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doi:10.2204/iodp.proc.323.103.2011

Geochemistry and microbiology

Interstitial water chemistry

Seven interstitial water samples were extracted from 10 cm whole-round sediment sections from Hole U1339A at a resolution of two samples per core in the first core, three samples per core in the second core, and one sample per core thereafter, covering a depth of ~33 mbsf.

In microbiology-dedicated Hole U1339B, samples from the uppermost 35 m were taken at high resolution; 10 cm whole rounds were taken every 25 cm for the first two sections of the first core. Sampling resolution then decreased to every 75 cm for Sections 323-U1339B-1H-3 through 1H-6 and Cores 2H through 4H. The uppermost 35 m of sediment was cut into whole rounds at a near in situ temperature of 7°C in the Cold Laboratory. To prevent oxidation, whole rounds were stored in a nitrogen-filled glove box at 7°C until squeezed. One sample per core was taken from Cores 323-U1339B-5H through 22H. Interstitial water samples were processed for routine shipboard geochemical analyses (see "Geochemistry" in the "Methods" chapter). Samples were also collected for shore-based analyses of sulfur and oxygen isotopes of sulfate and hydrogen sulfide, trace metals, dissolved organic carbon, and fatty acids.

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

Chloride concentrations in Holes U1339A and U1339B are fairly constant throughout the sediment column, averaging ~540 mM. Downhole salinity values are nearly constant throughout Hole U1339B, varying only between 34 and 36 (Fig. F18D, F18I).

Alkalinity increases from 3 to 30 mM in the uppermost 10 m and reaches a maximum of 53 mM at ~120 mbsf in Hole U1339B (Fig. F18C). Curvature is well defined throughout the profile. The dissolved inorganic carbon (DIC) profile is similar to the alkalinity profile in shape and concentration range (Fig. F18A). pH values range from 7.7 to 8.2, with two local maxima at 16 and 78 mbsf in Hole U1339B (Fig. F18B).

Dissolved sulfate and hydrogen sulfide

Sulfate concentrations decrease from seawater values to values below detection limit at ~10 mbsf (Fig. F18E). Dissolved hydrogen sulfide is below detection limit throughout the depths sampled in Hole U1339B, with the exception of 2.25 and 21.8 mbsf, where concentrations are 1.6 and 14.5 µM, respectively (data not shown).

Dissolved ammonium and phosphate

Ammonium concentrations increase throughout Hole U1339B and range from 0.02 to 5.6 mM (Fig. F18H). Phosphate concentrations increase to 186 µM throughout the uppermost ~30 m; from 30 to 200 mbsf, concentrations vary between ~100 and 200 µM (Fig. F18G).

Dissolved calcium, magnesium, sodium, and potassium

Calcium concentrations range from 15.8 to 1.3 mM, decreasing downhole in the uppermost 10 m and remaining relatively constant thereafter to 200 mbsf (Fig. F19D). Potassium, magnesium, and sodium concentrations are 19.7–6.5, 86.5–25.2, and 770–360 mM, respectively, throughout the sediment column, and their downhole profiles are roughly similar (Fig. F19A–F19C). These constituents are highly variable in the uppermost ~35 m.

Dissolved iron, manganese, boron, lithium, and strontium

Iron concentrations are less than ~6 µM. There are a few outliers in the ~10–40 µM range in the uppermost ~100 m (Fig. F19G). Manganese concentrations are less than ~10 µM and also remain relatively constant with depth (Fig. F19F, F19G).

Boron concentrations increase between near seawater values (416 µM) and 1800 µM at 140 mbsf (Fig. F19H) and then drop slightly to ~1400 µM below 140 mbsf. Lithium concentrations at the surface of the upper sediment column are near seawater values (26 µM), decrease rapidly toward a well-defined minimum at ~10 mbsf, and then increase monotonously to >30 µM with depth (Fig. F19I). Strontium concentrations, although scattered, decrease in the uppermost ~40 m. Deeper in the sediment column, strontium increases to local maxima of 101 and 125 µM at 60 and 185 mbsf, respectively (Fig. F19E).

Volatile hydrocarbons

Samples for volatile hydrocarbon analyses were taken from Holes U1339A and U1339B at the same resolution as the interstitial water samples described above. Methane concentrations are close to detection limit in the uppermost ~10 m. At the depth of sulfate depletion at ~10 mbsf, methane concentrations increase significantly (Fig. F18H). Below 10 mbsf, methane concentrations are minimum estimates. From 19.75 to 185 mbsf, ethane concentrations range from 0.25 to 2.6 µM. Throughout the rest of the hole, ethane concentrations are below detection limit.

Sedimentary bulk geochemistry

Splits of squeeze cakes from interstitial water whole rounds from Holes U1339A and U1339B were used for the 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) (Fig. F20). CaCO₃ concentrations range from 0 to 13.3 wt% (average = 2.1 wt%). TOC and TN contents range from 0.47 to 1.83 wt% (average = 0.98 wt%) and from 0.07 to 0.23 wt% (average = 0.12 wt%), respectively. CaCO₃, TOC, and TN concentrations are highest near the sediment/water interface and decrease relatively sharply in the uppermost 10 m of the sediment column. In contrast, TS concentrations increase in the upper portion of the sediment column and fluctuate between 0.2 and 0.8 wt% to 200 mbsf (Fig. F20D). 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 "Microbiology" in the "Methods" chapter. All microbiology analyses will be performed postcruise.

PFT concentrations are below detection limit throughout Hole U1339B, indicating that contamination from drill fluid is insignificant.

Conclusion

Dissolved sulfate, DIC, alkalinity, phosphate, and ammonium concentration profiles suggest relatively high rates of carbon turnover (i.e., microbial activity) at Site U1339, similar to other IODP shelf Sites U1343, U1344, and U1345. The relatively shallow sulfate–methane transition zone (SMTZ) is presently between 8 and 10 mbsf. The significance of anaerobic oxidation of methane (AOM) for carbon turnover is evident in the DIC concentration profile. The steepest DIC concentration gradient is directly above the SMTZ, suggesting that the highest DIC flux occurs from this zone.

Calcium and magnesium profiles show depletion at the depth of the present SMTZ. This suggests formation of authigenic Mg-rich carbonate, such as dolomite, driven by the production of DIC during AOM and an increase in pH and alkalinity, which leads to oversaturation of the interstitial water with respect to carbonate.

Accumulation of ammonium and phosphate is indicative of microbially mediated organic matter degradation. A local minimum in phosphate concentration at ~60 mbsf indicates inorganic consumption of phosphate.

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