulations of products from organic degradation processes. Alkalinity and DIC increase steeply with depth from nearly 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 an extreme concentration of 35 to 40 mM from 100 to 150 mbsf.

Below the interface of counter-diffusing sulfate and methane, there is a second diffusive interface between H2S and Fe2+ at 25 mbsf. The H2S 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 to reach zero at the sediment/water interface and at 25 mbsf. Iron is abundant in the pore water of the methane zone from 200 up to 25 mbsf, where it meets the H2S and is inferred to precipitate as ferrous sulfide and pyrite.

A diffusive interface between sulfate and Ba2+ is encountered at 8–9 mbsf. The barium concentration is only a few micromolar in the sulfate zone but increases steeply below that zone to plateau at 400 µM between 50 and 150 mbsf. At 250 mbsf the barium concentration approaches 1 mM, which may be the highest pore water concentration of Ba2+ ever recorded. 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 microorganisms. These volatile fatty acids are present at much higher concentrations at Site 1230 than at any other site studied during Leg 201. The acetate level is 5–20 µM in the sulfate reduction zone and reaches 230 µM 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 µM throughout the sediment column. Hydrogen concentration is low, in the 0.1- to 1.5-nM range.

The pore 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. Postcruise analyses will be conducted to characterize the dissolved organic substances from this and other sites.

Bacterial 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 bacterial density abruptly drops fourfold, from 7.9 x 106 to 1.9 x 106 cells/cm3. This shows that the concentration of subseafloor bacteria is closely related to sediment age rather than sediment depth. The factor that directly regulates population size may be the availability of energy substrates for microbial metabolism.

Samples were taken at regular depth intervals through the entire sediment column for DNA and FISH-SIMS analysis, measurements of sulfate reduction rates, hydrogen turnover, methanogenesis rates, acetate turnover, thymidine incorporation, and bacterial lipid biomarkers. Samples for cultivations and viable counts (MPN) target specific depths and geochemical zones, including the sulfate/methane interface and the hydrate-rich methane zone. Contamination tests with PFT and fluorescent beads show that the potential seawater contamination of microbiological samples is very low or undetectable. The only case of detectable bead contamination in a slurry used for bacterial cultivations is based on one single bead 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 DVTP deployments) over a depth interval of 0–255 mbsf defined a geothermal gradient of 34.3°C/km at Site 1230, with a mudline temperature of 1.7°C and an estimated temperature of 11.2°C 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 pore water advection of ~1 cm/yr may occur at this site.

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