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doi:10.2204/iodp.proc.341.102.2014

Geochemistry

Geochemical methods were used to quantify volatile gases and concentrations of chemical elements and ions in IW and in sediment samples (both IW squeeze cakes and discrete samples from the working halves). The applied methods followed recommendations published by Manheim and Sayles (1974), Gieskes et al. (1991), and Murray et al. (2000), as detailed below. Additional IW and sediment subsamples were taken for shore-based research. Headspace and IW sampling resolutions are detailed in the “Geochemistry” sections of the respective site chapters.

Hydrocarbon sampling and analyses

Sediment plugs were taken with a stainless steel sample coring tool from the cut section tops directly adjacent to the deepest IW whole-round samples immediately after the core was cut on the catwalk. When the sediment became too lithified to extract using the sample coring tool, fragments of the core were chiseled out. About 5 cm³ of sediment was put into a 20 cm³ serum glass vial that was immediately sealed with a septum and metal crimp on the catwalk. The sample was then heated at 70°C for 30 min, and a 5 cm³ headspace gas aliquot was removed from the vial with a glass syringe and injected into the gas chromatograph.

Headspace gas samples were analyzed using a Hewlett Packard 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, to remain at 110°C for 4.5 min, and then to ramp 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. The coefficients of variation for gas standards run after every 10 samples were 1%–5% and 1%–4% for methane and ethane, respectively.

Interstitial water sampling and chemistry

IW was extracted from 5 to 15 cm whole-round sediment samples, depending on the degree of induration—and hence expected IW content—of the sediment. Usually, IW whole-round samples were taken in the following depth resolution: at the base of Sections 2, 4, and 6 in Cores 1–3 (given full stroke; modified depending on mudline depth in Core 1); at the base of Sections 3 and 5 in Cores 4–6; and at the base of Section 3 from Core 7 downhole. In case of half APC system application, IW samples were taken at the base of Section 2. Before IW extraction, the whole-round samples were removed from the core liner using a plastic stamp and the outer surfaces were scraped with a clean steel 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 from 5,000 to 30,000 psi, using modified versions of the standard ODP stainless steel squeezer of Manheim and Sayles (1974). IW samples were collected in 50 mL plastic syringes and filtered through 0.45 ?m Whatman polyethersulfone disposable filters. Aliquots of IW samples were stored in several vials for shipboard and shore-based analyses (Table T2).

In addition to conventional IW squeezing, IW was also extracted in higher depth resolution (~50–100 cm) using rhizon samplers from APC Cores 1–5 at Sites U1417–U1419. These polymer-coated sticks (0.1 μm pore size) were inserted into the sediment through holes drilled into the core liners along the splitting line after the cores had passed through the WRMSL. Applying 12 mL plastic syringes with stoppers created a vacuum that extracted IW from the sediment.

The International Association of Physical Sciences Organizations (IAPSO) seawater standard was used for standardization of alkalinity and chloride concentrations for all measurements made on the ion chromatograph (IC) and for all inductively coupled plasma–atomic emission spectroscopy (ICP-AES) measurements (IAPSO composition in Gieskes et al., 1991). Sodium sulfide, ammonium chloride, potassium phosphate monobasic, and calcium carbonate were used to prepare calibration solutions and internal standards for ammonium and phosphate concentration measurements.

IW analyses followed the procedures outlined by Gieskes et al. (1991), Murray et al. (2000), and the user manuals for new shipboard instrumentation (www.iodp-usio.org/Tools_Labs/). IW was routinely analyzed for salinity with an Index Instruments digital refractometer. Alkalinity/pH and chloride concentrations were measured immediately after IW extraction by titration with 0.1 N HCl and 0.1 N AgNO₃ solutions, respectively, using Metrohm autotitrators.

Dissolved ammonium and phosphate were determined spectrophotometrically (Agilent Cary 100 double-beam UV/Vis spectrophotometer). Sulfate, chloride, calcium, sodium, magnesium, and potassium ion concentrations were determined with a Metrohm 850 Professional IC on 1:100 diluted aliquots in 18 MΩ water.

Minor elements (B, Ba, Fe, Li, Mn, Si, and Sr) were analyzed with a Teledyne Prodigy high-dispersion ICP-AES. Standards (IAPSO) and samples were diluted 1:20 with a 2% v/v HNO₃ solution in deionized water, containing 10 ppm yttrium (Y) as an internal standard. Wavelengths were selected according to the recommendations for shipboard analysis of Murray et al. (2000). Drift correction was automatically applied by the instrument software.

The coefficient of variation of the IAPSO standard (run after every tenth sample) on the IC was 3%–5% for all anions and cations, and the average ion concentrations measured throughout the expedition were within 1.5% (2.5% for K) of the standard values given by Gieskes et al. (1991). The coefficients of variation for ammonium and phosphate based on internal standards run after every tenth sample were 5% and 2%, respectively.

For ICP-AES analysis, quantification limits for the different elements/wavelengths (five times the absolute standard deviation of the calibration blank), as well as their relative standard deviations (calculated from triplicate measurements for samples with respective element concentrations above quantification limit), are given in Table T3. Relative standard deviations for multiple analyses of the IAPSO standard throughout the expedition were <5% for Sr, <6% for B, and <10% for Li and Si. For the elements Ba, Fe, and Mn, concentrations of the IAPSO standard were below quantification limit.

In case of limited IW recovery from sandy and/or lithified samples, the following list of analytical and sampling priorities was followed: ICP-AES, IC, ammonium/phosphate (spectrophotometric), chloride titration, alkalinity/pH titration, shipboard scientist IW splits, and shore-based scientist IW splits. This priority list, as well as the respective amounts and pretreatments of IW splits, are listed in Table T2.

Bulk sediment geochemistry

Total inorganic carbon (TIC) contents (wt%) were determined using a UIC 5015 CO₂ coulometer. Around 10 mg of freeze-dried ground sediment taken from IW squeeze cakes and discrete samples (collected by shipboard sedimentologists) were reacted with 1 M HCl. The liberated CO₂ was backtitrated to a colorimetric end point. Calcium carbonate content 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) and total nitrogen (TN) contents were determined on aliquots of the freeze-dried ground samples using a Thermo Electron Flash Elemental Analyzer 1112 equipped with a Thermo Electron packed column CHNS/NCS (polytetrafluoroethylene; length = 2 m; diameter = 6 × 5) and thermal conductivity detector. Aliquots of ~10 mg of freeze-dried ground sediment were weighed into tin cups and combusted at 900°C in an oxygen stream. Nitrogen oxides were reduced to N₂, and the mixture of N₂, CO₂, and H₂O gases was separated by GC, with detection performed by thermal conductivity detector. The GC oven temperature was set at 65°C. Notably, this column needed to be replaced after every 100 analyses to avoid a gradual increase of TN content due to saturation of the N₂ column. All measurements were calibrated by comparison to the standard, ThermoFisher Scientific Soil Reference Material NC (TN = 0.21 wt%; TC = 2.29 wt%), and average TN and TC contents throughout the expedition were within 0.4% of these standard values. Repeated measurements of this standard gave coefficients of variation of 7.6% and up to 19% for TC and TN, respectively. Contents of total organic carbon (TOC) (wt%) were calculated as the difference between TC and TIC:

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