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doi:10.2204/iodp.proc.323.109.2011

Geochemistry and microbiology

Interstitial water chemistry

In Hole U1345A, 17 samples for interstitial water analyses were retrieved at a resolution of two samples per core from Cores 323-U1345A-1H and 2H and one sample per core thereafter to 120.25 mbsf (Core 323-U1345A-14H). High-resolution sampling was conducted at the microbiology-dedicated hole (U1345B), with 77 whole-round samples taken. To prevent oxidation, whole rounds were stored in a nitrogen-filled glove box at 7°C until squeezed. Aliquot samples were processed for routine shipboard analyses (see "Geochemistry" in the "Methods" chapter) and collected for shore-based analyses of sulfur and oxygen isotopes of sulfate and hydrogen sulfide, trace metals, dissolved organic carbon (DOC), and fatty acids.

Chlorinity, salinity, alkalinity, dissolved inorganic carbon, and pH

Salinity at Site U1345 ranges from 35 to 37 (Fig. F21I), and chloride concentrations average 549.3 ± 30.4 mM (Fig. F22C), with no clear trend with depth.

Alkalinity increases significantly in the uppermost ~20 mbsf from 2.8 mM at 0.05 mbsf to ~46 mM, slightly increases and reaches a maximum value of 58.1 mM at 74.3 mbsf, and then decreases to 36.7 mM at the bottom of the hole (Fig. F21C). This trend is similar to the dissolved inorganic carbon (DIC) profile (Fig. F21A). DIC concentrations reach a maximum of 59.2 mM at 74.3 mbsf and decrease below this depth. pH values generally vary between 7.5 and 7.9 (Fig. F21B). A local maximum in pH occurs at ~6.2 mbsf.

Dissolved sulfate and hydrogen sulfide

Interstitial water sulfate concentrations decrease almost linearly from seawater values of 28.4 mM at 0.05 mbsf to values below detection limit at 6.25 mbsf (Fig. F21D). Hydrogen sulfide concentrations are above detection limit from 3 to 8 mbsf, with a maximum value of 2.3 mM at 6.25 mbsf (Fig. F21F).

Dissolved ammonium, phosphate, and silica

Dissolved ammonium concentrations increase throughout the sediment column to a maximum value of 9.1 mM at 139.43 mbsf (Fig. F21H). Dissolved phosphate concentrations increase in the uppermost 8.5 mbsf to 195 µM and then increase subtly to 215 µM from 8.5 to 21.45 mbsf (Fig. F21G). A local minimum of <200 µM occurs between 22.25 and 27.25 mbsf. Phosphate concentrations are ~280 µM at greater depths. Dissolved silica concentrations at Site U1345 are very scattered in the uppermost 40 mbsf and increase to >1000 µM between ~80 and 120 mbsf (Fig. F22K).

Dissolved calcium, magnesium, sodium, and potassium

Dissolved calcium concentrations decrease pronouncedly in the uppermost 7.75 mbsf to 2.7 mM (Fig. F22A). Below this depth, calcium concentrations continue to decrease, reaching a minimum value of 1.6 mM at 35.8 mbsf. Calcium concentrations increase slightly to >3 mM below ~40 mbsf. Dissolved magnesium concentrations decrease from 50 mM at 0.05 mbsf to 44.1 mM at 7.75 mbsf (Fig. F22B). Between 7.75 and 36.25 mbsf, magnesium concentrations have a scattered distribution but an overall trend to values higher than those in the uppermost layer. Below 74.31 mbsf, magnesium concentrations decrease again and reach a minimum of 42.1 mM at 120.21 mbsf. Dissolved sodium and potassium concentrations at Site U1345 are in the range of 464.8 ± 19.4 mM and 11.1 ± 0.8 mM, respectively (Fig. F22D, F22E).

Dissolved manganese, iron, barium, boron, lithium, and strontium

The dissolved iron profile has a distinct concentration peak between 8.3 and 15.8 mbsf, with a maximum value of 24.2 µM at 10.5 mbsf and a second, less defined, peak at ~35 mbsf (Fig. F22H). Below 40 mbsf, dissolved iron concentrations have scattered, high values. Despite its lower concentrations, dissolved manganese (Fig. F22G) constrains the two peaks in the iron profile. Peaks in manganese concentrations occur at ~15 and 25 mbsf. Maximum concentrations at these two peaks are 4.6 and 5.12 µM, respectively.

Dissolved barium concentrations increase just above the depth of the current SMTZ at ~5 mbsf and show the most pronounced increase between 5 and 9 mbsf. Below this layer, barium concentrations remain almost unchanged at <20 µM (Fig. F22L). Boron concentrations increase in the uppermost 20 m to ~730 µM and reach a maximum value of 880 µM at 83 mbsf. Lithium concentrations decrease in the uppermost 15 mbsf to <5 µM but steadily increase throughout the rest of the sediment column (Fig. F22I). Overall, dissolved strontium concentrations display a similar trend, with decreasing concentrations in the uppermost 35 m and increasing values below this depth, although the uppermost 40 m of this profile is scattered (Fig. F22F).

Volatile hydrocarbons

Samples for volatile hydrocarbon analyses were taken from Hole U1345A at the same resolution as interstitial water samples described above. Methane is detectable at all depths at Site U1345 except the uppermost 6 mbsf of the sediment column (Fig. F21E). Ethane and other volatile hydrocarbons were not detected at Site U1345.

Sedimentary bulk geochemistry

Eighteen samples from Hole U1345A were used for the preliminary analysis of solid-phase total carbon (TC), total nitrogen (TN), total sulfur (TS), and total inorganic carbon (TIC). From these analyses, total organic carbon (TOC) and calcium carbonate (CaCO₃) concentrations were calculated (see "Geochemistry" in the "Methods" chapter). CaCO₃ content at Site U1345 ranges from 1.2 to 6.1 wt% (average = 2.1 wt%) (Fig. F23A). TOC and TN values range from 0.40 to 1.43 wt% (average = 0.75 wt%) and from 0.07 to 0.14 wt% (average = 0.10 wt%), respectively (Fig. F23B, F23C). TS values range from 0.21 to 0.82 wt% (average = 0.42 wt%) (Fig. F23D). Splits of squeeze cakes were also collected and treated for shore-based analyses of bulk elemental composition, iron mineral phases, and iron-monosulfide and pyrite content and sulfur isotope composition.

Microbiology

Samples for community structure and total prokaryotic cell abundance were collected adjacent to interstitial water whole rounds. High-resolution sampling took place in the microbiology-dedicated cores, and additional samples were taken once per core to APC refusal.

Microbial activity and diversity in the uppermost 25 m of sediment column will be examined during postcruise study. We will focus on the function of Archaea in the uppermost zone of organoclastic sulfate reduction, the SMTZ, and the methanogenesis zone. The SMTZ is a "hot spot" for microbial activity and abundance in the subseafloor (D'Hondt, Jørgensen, Miller, et al., 2003), and we expect an increase in the abundance and activity of microorganisms there. To estimate the abundance of living microorganisms, samples were also taken at low resolution for catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) within the three zones and in the deepest section of Hole U1345A.

We will examine bacterial and archaeal diversity by a combination of conventional 16S ribosomal ribonucleic acid (rRNA) clone libraries, quantitative polymerase chain reaction (qPCR), and/or a new quantitative community fingerprinting method involving automated ribosomal intergenic spacer analysis (ARISA) (Ramette, 2009).

Conclusion

Of all the sites investigated, Site U1345 has the shallowest SMTZ, at 6–7 mbsf. This site is characterized by the steepest flux of methane into this zone and the highest interstitial water hydrogen sulfide concentrations. Similar to other shelf Sites U1343 and U1344, the almost linear sulfate and methane concentration profiles suggest that sulfate reduction coupled to AOM accounts for most of the sulfate consumption in the sediment column. Preliminary modeling of the DIC profile (data not shown) suggests that net DIC production in the SMTZ accounts for as much as 70% of DIC production in the uppermost sediment layers.

The occurrence of high concentrations of dissolved hydrogen sulfide in the SMTZ can be attributed to very high sulfate reduction rates at this depth and probably to an insufficient pool of reactive Fe mineral phases, such as Fe (oxyhydr)oxides, which can react with hydrogen sulfide on short timescales. Distinct peaks in dissolved iron and manganese concentrations immediately below the SMTZ might result from microbial dissimilatory iron reduction, which releases Fe²⁺, whereas the concurrent liberation of manganese might be attributed to the reoxidation of ferrous iron by Mn oxides (Canfield et al., 1993). Alternatively, downward-diffusing hydrogen sulfide from the SMTZ may react with the remaining Fe mineral phases to form FeS minerals and release dissolved Fe²⁺.

The organic matter degradation products phosphate and ammonium accumulate in the interstitial water. The distinct minimum in phosphate concentration between 22.25 and 27.25 mbsf, however, indicates that the consumption of this species is most likely caused by the formation of phosphate-bearing minerals such as apatite.

The calcium and magnesium profiles (Fig. F22A, F22B) show depletion at the depth of the present SMTZ. This suggests the formation of authigenic Mg-rich carbonate, such as dolomite, driven by the production of DIC during AOM and increased pH and alkalinity, leading to the oversaturation of interstitial water with respect to carbonate. Interestingly, dissolved calcium concentrations continue to decrease with depth to a minimum concentration at ~40 mbsf. This depth corresponds to a dolostone layer found at 40.27 mbsf (see "Lithostratigraphy").

Sites U1343, U1344, and U1345 also have high concentrations of dissolved barium in the interstitial water, indicating a sink of this ion just above the SMTZ. The distribution of barium at these sites can be attributed to diagenetic remobilization of barium, deposited as biogenic barite, into the sulfate-depleted interstitial water (von Breymann et al., 1992). The upper end of the SMTZ, where the sulfate and dissolved barium profiles overlap, marks the present front of authigenic barite formation.

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