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doi:10.2204/iodp.proc.323.107.2011

Geochemistry and microbiology

Interstitial water chemistry

In Hole U1343A, 24 samples for interstitial water analyses were retrieved at a resolution of two samples per core for Cores 323-U1343A-1H and 2H, followed by one sample per core down to ~200 mbsf. High-resolution samples were taken from microbiology-dedicated Hole U1343B, with a total of 58 whole rounds. In addition, 59 samples were taken from Cores 323-U1343E-25H through 83X (207–743 mbsf) at a resolution of one per core. 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

Chloride values are highly scattered but slightly decrease in the uppermost 300 m (Fig. F21C). In general, salinity decreases throughout the core but is fairly constant from 23 to 137 and 280 to 665 mbsf. Salinity ranges from 31 to 37. Alkalinity increases in the uppermost 81 m from 3.1 mM (at 0.05 mbsf) to 65 mM. Between 81 and 300 mbsf, alkalinity decreases by ~40 mM. From 300 mbsf to the bottom of Hole U1343E (743 mbsf), alkalinity slightly increases before gradually decreasing below ~460 mbsf to 16.3 mM (Fig. F22C). This trend is reflected in the dissolved inorganic carbon (DIC) profile (Fig. F22A). DIC concentrations increase from 3.3 to 66 mM in the uppermost 109 m and decrease to 16 mM at 734 mbsf. A local minimum in DIC concentrations of 20 mM occurs at ~337 mbsf. pH varies between 7.3 and 8.2 (Fig. F22B).

Dissolved sulfate and hydrogen sulfide

Dissolved sulfate at Site U1343 decreases from concentrations close to seawater values at 0.05 mbsf to undetectable values at 7.8 mbsf and below (Fig. F22D). Hydrogen sulfide concentrations have local maxima at ~2.3 and ~8 mbsf of 63.5 and 576 µM, respectively. Except for in these intervals of increased sulfide concentrations, values are below detection limit (Fig. F22F). Sulfate and hydrogen sulfide profiles are displayed for the uppermost 15 mbsf only (Fig. F22D, F22F).

Dissolved ammonium, phosphate, and silica

Ammonium concentrations increase with depth from 0.05 mM in the top of the sediment column to a maximum of 14.3 mM at 444 mbsf (Fig. F22H). Ammonium decreases slightly in the lowermost 140 m from 12.43 to 10.73 mM. Phosphate increases from 10 µM in the surface sediment to a maximum concentration of ~300 µM at 100 mbsf. Below this depth, phosphate decreases, most significantly from 100 to 200 mbsf (Fig. F22G). Dissolved silica concentrations are nearly constant throughout the sediment column (Fig. F21K).

Dissolved calcium, magnesium, sodium, and potassium

Calcium concentrations decrease from seawater values in the uppermost sediment to a minimum concentration of 2 mM at 32 mbsf (Fig. F21A). Between 30 and 300 mbsf, calcium concentrations are fairly constant at <5 mM. Below 300 mbsf, calcium concentrations slightly increase to a maximum of 6.9 mM at 444 mbsf. Below this depth, calcium concentrations show a scattered distribution but remain <8 mM (Fig. F21A). Magnesium concentrations reach a maximum of 56.8 mM at 100 mbsf and decrease to 8 mM at 740 mbsf (Fig. F21B). The most significant decrease in magnesium concentration is observed between 100 and 300 mbsf. Sodium concentrations do not show a significant trend with depth (Fig. F21D). Potassium concentrations decrease from seawater concentrations in the surface sediment to 6 mM at 740 mbsf (Fig. F21E).

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

Dissolved manganese and iron concentrations are higher near the top of the sediment column, with maximum values of 5.8 and 60.7 µM, respectively (Fig. F21G–F21H). The depth of maximum manganese concentrations is above the depth of maximum iron concentrations.

The lithium concentration profile shows a minimum in the uppermost ~10 m and increases steadily with depth (Fig. F21I). This profile resembles the lithium profiles of Sites U1339, U1344, and U1345. Dissolved boron concentrations increase from a minimum of 353 mM at 0.05 mbsf to a maximum of 1670 µM at 737 mbsf (Fig. F21J). Dissolved barium concentrations increase steeply in the uppermost ~10 m and vary between 40 and 80 µM below (Fig. F21L).

Volatile hydrocarbons

Samples for volatile hydrocarbon analyses were taken from Holes U1343A, U1343B, U1343C, and U1343E at the same resolution as interstitial water samples. Methane (C₁) is detectable at all depths except the uppermost 8 m (Fig. F22E). Low amounts of ethane (C₂) were detected below 33.5 mbsf. Ethene and propane were also intermittently detected below 189.05 mbsf. The headspace C₁/C₂ ratios generally decrease with depth and temperature from >10,000 at shallow sediment depths to ~200 at 712.6 mbsf. The very high C₁/C₂ ratios indicate biological methane formation. Significant losses of methane occurred during core retrieval and processing, especially below 11–12 mbsf; therefore, we only report methane data from the uppermost 15 m of Hole U1343B.

Sedimentary bulk geochemistry

Eighty-two samples from Holes U1343A and U1343E 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 content at Site U1343 ranges from 0.5 to 3.5 wt% (average = 1.6 wt%) (Fig. F23A). TOC and TN concentrations range from 0.42 to 1.56 wt% (average = 0.66 wt%) and from 0.06 to 0.16 wt% (average = 0.10 wt%), respectively (Fig. F23B–F23C). TS concentrations range from 0.14 to 0.82 wt% (average = 0.41 wt%) (Fig. F23D). CaCO₃ content varies downcore but does not display an overall trend. TOC and TN contents are relatively high in two samples at 2.9 and 108.7 mbsf. 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 compostion.

Microbiology

Samples for community structure and total prokaryotic cell abundance were collected adjacent to interstitial water whole rounds in sections drilled using the APC system. Additional samples were taken from XCB Cores 323-U1343E-78X through 80X to evaluate prokaryotic cell abundance and community structure in the deepest portion of Hole U1343E. PFT analyses performed in these cores show no contamination from drill fluid in the center of the whole round where microbiology samples were taken. Samples from all cores were fixed according to the protocol described in "Microbiology" in the "Methods" chapter.

Conclusion

Interstitial water sulfate, DIC, phosphate, and ammonium concentration profiles indicate that the sediment at Site U1343 is characterized by high rates of carbon turnover compared to the Bowers Ridge sites (U1340, U1341, and U1342). Concentrations of these interstitial water constituents are, in general, at least one order of magnitude higher than at Site U1342 (see "Geochemistry and microbiology" in the "Site U1342" chapter). Profiles of methane and sulfate suggest that sulfate reduction is largely driven by methane diffusing into the SMTZ. The sulfate profile is nearly linear in the uppermost 8 m, indicating minor sulfate consumption here. The methane flux into the SMTZ, as calculated from the methane concentration gradient between 8 and 11 mbsf, is ~50%–60% of the sulfate flux into the SMTZ. Hydrogen sulfide concentrations reach maximum values in the SMTZ. A relatively high flux of calcium into the SMTZ stresses the importance of sulfate reduction coupled to the anaerobic oxidation of methane (AOM), which commonly leads to the formation of CaCO₃ because of an increase in pH and alkalinity in the SMTZ. The calcium flux into the SMTZ is ~35% of the methane flux, indicating that a large fraction of the DIC produced through AOM is deposited as CaCO₃.

The curvature of the ammonium profile suggests that ammonium production results from organic matter degradation throughout the sediment column. This is confirmed through preliminary modeling exercises (data not shown) and suggests organic matter degradation and hence microbial activity even at depths below 400 mbsf. Organic matter degradation also leads to the accumulation of DIC and phosphate in the interstitial water. The accumulation of these constituents, however, is much lower than that predicted by the ammonium profile, assuming steady state and a constant ratio between carbon, nitrogen, and phosphorus of remineralized organic matter. This suggests both production and consumption of DIC and phosphate in the sediment. Consumption of these species is most likely caused by the formation of apatite and calcium carbonates (e.g., dolomite). The interstitial water profiles suggest that the net consumption of phosphate is highest between 180 and 200 mbsf and net DIC consumption is highest between 300 and 350 mbsf.

The decreases in salinity and interstitial water chloride concentrations indicate freshening of the interstitial fluids with depth. A possible explanation for this trend is the dissociation of gas hydrates during core recovery, which releases freshwater and causes dilution of dissolved ion concentrations (Kastner et al., 1998; Hesse et al., 2000). This process, however, is constrained to sediment intervals with in situ gas hydrate and therefore often causes a "scattering" of the chloride profile, with chloride depletion corresponding to high methane concentrations as artifacts of core recovery. Below 200 mbsf, where salinity and chloride profiles show the most pronounced decreases, the interstitial water profiles display the least scatter. On the other hand, a "soupy" core texture was observed in cores retrieved below 200 mbsf (A.C. Ravelo, pers. comm., 2009), and such a texture is typically an indication of the dissociation of gas hydrates (Westbrook, Carson, Musgrave, et al., 1994).

Alternatively, decreases in interstitial water salinity and chloride concentrations can result from meteoric water input, clay membrane ion filtration, and clay mineral dehydration (e.g., De Lange and Brumsack, 1998; Dählmann and de Lange, 2003). The hypothesis that depletion is caused by the release of mineral-bound water by clay mineral dehydration is strengthened by the significant decrease in potassium concentrations below 200 mbsf. Clay mineral dehydration (e.g., the transformation of smectite to illite) is known to result in strong potassium depletion because of the integration of potassium into the illite mineral structure (Dählmann and de Lange, 2003). Interestingly, potassium decreases significantly at ~180 and 280 mbsf. These depths correlate well with changes in lithology (see "Lithostratigraphy") characterized by increases in clay mineral content. This supports the idea that clay mineral transformations are the major cause of freshening of interstitial fluids at Site U1343. These findings highlight the potential importance of terrigenous material input at sites proximal to the Bering Sea shelf for geochemical processes during diagenesis.

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