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doi:10.2204/iodp.proc.323.105.2011

Geochemistry and microbiology

Interstitial water chemistry

Seventy-three interstitial water samples were extracted from 10 cm whole-round sediment sections from Holes U1341A and U1341B at a resolution of one sample per core for the first core, three samples per core in the second and third cores, two samples per core in the fourth core, and one sample per core thereafter. All samples from Hole U1341A were collected from APC cores (1.4–358 mbsf). Samples from Hole U1341B were collected from both APC cores (354–456 mbsf) and XCB cores (465.8–600 mbsf).

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.

Alkalinity, dissolved inorganic carbon, pH, chlorinity, and salinity

Alkalinity increases from 3.6 mM in the top of the sediment column to a maximum value of 11.9 mM at 31 mbsf. Below this depth, alkalinity decreases to a minimum of ~3.5 mM at 600 mbsf. Between 31 and 358 mbsf, local maxima in alkalinity concentration occur at 183, 240, and 318 mbsf (Fig. F21C). The dissolved inorganic carbon (DIC) profile is similar to the alkalinity profile, with a maximum concentration of 12.4 mM at 40 mbsf (Fig. F21A). pH increases in the uppermost 15 m to 7.74. Below this depth, pH decreases and becomes fairly constant at 7.47 between 50 and 220 mbsf. Below 220 mbsf, pH increases to a local maximum of 7.61 at 240 mbsf and then decreases to 7.2 at 600 mbsf (Fig. F21B). Salinity varies between 36 and 37 in the uppermost 200 m and is 36 down to 360 mbsf. Below this depth, salinity decreases to 34–35. Chloride concentrations slightly decrease with depth (Fig. F22C).

Dissolved sulfate and hydrogen sulfide

Dissolved sulfate concentrations decrease from seawater concentration at 1.4 mbsf to 12 mM at 450 mbsf (Fig. F21D). The steepest sulfate concentration gradient is near the surface (1.4–24 mbsf). Between 300 and 400 mbsf, a second steep sulfate concentration gradient is present. Below 450 mbsf, sulfate concentrations slightly increase. Hydrogen sulfide concentrations in the uppermost 20 m range from 1 to 2 µM. Below 20 mbsf, sulfide concentrations are consistently low, averaging 0.5 µM (Fig. F21E).

Dissolved ammonium, phosphate, and silica

Ammonium concentrations at this site generally increase from 0.05 mM at 1.4 mbsf to ~5 mM at 600 mbsf (Fig. F21H). The slope of the ammonium profile, however, changes distinctly with depth, including zones of almost constant concentration at the top and bottom of the profile. The phosphate profile is similar to both the alkalinity and DIC profiles. Phosphate concentrations gradually increase throughout the uppermost 31 m from 19.1 to 50 µM. (Fig. F21G). Below 31 mbsf, phosphate concentrations decrease to ~2 µM at 600 mbsf. Dissolved silica concentrations increase from 140 µM at 1.4 mbsf to ~260 µM at 270–360 mbsf.

Dissolved calcium, magnesium, sodium, and potassium

Dissolved calcium concentrations decrease from near-seawater values at the top to <8 mM between 200 and 300 mbsf (Fig. F22A). Calcium concentrations increase below 300 mbsf to maximum values of ~14 mM. Magnesium concentrations gradually decrease from seawater concentration to a minimum at ~500 mbsf (Fig. F22B). Sodium and potassium concentrations are fairly constant throughout the sediment column (Fig. F22D, F22E).

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

Barium, iron, and manganese concentrations are relatively low, with values close to or below detection limit, particularly in the deeper portion of the sediment column (Fig. F22G, F22H, F22L). Barium values below detection limit for the uppermost ~350 m are not shown.

Interstitial water concentrations of lithium, boron, silica, and strontium increase with depth from seawater values to ~60, 1100, 300, and 130 µΜ, respectively (Fig. F22). There is a slight decrease in lithium in the uppermost 10 m (Fig. F22I), and boron concentrations decrease in the bottom 400–600 mbsf (Fig. F22J).

Volatile hydrocarbons

Headspace samples were taken adjacent to whole rounds. Methane was the only hydrocarbon gas detected with values of 2–4 ppmv (Fig. F21F).

Sedimentary bulk geochemistry

Seventy samples from Holes U1341A and U1341B were analyzed for 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) (Fig. F23). CaCO₃ concentrations at Site U1341 range from 0 to 29.4 wt% (average = 1.6 wt%) (Fig. F23A). Three maxima in CaCO₃ concentrations in the uppermost 80 m and one local maximum at 514.8 mbsf were observed. These maximum values correlate well with increased abundance of calcareous nannofossils (see "Biostratigraphy"). TOC and TN contents range from 0.22 to 1.51 wt% (average = 0.51 wt%) and from 0.05 to 0.17 wt% (average = 0.08 wt%), respectively (Fig. F23B, F23C). TS contents range from 0.19 to 1.79 wt% (average = 0.61 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 isotopic composition.

Microbiology

Samples for total prokaryotic cell abundance were collected adjacent to interstitial water whole rounds at the resolution described above. Samples were fixed according to the protocol described in "Microbiology" in the "Methods" chapter.

Conclusion

Low methane concentrations, deep sulfate penetration, and low concentrations of DIC, alkalinity, ammonium, and phosphate suggest lower present-day microbial activity at this site than at sites located on the shelf (i.e., IODP Sites U1339, U1343, U1344, and U1345). Organoclastic sulfate reduction is the major metabolic pathway at Site U1341 and is likely most significant in the uppermost 30 m based on the DIC, alkalinity, and sulfate profiles.

The DIC, alkalinity, and phosphate profiles suggest that these constituents are not in steady state and/or that they are produced in the uppermost ~30 m and are consumed in deeper sediment. The inconsistency between the ammonium, DIC, and phosphate profiles indicates that nonsteady state cannot be the sole mechanism affecting these profiles. Most likely, DIC, alkalinity, and phosphate are produced in the uppermost ~30 m by organic matter degradation. The formation of carbonates and iron phosphates in layers below ~30 mbsf could be responsible for the decrease in DIC, alkalinity, and phosphate.

The sulfate profile suggests a curiously high net consumption between 300 and 400 mbsf, despite the lack of methane and presumably the presence of only refractory organic carbon. Further studies and modeling are needed to explain this pattern. Furthermore, an increase in sulfate concentration is observed in the deepest part of the profile, which could indicate intrusion of seawater through, for example, channelized fluid flow in the crust. Similar patterns in the sulfate profile have been observed for Ocean Drilling Program (ODP) Leg 201 Sites 1225 and 1226 and have been attributed to water flow through the underlying basalts (D’Hondt, Jørgensen, Miller, et al., 2003).

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