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doi:10.2204/iodp.proc.303306.110.2006

Geochemistry

The shipboard geochemistry program for Expedition 306 included characterization of (1) volatile hydrocarbons, (2) interstitial water, (3) sedimentary inorganic and organic carbon and total nitrogen, and (4) examination of solvent-extractable organic compounds. Volatile hydrocarbon analyses were carried out as part of the routine shipboard safety and pollution prevention requirements.

Sediment gas sampling and analysis

During Expedition 306, the compositions and concentrations of volatile hydrocarbons in the sediments were analyzed once per core using the routine headspace procedure (Pimmel and Claypool, 2001). This procedure involved placing ~5 cm³ of sediment sample in a 21.5 cm³ glass serum vial that was sealed with a septum and metal crimp cap immediately after core retrieval on deck. In the shipboard laboratory, the glass vial was heated at 60°C for 30 min. Five cubic centimeters of gas was removed from the headspace in the vial with a glass syringe for analysis by gas chromatography.

Headspace gas samples were analyzed using a Hewlett-Packard 6890 Plus gas chromatograph (GC) equipped with a 2.4 m × 3.2 mm stainless steel column packed with 80/100 mesh HayeSep S and a flame ionization detector (FID). This analytical system measures the concentrations of methane (C₁), ethane (C₂), ethene (C₂₌), propane (C₃), and propene (C₃₌). The sample gas syringe was directly connected to the GC via a 1 cm³ sample loop. Helium was used as the carrier gas, and the GC oven temperature was held at 90°C. Calibrations were conducted using analyzed gases, and gas concentrations were measured in parts per million by volume.

Interstitial water sampling and chemistry

Interstitial waters were extracted from 5 cm long whole-round sediment sections that were cut and capped immediately after core retrieval on deck. In one hole at each site, samples were taken from each core for the upper 60 mbsf and at intervals of every third core thereafter to a depth of 110 mbsf. In addition to whole-round samples, small plug sediment samples of ~10 cm³ were taken with a syringe at Site U1313 for shore-based analyses of oxygen isotopes, deuterium, and chloride. These samples were taken at a resolution of two per core for Cores 1 and 2, one per section for Cores 3 through 5, two for Core 6, and one per core for Cores 7, 8, 10, and 11. The small plug samples were taken from the working half at the time of sectioning on deck. This sampling technique was used to obtain high-resolution interstitial water samples while maintaining the integrity of the composite section.

In the shipboard laboratory, whole-round sediment samples were removed from the core liner, and the outside surfaces of the sediment samples were carefully scraped off with spatulas to minimize potential contamination with drill fluids. Sediment samples were then placed into a Manheim titanium squeezer and squeezed at ambient temperature with a Carver hydraulic press (Manheim and Sayles, 1974). Interstitial water samples discharged from the squeezer were passed through 0.45 µm Whatman polyethersulfone membrane filters, collected in plastic syringes, and stored in plastic sample tubes for shipboard analyses or archived in glass ampoules and/or heat-sealed acid-washed plastic tubes for shore-based analysis. For shipboard inductively coupled plasma–atomic emission spectroscopy (ICP-AES) analysis, 25 µL of concentrated HNO₃ was added to the samples prior storage. The squeezed sediment residues were split in portions of approximately one-quarter and three-quarters. The one-quarter portion was freeze-dried, ground, and used for shipboard organic geochemical analyses (see below). The rest of the portion was stored for shore-based analysis.

Shipboard analyses of whole-round interstitial water samples followed the procedures outlined by Gieskes et al. (1991) and Murray et al. (2000) with modifications as summarized below. Salinity was measured with an Index Instruments digital refractometer. Alkalinity and pH were measured by Gran titration with a Brinkman pH electrode and a Metrohm autotitrator. NH₄⁺ concentrations were analyzed by spectrophotometric methods with a Milton Roy spectrophotometer. For the analyses of Mg²⁺, Ca²⁺, K⁺, Na⁺, Mn²⁺, Fe²⁺, B, Sr²⁺, Ba²⁺, Li⁺, and H₄SiO₄ concentrations, a Jobin Yvon JY2000 ICP-AES was used. Concentrations of SO₄^2– were determined with a Dionex ion chromatograph. Cl^– was measured by silver nitrate titration method using a Metrohm titrator.

Analytical precision for each of the techniques was monitored through multiple analyses of International Association of Physical Sciences Organization (IAPSO) standard seawater solution or other standard solutions and is reported in Table T3.

For calibrations of major elements measured by ICP-AES, IAPSO solutions of Mg²⁺, Ca²⁺, K⁺, and Na⁺ diluted with 18 MS deionized water to 0%, 20%, 40%, 60%, 80%, 100%, 120%, 140%, and 160% were used at the beginning of each batch (i.e., site) run.

Minor elemental analysis by ICP-AES was modified from Murray et al. (2000) as outlined in Shipboard Scientific Party (2003a). Specifically, a "master standard" was prepared by blending certified standard solutions of Mn²⁺, Fe²⁺, B, Sr²⁺, Ba²⁺, and Li⁺ and a silicon reference solution (sodium silicate) in an acidified (1.25% HNO₃, by volume) sodium chloride matrix (17.5 g NaCl/L) diluted with 18 MS deionized water. This solution was diluted to 0%, 3%, 5%, 10%, 30%, 50%, and 100% with a NaCl/HNO₃ matrix (35 g NaCl + 2.5% HNO₃ per 1 L) to generate “working standard” solutions. Calibrations were made for each of the chemical constituents measured on ICP-AES using the working standard solutions diluted with a matrix solution (2.5% HNO₃ + 10 ppm yttrium as an internal standard) prior to each batch (i.e., site) run. Samples were diluted 1:50 for major and 1:10 for minor elements prior to ICP-AES analyses. Chemical data for interstitial water are reported in molar concentration units.

Sedimentary inorganic and organic carbon and nitrogen concentrations

Inorganic carbon concentrations were determined using a Coulometrics 5011 carbon dioxide coulometer equipped with a carbonate analyzer. Two carbonate analyses were performed for each core. Samples of ~10 mg of freeze-dried, ground sediment were reacted with 2N HCl. The liberated CO₂ was backtitrated to a colorimetric end point. Calcium carbonate (CaCO₃) content, as weight percent, was calculated from the inorganic carbon (IC) content with the assumption that all inorganic carbon is present as calcium carbonate:

No corrections were made for other carbonate minerals. The coulometer was calibrated with pure CaCO₃ powder during the expedition, and the analytical precision (expressed as 1F standard deviations of means of multiple determinations of a standard) for IC was ±0.45% (for Site U1312).

Total carbon (TC), total nitrogen (TN), and total hydrogen were determined using a Carlo Erba 1500 CHNS analyzer on the same samples used for inorganic carbon concentrations. Aliquots of ~10 mg of freeze-dried, ground sediment and ~10 mg V₂O₅ as co-oxidant were combusted at 1000°C in a stream of oxygen. Nitrogen oxides were reduced to N₂, and the mixture of N₂, CO₂, and H₂O gases was separated by GC and detection performed by a thermal conductivity detector. All measurements were calibrated by comparison to pure sulfanilamide as standard. Analytical precision for TC and TN was 1.99% and 1.07%, respectively (1F standard deviation of means of multiple determinations of sulfanilamide for Sites U1312, U1313, and U1314). Despite low 1F standard deviation for TN, we noticed in sediments from Hole U1314A and partly Hole U1313A that systematic offsets in TN of sample sets run within different batches occurred and initially generated obvious staircase-like downhole profiles. In case of such observations, samples were submitted for rerun. However, shore-based elemental reanalysis of the shipboard samples will be additionally performed and reported elsewhere.

Contents of total organic carbon (TOC), as weight percent, were calculated as the difference between TC and IC:

As most of the samples had very high CaCO₃ contents and very low TOC values (see respective site chapters), the TOC data determined during Expedition 306 should be considered as preliminary, keeping in mind its methodological (i.e., calculated rather then measured) limitation. Indeed, postcruise analysis of shipboard samples from all three sites by means of direct TOC measurement using a LECO CS-125 analyzer (J. Hefter, unpubl. data, 2006) showed no correlation (Fig. F10), especially for samples rich in carbonate (>90 wt%, e.g., most of the samples from Hole U1312A).

Solvent-extractable organic compounds

A limited number of sediment samples were analyzed onboard for solvent-extractable organic compounds to determine initial organic matter characteristics for subsequent shore-based analyses (see individual site chapters for numbers and depths). For the shipboard organic geochemistry analyses, freeze-dried splits of one-quarter of whole-round sediment squeeze cakes, used for interstitial water (sampling code IWSG) sampling, were used. To perform the extraction, 10–25 g of sediment was ultrasonically extracted in two 40 mL glass tubes using 20–40 mL CH₂Cl₂ for 15 min. Sediments and solvents were separated in a centrifuge at 2500 rpm for 5 min, and the solvent phase was decanted and collected. The extraction step was repeated by adding 10–20 mL of fresh CH₂Cl₂ to the sediment remnants. The combined total extracts were reduced to near dryness by a gentle stream of N₂. Depending on the amount of TOC and the color of the extracts, total extracts were either directly injected into the GC-mass spectrometer (MS) or further separated by column chromatography. Silica gel (100–200 mesh, dried at 120°C overnight) was placed in a Pasteur pipette plugged with cotton wool and preconditioned by successive elution with 4 mL of CH₂Cl₂ and 4 mL of hexane. Total extracts, redissolved in a small amount (100–200 µL) of hexane, were placed on top and separated into two subfractions by elution with 4 mL of hexane and 4 mL of CH₂Cl₂, respectively. Note that the above analytical procedure will not allow the detection of certain types of compound classes (e.g., fatty acids, alcohols, sterols, etc.) because of the lack of derivatization steps.

The GC-MS system used for compound identification consists of an HP 6890 GC equipped with an HP 7683 automatic liquid sampler and is interfaced via a heated (280°C) transfer line to an HP 5973 mass-selective detector (MSD). Total extracts or subfractions dissolved in a final volume of 100 µL hexane were injected into an electronic pneumatic-controlled split-splitless injector (operated in splitless mode at 300°C; injection volume = 1 µL). Compounds were separated on an HP-5MS capillary column (30 m × 0.25 mm; film thickness = 0.25 µm), using He in constant flow mode (1 mL/min) as carrier gas. The GC temperature program was as follows: 40°C (hold 2 min), heat to 130°C (rate = 20°C/min), heat to 320°C (rate = 4°C/min), and hold 20 min at 320°C. The source temperature of the MSD was set to 230°C and the quadrupole mass analyzer to 150°C. Mass spectra were recorded in full-scan mode from m/z 50 to 600 at a scan rate of 2.7 spectra/s. Compounds were identified by their mass spectral characteristics and GC retention times and by comparison with published mass spectral data or mass spectral databases (NIST, Wiley). Unfortunately, the MS could only be used for Site U1312. Later, the high-voltage supply of the mass analyzer failed and was found not to be repairable for the remaining of the expedition due to the lack of spare electronic parts. As an alternative, we had to use the FID installed on the HP 6890 GC. Identification of n-alkanes at the following sites, however, could be established relatively easily by comparison to retention times obtained from a standard mixture containing all homologs from n-C₁₀ to n-C₃₆. The identification of C₃₇ and C₃₈ alkenones is based on their well-known retention times (e.g., >C₃₈ n-alkane under the GC conditions used), their specific elution order, and their overall characteristic distribution pattern. Identification of compounds designated as contaminants (mainly plastic-derived, e.g., homologs of phthalate esters) for GC-FID runs is based on previous GC-MS identifications of those compounds in blank and analytical procedure tests and intercomparison of retention times after reruns of the appropriate fractions under GC-FID conditions.

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