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doi:10.2204/iodp.proc.323.106.2011

Geochemistry and microbiology

Interstitial water chemistry

Samples for interstitial water analyses were retrieved from Hole U1342A at 1.48, 4.7, 14.2, 20.2, 29.7, 39.2, 44.2, and 54.2 mbsf by the whole-round squeezing technique. Furthermore, high-resolution samples were taken from microbiology-dedicated Hole U1342B with a total number of 56 whole-round samples. 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

Interstitial water chloride concentrations at Site U1342 average 543 ± 16 mM (Fig. F25F), and salinity ranges from 35 to 37. Alkalinity increases subtly with depth in the uppermost 12 m from 3.0 to 5.1 mM and decreases to 3.5 mM at 54.2 mbsf (Fig. F25C). This trend is similar to the dissolved inorganic carbon (DIC) profile (Fig. F25A). DIC concentrations range from 2.7 to 5.1 mM in the uppermost 12 m and decrease to a minimum of 2.9 mM at 54.2 mbsf. At ~19 mbsf, DIC decreases to a local minimum of 3.9 mM. Interstitial water pH varies between 7.4 and 8.1, with a trend toward higher values below 30 mbsf (Fig. F25B). Dissolved sulfate and hydrogen sulfide.

Dissolved sulfate concentrations at Site U1342 decrease from concentrations close to seawater values at 0.05 mbsf to ~24 mM below 20 mbsf (Fig. F25D). Hydrogen sulfide concentrations above the detection limit of 0.5 µM were only observed sporadically and remain <1.5 µM (data not shown).

Dissolved ammonium, phosphate, and silica

Ammonium concentrations increase with depth from 0.02 mM in the uppermost 10 m to maximum values averaging 0.33 mM between 11 and 24 mbsf (Fig. F25H). Concentrations decrease slightly below 24 mbsf. Phosphate concentrations gradually increase from 3.2 µM at 0.05 mbsf to a maximum of 22.4 µM at 6.1 mbsf (Fig. F25G). Phosphate concentrations below this depth decrease to <5 µM at 33 mbsf. Dissolved silica concentrations increase from 470 µM at 0.05 mbsf to 940 µM at 25 mbsf and then decrease to 330 µM at 54 mbsf (Fig. F26J).

Dissolved calcium, magnesium, sodium, and potassium

Dissolved calcium, magnesium, sodium, and potassium concentrations were determined by ion chromatography. Calcium concentrations increase linearly from seawater values in the uppermost centimeters to a maximum concentration of 18 mM at 42.85 mbsf (Fig. F26A). In contrast, magnesium concentrations decrease from 52.5 mM at 0.05 mbsf to <45 mM below 30 mbsf (Fig. F26B). Sodium and potassium behave conservatively, with average values of 467 ± 15 mM and 11.3 ± 0.7 mM, respectively (Fig. F26C, F26D).

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

Concentrations of dissolved minor elements, analyzed by inductively coupled plasma–atomic emission spectrometry (ICP-AES), are displayed in Figure F26E–F26I. Manganese concentrations increase throughout the uppermost 12 m to ~4.6 µM, are fairly constant with depth to ~25 mbsf, and increase again to a maximum of 9.8 µM at 37 mbsf. Dissolved iron concentrations are scattered and low, with no values exceeding 15 µM.

Dissolved barium is below detection limit at all depths. Dissolved lithium concentrations are fairly constant (~21 µM), with some scatter. Dissolved boron concentrations are fairly constant in the uppermost ~30 mbsf and decrease below this depth. Dissolved strontium concentrations slightly increase below 25 mbsf.

Volatile hydrocarbons

Forty-nine headspace samples were taken adjacent to whole rounds. Methane was the only hydrocarbon gas detected, and values are ≤7 ppmv (Fig. F25E).

Sedimentary bulk geochemistry

Forty-nine samples from Holes U1342A and U1342B 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 concentrations at Site U1342 range from 0.2 to 47.6 wt% (average = 6.5 wt%) (Fig. F27A). TOC concentrations range from 0.03 to 2.19 wt% (average = 0.85 wt%) (Fig. F27B). TN concentrations are 0.01–0.25 wt% (average = 0.11 wt%) (data not shown). TS concentrations are 0.01–1.35 wt% (average = 0.29 wt%) (Fig. F27C). The highest CaCO₃ and TOC concentrations were determined for the shallowest sample collected from Hole U1342B at 0.05 mbsf. Low values of CaCO₃, TOC, and TS were detected in the deepest sediment of Hole U1342A. A higher sampling resolution in Hole U1342B reveals that the values of each solid-phase constituent fluctuate within narrow depth intervals below ~10 mbsf. The variations of geochemical records were compared with the position of laminated sediment intervals, which were estimated by GRA bulk density in Hole U1342B. The results suggest that TOC and TS are markedly higher in the laminated layers (Fig. F28), in contrast to CaCO₃, which is not always high. To consider the formation processes of the laminated sediment in the Bering Sea, these data will provide useful information during shore-based research. 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 at the resolution described above. Samples were fixed according to the protocol described in "Microbiology" in the "Methods" chapter.

Conclusion

Interstitial water sulfate, DIC, phosphate, and ammonium profiles indicate that the sediment at Site U1342 is characterized by low rates of anaerobic carbon mineralization predominantly driven by organoclastic sulfate reduction. Additionally, small increases in manganese concentrations may indicate microbial manganese oxide reduction as an additional mineralization pathway. It is more likely, however, that dissolved manganese is released during the reaction of hydrogen sulfide with Fe/Mn (oxyhydr)oxides.

In contrast to Site U1341, a deeper site on Bowers Ridge, Site U1342 sediments have one order of magnitude lower ammonium concentrations and 50% lower phosphate concentrations. This highlights the extremely low mineralization rates at Site U1342 despite its shallower depth and similar TOC concentrations. The low extent of anaerobic carbon mineralization at this site may be attributed to the extremely low sedimentation rates (see "Biostratigraphy"). Very low sedimentation rates prolong the time that organic matter is degraded by oxic respiration and nitrate reduction in the oxic/suboxic sediment zone. This leaves refractory organic material that is inefficiently degraded during anaerobic carbon mineralization (Hulthe et al., 1998). Dissimilar ammonium and phosphate profiles indicate the removal of phosphate from the interstitial water due to the formation of solid-phase phosphate mineral phases rather than different mechanisms of production.

Solid-phase data, however, suggest that present-day interstitial water geochemistry may give limited insight into past conditions. High TOC concentrations that correlate strongly with high TS concentrations in several laminated intervals throughout the sediment column indicate events of high organic carbon input that resulted in high sulfate reduction rates and hence elevated hydrogen sulfide production. This is reflected in high pyrite (TS) concentrations.

Another striking feature at Site U1342 is the inverse relationship of the (almost linear) calcium and magnesium profiles, which indicates the influence of signals linked to the alteration of the underlying basalt on the interstitial water calcium and magnesium concentrations. Both profiles are most likely the result of diffusion between seawater and the relatively shallow basaltic basement. Low-temperature interactions of seawater with the basaltic basement, such as the dissolution of basaltic glass, calcic plagioclase, and olivine, result in the liberation of calcium, whereas the precipitation of smectite leads to the consumption of magnesium (e.g., Gieskes, 1981; Staudigel and Hart, 1983; Thompson, 1983; Lyons et al., 2000).

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