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doi:10.2204/iodp.proc.323.102.2011

Geochemistry

Geochemical methods were used to quantify volatile gases and concentrations of chemical species in interstitial water and sediment. Routine shipboard safety measurements and standard shipboard analyses were used to characterize the geochemistry of the sediment. Nonstandard sampling and analyses were carried out to satisfy the various research goals of this expedition. Subsamples of interstitial water and sediment were also taken for shore-based research.

Hydrocarbon sampling and analyses

Concentrations of volatile hydrocarbons (C₁–C₆) from sediment headspace samples were measured once per core in Hole A (all sites) and at higher resolution in microbiology-dedicated Holes U1339B, U1342B, U1343B, U1344C, and U1345B. In the latter two holes, high-resolution headspace sampling took place on the catwalk to prevent methane (CH₄) loss and to produce more accurate CH₄ profiles, particularly near the sulfate–methane transition zone (SMTZ). Sediment plugs were taken directly from holes drilled in the core liner immediately after the core was placed on the catwalk. Two cubic centimeters of sediment was put into vials containing 4 mL of sodium hydroxide. The vials were immediately capped and stored upside down until volatile hydrocarbons were measured. The holes drilled into the core liner were taped. Headspace samples were taken at a resolution of 25 cm in the uppermost 15 mbsf, avoiding areas of the core that would be sampled for interstitial water and microbiology. In all other microbiology-dedicated holes, high-resolution headspace sampling took place in the Cold Laboratory. These headspace samples were taken in the same manner as the low-resolution catwalk samples by placing ~3–5 cm³ of sediment in a 20 cm³ glass serum vial and sealing it with a septum and metal crimp cap. The samples were then heated at 70°C for 30 min. A 5 cm³ headspace gas aliquot was removed from the vial with a glass syringe for analysis by gas chromatography.

Headspace gas samples were analyzed using an Agilent 6890 gas chromatograph (GC) equipped with a 2.4 m × 3.2 mm stainless steel column packed with 100/120 mesh HayeSep R and a flame ionization detector. This instrument measures the concentrations of methane (C₁), ethane (C₂), ethene (C₂=), propane (C₃), and propene (C₃=). The glass syringe was directly connected to the GC with a 1 cm³ sample loop. The carrier gas was helium. The GC oven temperature was programmed to ramp at 30°C/min from 90° to 100°C and at 15°C/min from 100° to 110°C and to remain at 110°C for 4.5 min before ramping at 50°C/min to 150°C with a final holding time of 1.8 min. Data were collected and evaluated with an Agilent Chemstation data-handling program. Chromatographic response was calibrated against preanalyzed standards.

Interstitial water sampling and chemistry

Interstitial water was extracted from 5, 10, or 15 cm whole-round sections. For microbiology-dedicated holes (U1339B, U1342B, U1343B, U1344C, and U1345B), unique sampling strategies were used (see Fig. F10 for an example of the strategy used in Hole U1345B). Samples were centered 25 cm apart for the uppermost 3 m in Holes U1339B and U1342B, the uppermost 9 m in Hole U1343B, the uppermost 12 m in Hole U1344C, and the uppermost 13 m in Hole U1345B. The length of this very high resolution sampling interval varied in order to create high-resolution chemical profiles through the SMTZ at each site. Cores from the microbiology-dedicated holes were cut into whole rounds in the Cold Laboratory (~7°C). Whole rounds were stored in a nitrogen-filled glove box in the Cold Laboratory until squeezed.

In Hole A at every site, three samples were taken from the top core (1H), two were taken from the second core (2H), and one was taken from the cores below Core 2H. These whole rounds were cut on the catwalk shortly after the core was brought on deck. Squeezing occurred immediately after the whole rounds were cut.

Before squeezing, samples were removed from the core liner and the outer surfaces were carefully scraped off with a clean spatula to minimize potential contamination by the coring process. Whole rounds were placed into a titanium and steel squeezing device and squeezed at ambient temperature with a hydraulic press at pressures as great as 35,000 psi using modified versions of the standard ODP stainless steel squeezer of Manheim and Sayles (1974). Interstitial water samples were collected in plastic syringes and filtered through 0.45 µm Whatman polyethersulfone disposable filters. Aliquots of interstitial water were treated and stored for shore-based analyses.

Rhizon CSS-F 5 cm core solution samplers (Rhizosphere Research Products) were also used to collect interstitial water (Seeberg-Elverfeldt et al., 2005). Rhizon samplers were soaked in distilled water for ~1 h before use. They were then carefully inserted through holes drilled in the core liner. Acid-washed syringes were attached to each Rhizon sampler, pulled to generate vacuum, and held open with wooden spacers. The first ~1 mL was discarded, and the rest was collected and aliquoted for shore-based analyses.

Interstitial water analyses followed the procedures outlined by Gieskes et al. (1991), Murray et al. (2000), and user manuals for new shipboard instrumentation, with modifications as indicated. Interstitial water was routinely analyzed for salinity with an INDEX Instruments digital refractometer. After squeezing, alkalinity (and pH) and dissolved inorganic carbon (DIC) were measured immediately by Gran titration (Gieskes and Rogers, 1973) with a Metrohm autotitrator and a UIC 5011 CO₂ coulometer, respectively.

Dissolved ammonium and total hydrogen sulfide (ΣH₂S = H₂S + HS^–) were determined spectrophotometrically (Shimadzu ultraviolet [UV] mini 1240 UV-Vis spectrophotometer) based on methods by Solórzano (1969) and Cline (1969), respectively. Phosphate concentrations were determined using an OI Analytical discrete analyzer (DA3500) spectrophotometer unit. Sulfate, chloride, calcium, sodium, magnesium, and potassium concentrations were determined with a Dionex ICS-3000 ion chromatograph on 1:200 diluted aliquots in 18 MΩ water.

The International Association for the Physical Sciences of the Oceans (IAPSO) seawater standard was used for standardization of alkalinity and for all measurements made on the ion chromatograph. Sodium sulfide, ammonium chloride, potassium phosphate monobasic, and calcium carbonate were used for calibration curves and as internal standards for sulfide, ammonium, phosphate, and DIC concentration measurements, respectively. There was <1% coefficient of variation (the ratio of the standard deviation to the mean) in the DIC measurements based on calcium carbonate standards run after every fifth sample. Variation for alkalinity was 1%–2% based on IAPSO runs after every fifth sample. The coefficient of variation (based on IAPSO runs after every fifth sample) measured on the ion chromatograph was 1% for anions and 1.3% for cations. The coefficients of variation for sulfide, ammonium, and phosphate (based on internal standards run after every fifth sample) were 5%, 0.5%–2%, and 0.8%, respectively.

The minor elements manganese, iron, boron, strontium, barium, silica, and lithium were determined by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) with a Teledyne Prodigy high-dispersion ICP-AES. ICP-AES techniques for minor elements were modified from those described by Murray et al. (2000) by preparing calibration standards in an acidified (2% HNO₃, by volume) sodium chloride matrix (35 g NaCl/L). Samples and standards were diluted 1:10 using the 2% HNO₃ matrix solution with 10 ppm yttrium (Y) as an internal standard. When necessary, drift correction was made using the factor from a drift monitor solution (middle value standard solution), which was analyzed after every seventh sample. The coefficient of variation based on duplicate samples was typically <5% but in some cases was as high as 10%. Quantification of the major cations sodium, magnesium, calcium, and potassium by ICP-AES was only done for Site U1339 interstitial water samples. For major cations, standards (IAPSO) and samples were diluted 1:100 with 2% HNO₃ containing 10 ppm Y.

Bulk sediment geochemistry

Total inorganic carbon (TIC) concentrations were determined using a UIC 5011 CO₂ coulometer. Samples of ~10 mg of freeze-dried ground sediment were reacted with 1 N HCl. The liberated CO₂ was backtitrated to a colorimetric end point. Calcium carbonate content, as weight percent, was calculated from the TIC content with the assumption that all TIC is present as calcium carbonate (CaCO₃):

The coefficient of variation was ~1%.

Total carbon (TC), total nitrogen (TN), and total sulfur (TS) contents were determined for a subset of the samples using a Thermo Electron Flash EA 1112 elemental analyzer equipped with a Thermo Electron packed column CHNS/NCS (polytetrafluoroethylene [PTFE]; length = 2 m; diameter = 6 × 5) and thermal conductivity detector (TCD). Aliquots of 10 mg of freeze-dried ground sediment in tin cups were combusted at 900°C in a stream of oxygen. Nitrogen oxides were reduced to N₂, and the mixture of N₂, CO₂, H₂O, and SO₂ gases was separated by GC, with detection performed by TCD. The GC oven temperature was set at 65°C. All measurements were calibrated by comparison to the standard, pure sulfanilamide. Repeated measurements of this standard gave coefficients of variation of 1%, 2%, and 6% for TC, TN, and TS, respectively. Contents of total organic carbon (TOC), as weight percent, were calculated as the difference between TC and TIC:

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