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doi:10.2204/iodp.proc.311.107.2006

Interstitial water geochemistry

A total of 52 interstitial water (IW) whole-round samples, 10–30 cm long, were collected at Site U1329. Forty samples were collected from Hole U1329C, with a sampling frequency of four per core in the second core, three in the first and third cores, two per core to a depth of 140 mbsf, and one per core for the remaining cores. One sample was collected from pressure Core 311-U1329C-7P, retrieved from a depth of 55.6 mbsf. Sulfate analysis indicates that this sample was contaminated during coring and, therefore, was only analyzed for Cl concentration. The Cl concentration value corrected for seawater contamination is reported in Table T3. In Hole U1329D, only one whole-round sample was obtained from a depth of 201.7 mbsf. Hole U1329E was dedicated to high-resolution microbiological and geochemical sampling, from which a total of 11 samples were recovered, covering the interval from the seafloor through the sulfate/methane interface (SMI). Whole-round samples were taken adjacent to core sections sampled for microbiological studies, and headspace (HS) samples were taken adjacent to each whole-round IW sample. The sampling resolution was one sample per section in the first two cores. Two additional samples were collected in Core 311-U1329E-3H, but only one of them was processed because whole-round Sample 311-U1329E-1H, 135–150 cm, was highly disturbed and unsuitable for IW extraction. The IW data for Site U1329 are listed in Table T3 and illustrated in Figures F20, F21, and F22. The microbially mediated reactions and nonbiogenic, inorganic, and fluid-rock reactions that modified the IW chemistry at this site are discussed below.

Salinity and chlorinity

Salinity decreases from 33.8 to 30.3 at 44 mbsf. Below this depth, salinity values stabilize between 30.5 and 30.0 to 176.5 mbsf (Fig. F20). The last sample collected from Hole U1329C shows a sharp drop in salinity to 28. This depth corresponds to a marked change in resistivity and density in the LWD data (see "Downhole logging"). To confirm this low salinity value, further coring was attempted to sample the sediments corresponding to the anomalous LWD-inferred section. Unfortunately, only a 36 cm long sample was retrieved in the single XCB core collected from this hole because of drilling difficulties. We took a whole-round sample at 201.7 mbsf, which yielded a salinity value of 22, confirming the presence of highly freshened fluid below 176 mbsf. The anomalous composition of this fluid is also apparent in the concentration of major and minor elements (Table T3). Analyses of hydrocarbons collected from this zone also revealed anomalously low C1/C2 ratios in void gas and pressure core samples, indicative of a partial contribution from a thermogenic hydrocarbon source (see "Organic geochemistry").

The overall freshening trend is also observed in the chlorinity data. It is important to note, however, that the salinity versus depth profile does not mimic the dissolved Cl concentration profile. Chlorinity decreases from 547 mM at 1.5 mbsf to 480 mM at 183.6 mbsf and reaches a value of 380 mM in Sample 311-U1329D-1X-1, 52–88 cm, at 201.7 mbsf. Superimposed on the freshening trend with depth, there is a distinct, low-chloride anomaly in the pore fluids recovered from 111 to 135 mbsf, relative to a constant chloride concentration of 519 ± 1 mM that extends from 98 to 139 mbsf. The minimum value in dissolved chloride lies at ~125 mbsf, coincident with the depth of the BSR. If we assume that this anomaly was produced by in situ dissociation of disseminated gas hydrate, the calculated pore volume occupancy by gas hydrate is <2% based on 60% porosity (see "Physical properties"). However, because no direct visual or infrared (IR) evidence for methane hydrate presence at this depth interval was obtained, this Cl minimum may represent a paleohydrate dissociation zone. Assuming a sediment diffusion coefficient of 5 x 10–6 cm2/s, this low-Cl anomaly would dissipate in ~15,000 y.

The lower than seafloor chlorinity at the sediment/seawater interface probably reflects the modern bottom water value at this site, and the slight decrease in chlorinity in the uppermost ~20 m may reflect diffusion of low-Cl interglacial water into the sediment. From ~140 mbsf to TD, Cl concentration decreases more steeply, suggesting communication with a deep-seated, fresher fluid. At this depth interval, in addition to Cl, the concentrations of Na and K (Fig. F20) and Ca and Mg (Fig. F22) also decrease with depth, and each has a greater concentration gradient than above ~140 mbsf, supporting the suggestion of communication with a deeper sourced, fresher fluid. The discontinuity in the concentration profiles at this depth coincides with the lithostratigraphic Unit II/III boundary (see "Lithostratigraphy"). The conglomerate at the base of Unit II seems to act as a semipermeable diffusional barrier.

Biogeochemical processes

The concentration versus depth profiles indicate that the IW chemistry has been extensively modified from seawater by both microbially mediated reactions and by abiological, inorganic fluid-rock reactions. The microbially mediated reactions, such as bacterial sulfate reduction followed by carbonate reduction or methane oxidation and associated authigenic carbonate formation, are particularly intense in the uppermost ~15 m of the sediment section, as reflected in the concentration profiles of sulfate, alkalinity, ammonium, and phosphate (Fig. F21).

In Hole U1329E, sulfate concentrations decrease almost linearly from 22 mM at 1.5 mbsf to 0.2 mM at 9.4 mbsf at a rate of 2.8 mM/m and reach zero concentration between 9 and 10 mbsf. The decrease in sulfate concentration is coupled with a sharp increase in alkalinity, suggesting that sulfate is consumed by microbially mediated sulfate reduction processes. The depth of sulfate depletion is concomitant with increasing methane concentration (Fig. F28), consistent with the anaerobic methane oxidation at the SMI.

Although utmost care was taken during cleaning of the samples prior to squeezing, occasional contamination by drilling fluid could not be avoided. For example, in Sample 311-U1329D-1X-1, 52–88 cm, where the sediment was highly disturbed, a dissolved sulfate concentration of 4.7 mM was measured, which reflects drilling fluid contamination. Reported sulfate concentrations below 0.4 mM are most likely an instrumental precision problem of the ion chromatograph.

Note that there is a clear offset in the uppermost 9 m between the sulfate profiles of Holes U1329C and U1329E (Fig. F21). It was initially thought that this was caused by double penetration of the APC at the onset of coring operations during high heave conditions. However, because none of the other concentration data show this offset, this discrepancy is most likely caused by unreliable ion chromatograph sulfate data. Nevertheless, we use the data from Hole U1329E and place the SMI at 9.5 mbsf pending postcruise revision of the sulfate data for Holes U1329C and U1329E (Fig. F23).

Anaerobic oxidation of organic matter is a continuous electron generating process. First, dissolved sulfate is exhausted by bacterial sulfate reduction; the next available electron acceptor is CO2 (Claypool and Kaplan, 1974). At the depth where sulfate is exhausted, the alkalinity profile shows a reversal caused by the onset of CO2 reduction by microbial methanogenesis. The net CO2 produced by organic matter remineralization can be calculated by summing the CO2 removed by authigenic carbonate precipitation and the CO2 removed by methane production to the measured alkalinity.

Within the sulfate reduction and alkalinity generation zone, calcium and magnesium profiles show a rapid decrease with depth that reflects authigenic carbonate formation. In the uppermost 28 m, calcite and/or Mg-calcite formation preferentially consumes calcium, leading to the observed increase in Mg/Ca ratios to a value of 16, about three times that of seawater (Fig. F22; Table T3). At such high Mg/Ca ratios, high alkalinity values, and low sulfate values, dolomitization is the favored authigenic carbonate reaction. Indeed, from ~10 to ~70 mbsf, the decrease in alkalinity is coincident with a low-Ca and -Mg concentration zone, which includes the minima in calcium, magnesium, and Mg/Ca at ~30 mbsf, suggesting dolomitization reactions at this depth. The above reactions most likely were responsible for the formation of the dolomite concretions recovered in the core (see "Lithostratigraphy").

From ~60 mbsf through the BSR depth, Ca concentrations steadily increase, suggesting communication with a fluid at greater depth. Accordingly, the Mg concentrations should have decreased with depth, but, as seen in Figure F22, they are instead almost constant and even increase slightly with depth from 30 to ~55 mbsf. This is caused by Mg expulsion from clay-mineral ion exchange sites by ammonium, which reaches concentrations up to 7.7 mM at 108 mbsf (Fig. F21). Below the BSR and the zone of maximum ammonium concentrations, Ca increases at a steeper gradient and Mg decreases with depth.