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doi:10.2204/iodp.proc.323.104.2011

Geochemistry and microbiology

Interstitial water chemistry

Nine interstitial water samples were extracted by the whole-round squeezing technique from Hole U1340B at 2.9, 5.9, 9.3, 12.3, 15.3, 24.8, 34.3, 42.3, and 53.3 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. Samples for hydrocarbon and solid-phase analyses were taken down to 600 mbsf in Hole U1340A.

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

Interstitial water chloride concentrations vary between 528 and 570 mM in Hole U1340B (Fig. F26G), whereas downhole salinity remains constant at 36 (Fig. F26H).

Alkalinity increases from 4.2 mM at 2.9 mbsf to 9.3 mM at 24.8 mbsf. Below this depth, alkalinity increases subtly, with a maximum concentration of 10 mM at 53.3 mbsf (Fig. F26C). Dissolved inorganic carbon (DIC) shows a similar trend as alkalinity, with a maximum concentration of 10 mM at 42.3 mbsf (Fig. F26A). pH is constant throughout all cores from Hole U1340B, averaging 7.7 (±0.05) (Fig. F26B).

Dissolved sulfate and hydrogen sulfide

The dissolved sulfate concentrations in Hole U1340B decrease from 28.2 mM at 2.9 mbsf to 23.1 mM at 53.3 mbsf (Fig. F26E). Hydrogen sulfide was detected at low concentrations, averaging 4.4 µM (Fig. F26D).

Dissolved ammonium, phosphate, and silica

Ammonium concentrations increase with depth from 0.25 mM at 2.9 mbsf to 0.93 mM at 53.3 mbsf (Fig. F26J). Phosphate concentrations gradually increase throughout the uppermost ~12 m from 2.9 to 23.3 µM, followed by a gradual decrease to 12.8 µM at 53 mbsf (Fig. F26I). Dissolved silica measured by inductively coupled plasma–atomic emission spectrometry (ICP-AES) varies between 120 and 200 µM (Fig. F27I).

Dissolved calcium, magnesium, sodium, and potassium

Major cations were analyzed using ion chromatography. Calcium concentrations decrease slightly from 10.6 to 9.0 mM (Fig. F27D). Potassium, magnesium, and sodium concentrations are 12.1–11.5, 55.1–52.6, and 501–482 mM, respectively, throughout the profile (Fig. F27A–F27C).

Dissolved minor elements

In this low-resolution sampling of the uppermost 53.3 m, all minor elements are scattered without a distinctive trend with depth (Fig. F27E–F27I). Iron is below detection limit.

Volatile hydrocarbons

Sixty headspace samples from Hole U1340A with a sample resolution of one sample per core were analyzed. Eight headspace samples from Hole U1340B were also analyzed. Methane was the only hydrocarbon gas detected at Site U1340. The concentration of methane in Holes U1340A and U1340B is <1 µM (Fig. F26F). Ethane and other volatile hydrocarbons are below detection limit in Holes U1340A and U1340B.

Sedimentary bulk geochemistry

Fourteen core catcher samples from Hole U1340A 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 concentrations in Hole U1340A range from 0 to 12.4 wt% (average = 2.2 wt%) (Fig. F28A). The increase in CaCO₃ content between 300 and 400 mbsf corresponds to an interval where authigenic dolomite and calcite were observed in the sediment (see "Lithostratigraphy"). TOC and TN contents range from 0.27 to 0.99 wt% (average = 0.61 wt%) and from 0.05 to 0.11 wt% (average = 0.08 wt%), respectively, in sediment from Hole U1340A (Fig. F28B, F28C). TOC concentrations decrease below ~300 mbsf. TS contents range from 0.17 to 1.19 wt% (average = 0.54 wt%) (Fig. F28D). 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 total prokaryotic cell abundance were collected adjacent to interstitial water whole rounds at the same resolution. Samples were fixed according to the methods described in "Microbiology" in the "Methods" chapter.

Conclusion

Undetectable methane, deep sulfate penetration, and low values of DIC, alkalinity, ammonium, and phosphate suggest low microbial activity at Site U1340 compared to Site U1339, despite similar TOC content and water depth. These data also suggest that microbial carbon turnover is driven by organoclastic sulfate reduction, whereas sulfate reduction coupled to the anaerobic oxidation of methane is absent in the uppermost 60 m at present.

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