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doi:10.2204/iodp.proc.334.104.2012

Geochemistry and microbiology

Geochemistry

We collected 110 whole-round samples for pore fluid analysis. In Hole U1379B, we collected two samples per section. In Hole U1379C, the sample frequency was two samples per core in the uppermost 235 mbsf followed by one sample per core for the rest of the hole. Samples from the uppermost interval (Cores 334-U1379B-1H through 2H [0–7.98 mbsf] and Cores 334-U1379C-1H through 6H [0–39.38 mbsf]) were processed under a nitrogen atmosphere to preserve the reduced dissolved species and their isotopic composition (Fe and Mo), which will be analyzed postcruise. The remaining samples were exposed to the atmosphere prior to squeezing. For the organic geochemistry program, we collected 118 sediment plugs (HS) for safety monitoring of headspace hydrocarbon gases and analyzed them by gas chromatography–flame ionization detection (GC-FID) on the natural gas analyzer. In addition, 74 void gas samples (VAC) were collected from voids, whenever present. Most of these samples were analyzed by GC-FID (102 samples), and a replicate sample was preserved for isotope analyses at onshore laboratories. The inorganic and organic geochemistry data are listed in Tables T6, T7, and T8 and are plotted in Figures F22, F23, F24, F25, and F26. Microbiology data (cell counts) are shown in Figures F27 and F28.

The pore fluid profiles of sulfate, alkalinity, ammonium, and calcium in the uppermost 50 mbsf reflect typical changes associated with organic carbon cycling. Alkalinity and ammonium increase from seawater values to maxima at 12 mbsf, reflecting organic matter diagenesis. At ~14 mbsf, the sulfate gradient decreases associated with a concomitant decrease in NH₄ concentrations and a decrease in the alkalinity gradient. Based on the sulfate data, the depth of the sulfate–methane transition zone (SMTZ) is estimated to be ~30 mbsf (Fig. F22). This zone is characterized by a decrease in dissolved calcium to values as low as 2.5 mM, suggesting authigenic carbonate precipitation between 20 and 30 mbsf. Below the SMTZ, methane concentrations increase with depth (Fig. F22) and reach the highest concentrations from 42.31 to 67.18 mbsf (Fig. F25). Methane at these depths is dominated by microbial production, as indicated by the high ratio of methane to heavier homologs (ethane and propane), with CH₄/(C₂H₆ + C₃H₈) ratios ranging from 8,000 to 10,000 (Fig. F26).

Based on the limited shipboard data set collected at Site U1379, the pore fluid geochemistry below the zone of most intense biogeochemical cycling in the uppermost 50 m can be split into three zones. The first zone extends from ~50 to 500 mbsf and is characterized by a steady increase in Ca concentration and a decrease in Mg and K concentrations with depth. Within this zone there are two maxima in NH₄ concentration of ~10 mM at 158 and 386 mbsf (Fig. F23). There are small maxima in potassium and alkalinity associated with the NH₄ peak at 386 mbsf. The high NH₄ concentrations observed in the uppermost 400 m of the sediment column, generally between 3 and 10 mM, is likely the result of the high sedimentation rates at this site (see “Lithostratigraphy and petrology” and “Paleontology and biostratigraphy”) causing the ammonium produced by organic matter diagenesis to remain buried in the section. Within this zone, ethane concentrations increase progressively below ~100 mbsf, whereas propane to pentane are only detected in insignificant amounts to ~360 mbsf (Fig. F25). Two discrete peaks in C₂ to C₅ gas concentrations are observed at ~360 and 440 mbsf, which roughly correlate with an increase in NH₄ concentration observed in pore fluids. The profiles of C₂ to C₅ indicate the presence of thermogenic gases at and below 360 mbsf.

The second geochemical zone occurs between ~500 and 800 mbsf and is characterized by a broad interval of low Cl, Ca, Mg, NH₄, and alkalinity concentrations, with the lowest concentrations occurring at ~680 mbsf (Fig. F23). The K concentration-depth profile, however, steadily decreases in this interval, similar to the gradient observed in the uppermost 500 m of the sediment section. The lowest CH₄/(C₂H₆ + C₃H₈) ratios observed at this site (137–327) occur in the zone between 598.49 and 656.55 mbsf, with methane concentration ranging between ~3000 and 6000 ppmv (Fig. F25). A strong peak in C₂ to C₅ concentrations occurs at ~650 mbsf and is coincident with the minima in Cl concentration observed in the pore fluids. The broad decrease in the major elements and decrease in the ratio of methane to the heavier hydrocarbons correlates with lithostratigraphic Unit III, which is dominated by coarser grained sediments as well as several fault zones identified below ~600 mbsf (see “Structural geology”). Collectively, the inorganic and organic geochemical data suggest lateral flow of a freshened fluid with elevated concentrations of thermogenic hydrocarbons (up to iso-pentanes) and potassium. The geothermal gradient at Site U1379 is 40°C/km (see “Physical properties”); thus, the temperature between 600 and 800 mbsf ranges from 24° to 32°C. This temperature range is too low for in situ production of thermogenic hydrocarbons or for extensive clay dehydration, suggesting the fluid sampled in Unit III originated from a deeper source and is migrating laterally and upward along the permeable sand horizons and faults. This inference will be confirmed by isotopic analyses of the pore fluids postcruise.

The third geochemical zone occurs from ~800 to ~900 mbsf and is characterized by a strong linear increase in Ca concentrations and a decrease in Mg concentrations (Fig. F23). Within this interval calcium concentrations increase from 14 to 35 mM (~3 times seawater value) and Mg concentrations decrease from 22 to 10 mM (~20% of seawater value). This zone extends from the base of lithostratigraphic Unit III to basement (Unit IV; see “Lithostratigraphy and petrology”). The Ca and Mg trends (Fig. F24) suggest that the deepest sampled fluid is distinct from that of Unit III and is dominated by fluid-rock reaction with basalt in the basement mélange. Within this interval, CH₄ concentrations reach 9630 ppmv and the CH₄/(C₂H₆ + C₃H₈) ratio increases to 834. With the limited shipboard data set, it is difficult to determine whether the fluids in the sampled basement section have migrated from depth or simply reflect in situ diagenesis. The source of the deepest sampled fluids will be constrained by shore-based chemical and isotopic analyses.

Microbiology

Microbiological sampling consisted of 5 cm whole-round samples cut on the catwalk and subsampled in the laboratory using sterile techniques. In Hole U1379B, whole-round samples were taken at a frequency of one per meter of core. In Hole U1379C, whole-round samples were taken at a frequency of two per core during APC coring and once per 10–15 cores during XCB coring. The deepest sample was taken from Core 334-U1379C-92X at ~770 mbsf. Subsampling of the whole-round samples was performed for three different categories of postcruise studies: (1) frozen samples for molecular analyses, (2) refrigerated samples for cultivation studies, and (3) paraformaldehyde-fixed samples for cell counting and contamination testing.

Fixed samples for cell counting were further prepared on board with a SYBR Green I staining procedure, and enumeration estimates performed for samples from 0.7 to 211 mbsf revealed cell concentrations ranging from 10⁶ to 10⁸ cells/cm³, generally declining with depth. (Fig. F27). Closer inspection of the uppermost 45 mbsf shows a localized trend of increasing cell concentrations from 15 to 33 mbsf, near the proposed SMTZ, before decreasing again (Fig. F28). Contamination was assessed qualitatively in XCB cores using fluorescent microspheres. Microscopic analysis of the fluorescent microspheres revealed at least some drill fluid contamination of XCB-cored sediment into the centre of the core. Hence, cell enumeration estimates became increasingly unreliable with great depth, and those beyond 200 mbsf were not included in this analysis.

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