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doi:10.2204/iodp.proc.334.103.2012

Geochemistry and microbiology

Geochemistry

We collected 84 whole-round samples in Hole U1378B for pore fluid analysis. The sample frequency was two samples per core in the uppermost 87 mbsf followed by one sample per core in the remainder of the hole. The uppermost interval (Core 334-U1378B-1H to Section 8H-2; 0–64 mbsf) was 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. Because of time constraints, we focused our efforts on collecting samples for postcruise studies and only a limited number of analyses were carried out onboard. For the organic geochemistry program, 65 headspace (HS) samples were collected for safety monitoring. In addition, 99 void gas samples (VAC) were collected, when present, and 173 gas samples (NZ) preserved with 10% KCl solution were sampled for shipboard analysis. The data reported are from 69 NZ samples analyzed at this site. The inorganic and organic geochemistry data are listed in Tables T4, T5, and T6 are plotted in Figures F17, F18, F19, and F20. Microbiology data (cell counts) are shown in Figures F21 and F22.

The pore fluid profiles of sulfate, alkalinity, ammonium, methane, and calcium in the uppermost 20 mbsf at this site reflect typical changes associated with organic carbon cycling (Fig. F17). Sulfate concentrations decrease almost linearly from the seafloor to the sulfate–methane transition zone (SMTZ) at ~13 mbsf. Alkalinity increases from the seawater value at the seafloor to a maximum of 36 mM at 24 mbsf. Likewise, ammonium concentrations increase in the zone of active SO₄ reduction and reach a local maximum of ~4.5 mM below the SMTZ at 24 mbsf, reflecting ongoing organic matter diagenesis. Calcium concentrations decrease from a seawater value at the seafloor to ~2.5 mM in this zone, reflecting precipitation of authigenic carbonates. The highest methane concentrations were observed just below the SMTZ between 14.1 and 23.6 mbsf. The gas at these depths results from biogenic production, as indicated by the high ratio of methane to heavier homologs (ethane and propane), with CH₄/(C₂H₆ + C₃H₈) values ranging from 8,000 to 15,000 (Fig. F19). Except perhaps for the sample recovered at 68.2 mbsf, in the interval from 20.7 to 200 mbsf, the CH₄/(C₂H₆ + C₃H₈) ratio steadily decreases and is interpreted as a mixing zone between shallow biogenic and deep-sourced thermogenic gas transported upward by diffusion.

Between 40 and 60 mbsf, a marked discontinuity is characterized by a decrease in alkalinity that likely reflects carbonate precipitation. From 100 to ~440 mbsf, salinity, Cl, Mg, K, and Na concentrations show a monotonic decrease with depth (Fig. F18). Dissolved calcium concentrations are variable to ~200 mbsf and then increase with depth to a peak concentration of ~10 mM at 440 mbsf that is coincident with minima of 390, 9, and 4 mM in Cl, Mg, and K concentrations, respectively. The Cl, Mg, and K concentrations at 440 mbsf are 67%, 17%, and 38% of seawater values, respectively. Collectively, the pore fluid chemical profiles suggest that a unique fluid exists between 420 and 500 mbsf characterized by relatively low salinity, Cl, Mg, K, and alkalinity concentrations and elevated Ca concentrations. This depth interval also corresponds to a marked increase in thermogenic hydrocarbons (propane, n-butane, and iso-butane; Figs. F19, F20). The in situ temperature at this depth (see “Physical properties”) is too cold for local generation of thermogenic hydrocarbons, suggesting lateral migration of a fluid sourced in a region with temperatures high enough to support clay dehydration and thermogenic hydrocarbon production.

The three samples recovered below 500 mbsf show a steep gradient with depth in salinity and chloride (Fig. F18), indicating diffusional communication with another fluid below the cored section. Since the Cl and salinity profiles decrease below 500 mbsf, this fluid must be fresher than the deeper sourced fluid sampled between 420 and 500 mbsf. Ethane, propane, iso-butane, and n-butane all show a maximum at 518.7 mbsf. The increasing concentrations and maxima of these longer chain hydrocarbons (C₂₊) and the CH₄/(C₂H₆ + C₃H₈) ratios indicate the dominance of thermogenic gas at depth in Hole U1378B.

Microbiology

Microbiological sampling consisted of 5 cm whole-round samples cut on the catwalk and subsampled in the laboratory using sterile techniques. Whole-round samples were taken at a frequency of two per core during APC coring of Hole U1378B and once every 10 cores during XCB coring. The deepest sample was taken from Core 334-U1378B-60X at ~500 mbsf. Whole-round samples were not collected from deeper cores because of poor recovery. 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 ship with a SYBR Green I staining procedure; and enumeration estimates revealed cell concentrations ranging from 9 × 10⁶ to 6 × 10⁷ cells/cm³, generally declining with depth (Fig. F21). Data have lots of scatter; however, there is one sample with significantly higher cell counts at the depth corresponding to the SMTZ (Fig. F22). Contamination was assessed qualitatively in XCB cores using fluorescent microspheres. These revealed at least some drill fluid contamination of XCB-cored sediment in the center of the core; therefore, all analyses on XCB samples will have to be interpreted accordingly. Cell enumeration estimates became increasingly unreliable with great depth, and those from depths below 250 mbsf were not included in this analysis.

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