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doi:10.2204/iodp.proc.323.108.2011

Geochemistry and microbiology

Interstitial water chemistry

In Hole U1344A, 59 samples for interstitial water analyses were retrieved at a resolution of three samples per core for Core 1H, two samples per core for Core 2H, and one sample per core thereafter to 736.15 mbsf. In addition, high-resolution sampling was conducted at microbiology-dedicated Hole U1344C, totaling 69 whole rounds processed. 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 concentrations decrease fairly linearly from 590 mM in the top of the sediment column to 482 mM at 717 mbsf (Fig. F24C). Salinity at Site U1344 generally decreases from ~36 in the upper sediment column to 33 at ~320 mbsf and remains at this value downhole to 736 mbsf (Fig. F25I). The salinity profile is fairly similar in shape to the alkalinity and DIC profiles described below.

Alkalinity increases pronouncedly in the uppermost 106 m from 4.1 mM at 0.05 mbsf to a maximum of 71.1 mM (Fig. F25C). Below 106 mbsf, alkalinity concentrations decrease downhole to 362 mbsf, below which concentrations are relatively constant, averaging 19.8 mM. This trend is similar to the dissolved inorganic carbon (DIC) profile (Fig. F25A). DIC concentrations reach a maximum of 74.1 mM at 87.78 mbsf and average 20.1 mM between 362 and 736 mbsf. pH varies between 7.5 and 7.9 throughout the sediment column (Fig. F25B).

Dissolved sulfate and hydrogen sulfide

Interstitial water sulfate concentrations decrease almost linearly from seawater values of 28 mM at 0.05 mbsf to values below detection limit at 8.35 mbsf (Fig. F25D). Hydrogen sulfide has a well-defined concentration peak from 6.75 to 10.85 mbsf, with a maximum of 1.1 mM at 8.35 mbsf (Fig. F25F). Concentrations that are close to detection limit are above and below this peak.

Dissolved ammonium, phosphate, and silica

Dissolved ammonium concentrations increase with depth from 0.4 mM in the top of the sediment column to a maximum of 11.4 mM at 717 mbsf. The steepest increase is observed in the uppermost ~200 m (Fig. F25H). Dissolved phosphate concentrations increase with depth to a maximum concentration of 440 µM at 144 mbsf, below which phosphate concentrations decrease (Fig. F25G). The most pronounced decrease is from 140 to 300 mbsf. Silica is highly scattered but generally increases throughout the sediment column to 1300 µM at 736 mbsf (Fig. F24K).

Dissolved calcium, magnesium, sodium, and potassium

Calcium concentrations decrease from 11 mM in the surface sediment to 3.4 mM at 8.35 mbsf (Fig. F24A). From 12 to 170 mbsf, calcium concentrations are fairly constant. Below 170 mbsf, they increase to 9 mM at 736 mbsf. Magnesium concentrations strongly decrease in the uppermost 8 m of the sediment column, increase to a maximum of 60 mM at 60 mbsf, and decrease to 10 mM at 735 mbsf (Fig. F24B). Sodium is fairly constant (~475 mM) throughout the sediment column (Fig. F24D). Potassium concentrations decrease almost linearly with depth to a minimum of 5.2 mM at 682 mbsf (Fig. F24E).

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

Dissolved iron is low from the seafloor to ~10 mbsf, increasing to 20 µM below the hydrogen sulfide peak (Fig. F24H). Iron concentrations increase below 20 mbsf to higher but very scattered values. The dissolved manganese profile decreases in the uppermost 8 mbsf and is below detection limit between 8 and 10 mbsf (Fig. F24G). Manganese concentrations decrease with depth and remain low (<5 µM).

Lithium concentrations decrease in the uppermost 10 m but steadily increase throughout the rest of the sediment column (Fig. F24I). Boron concentrations increase from 405 µM at 0.05 mbsf to 1624 µM at 736.15 mbsf (Fig. F24J). The most pronounced boron increase is in the uppermost ~20 m. Barium concentrations at this site are the highest of all sites investigated, with values >80 µM below ~300 mbsf (Fig. F24L). Strontium concentrations are highly scattered throughout the uppermost 400 m. Below this depth, strontium concentrations are 80–100 µM (Fig. F24F).

Volatile hydrocarbons

Samples for volatile hydrocarbon analyses were taken from Hole U1344A at the same resolution as interstitial water samples described above. Methane (C₁) is detectable throughout Site U1344, except in the uppermost 8 m of the sediment column (Fig. F25E). Low amounts of ethane (C₂) were detected below 315 mbsf. Propane was also intermittently detected below 544 mbsf. The headspace C₁/ C₂ ratios generally decrease with increasing depth and temperature from >10,000 in shallow depths to 350 at the bottom of Hole U1344A. The very high ratios indicate biological methane formation.

Sampling of methane from Hole U1344C was performed more efficiently and at a higher resolution than at other sites. Sediment plugs were taken directly from holes drilled in the core liner immediately after the core was brought onto the catwalk. Two cubic centimeters of sediment was placed into vials containing 4 mL of sodium hydroxide. The vials were immediately capped and stored upside down until they were measured. The holes drilled into the core liner were taped. Methane samples were taken at a resolution of 25 cm in the uppermost 15 m, avoiding areas of the core that would be sampled for interstitial water and microbiology.

This sampling procedure allowed for a more accurate representation of the methane profile, particularly near the SMTZ. A steep methane concentration gradient was observed between 8 and 13.3 mbsf. At 8 and 13.3 mbsf, methane was 0.06 and 14.2 mM, respectively. The latter value is the maximum concentration recorded and is likely a minimum estimate of the in situ concentration. At 1 atm pressure, 5°C, and a salinity of 35, the saturation concentration of dissolved methane is ~1.7 mM. Higher in situ concentrations in the interstitial water may lead to bubble formation and, consequently, loss of gaseous methane when cores are brought to the surface. Although sampling was performed immediately after core retrieval, some loss of methane from the interstitial water is evident from the scatter in the profile (Fig. F25E).

Sedimentary bulk geochemistry

Sixty samples from Hole U1344A 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). CaCO₃ content at Site U1344 ranges from 0.7 to 4.4 wt% (average = 1.4 wt%) (Fig. F26A). TOC and TN concentrations range from 0.48 to 1.32 wt% (average = 0.69 wt%) and from 0.08 to 0.15 wt% (average = 0.10 wt%), respectively (Figs. F26B–F26C). TS contents range from 0.11 to 1.59 wt% (average = 0.45 wt%) (Fig. F26D). 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 in sections drilled using the APC system. High-resolution sampling took place in the microbiology-dedicated cores, and additional samples were taken once per core to APC refusal. Samples were also taken from XCB Cores 323-U1344E-78X through 80X to evaluate prokaryotic cell abundance and community structure in the deepest portion of Hole U1344E. PFT analyses performed on these cores show no contamination from the drill fluid. Samples from all cores were fixed according to the protocol described in "Microbiology" in the "Methods" chapter.

During postcruise study, we will examine microbial activity and diversity in the uppermost 25 m of sediment. 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 microbial life 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 U1344E.

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

The rate of carbon turnover in the sediment at Site U1344 is similar to or slightly higher than rates at Site U1343, based on similar sulfate, DIC, phosphate, and ammonium concentration profiles. Similar to Site U1343, profiles of methane and sulfate at Site U1344 suggest that sulfate reduction is largely driven by methane diffusing into SMTZ. The methane flux into the SMTZ, as calculated from the concentration gradient between 8 and 13 mbsf, is ~70%–80% of the sulfate flux into the SMTZ. The importance of AOM for overall carbon turnover is also stressed by the curvature in the DIC profile. The steepest concentration gradient in the uppermost 10 m is observed directly above the SMTZ, suggesting that the highest DIC flux occurs in this zone. Preliminary modeling of the DIC profile (data not shown) suggests that net DIC production in the SMTZ accounts for ~80% of the DIC production in the uppermost 30 m. Hydrogen sulfide concentrations are also at a maximum in the SMTZ because sulfate reduction rates are highest and hydrogen sulfide removal by reaction with reactive Fe (oxyhydr)oxide mineral phases is limited because these mineral phases are depleted, probably due to prior pyritization. Magnetic susceptibility data obtained during the fast scan of the cores confirm low content of oxidized iron in the SMTZ (see "Paleomagnetism").

AOM favors the precipitation of carbonates in the SMTZ. A relatively high flux of calcium into the SMTZ is observed at Site U1344, which indicates the formation of calcium carbonate. There is also a magnesium flux into the SMTZ, which may suggest the formation of a Mg-rich carbonate such as dolomite.

The curvature of the ammonium profile suggests production during organic matter degradation throughout the sediment column. Microbially mediated organic matter degradation occurs through either a respiratory or a fermentative pathway. According to the classic reduction scheme in sediment, only fermentation and hydrogenotrophic methanogenesis occur below the SMTZ; however, at this site, the iron concentration profile suggests that dissimilatory iron reduction may also occur below the SMTZ.

Organic matter degradation also leads to the accumulation of DIC and phosphate in the interstitial water. The accumulation of these species, however, is much lower than 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 due to the formation of apatite and calcium carbonates. The interstitial water profiles suggest that rates of net consumption of phosphate and DIC are highest between 300 and 350 mbsf. Calcium and magnesium concentration profiles likewise indicate net consumption of these species between 300 and 350 mbsf.

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