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doi:10.2204/iodp.proc.303306.102.2006

Geochemistry

The shipboard geochemistry program for Expedition 303 included characterization of (1) volatile hydrocarbons and other gases, (2) interstitial water, and (3) sedimentary inorganic and organic carbon, nitrogen, and sulfur. These analyses were carried out as part of the routine shipboard safety and pollution prevention requirements and to provide the standard shipboard data set and preliminary information for shore-based research.

Sediment gas sampling and analysis

During Expedition 303, the compositions and concentrations of volatile hydrocarbons in the sediments were monitored once per core. Gas samples were obtained by two different methods. First, the routine headspace procedure (Pimmel and Claypool, 2001) 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. A 5 cm³ volume of gas from the headspace in the vial was removed with a glass syringe for analysis by gas chromatography. A second gas sampling procedure was used for gas pockets or expansion voids that appeared in the core while it was still in the core liner. A device with a heavy-duty needle was used to penetrate the core liner, and an attached syringe was employed to collect the gas. Thus method was applied only once during Expedition 303 on Sample 303-U1304A-14H-2, 50–51 cm.

Headspace and gas samples were both 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 quickly measures the concentrations of methane (C₁), ethane (C₂), ethene (C₂₌), propane (C₃), and propene (C₃₌). The 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. Data were collected and evaluated with the Hewlett-Packard 3365 Chemstation data-handling program. Calibrations were conducted using analyzed gases, and gas concentrations were measured in parts per million by volume.

A natural gas analyzer (NGA) was used to measure concentrations of hydrocarbons through C₆ and nonhydrocarbon gases on void gas Sample 303-U1304A-14H-2, 50–51 cm. The NGA system consists of a Hewlett-Packard 6890 Plus GC equipped with multiport valves that access two different column and detector combinations. Hydrocarbons from methane to hexane were measured with a 60 m × 0.32 mm DB-1 capillary column and an FID. The GC oven holding this column was heated from 80° to 100°C at 8°C/min and then to 200°C at 30°C/min. Nonhydrocarbon gases were isothermally analyzed at the same time using a sequence of packed columns (15 cm HayeSep-R column connected to a 1 m molecular sieve column and a 2 m Poropak-T column) and thermal conductivity detectors (TCDs). Helium was the carrier gas in both systems, and Hewlett-Packard Chemstation was used for data acquisition and processing. Chromatographic response was calibrated against preanalyzed standards. Gas contents are reported in ppmv.

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. At each site, samples were taken from each core for the upper 60 m and at intervals of every third core thereafter to total depth. In addition to whole-round samples, small plug sediment samples of ~10 cm³ were taken with a syringe at two per core for the upper 60 m and at one per core for 60–100 mbsf for shore-based analyses of oxygen isotopes, deuterium, and chlorinity at high precision. 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 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.

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 702 SM autotitrator. NH₄⁺ concentrations were analyzed by spectrophotometric methods with a Milton Roy Spectronic 301 spectrophotometer. For the analyses of Mn²⁺, Fe²⁺, B, Sr²⁺, Ba²⁺, Li⁺, and H₄SiO₄ concentrations, a Jobin Yvon JY2000 inductively coupled plasma–atomic emission spectrometer (ICP-AES) was used. Concentrations of SO₄^2–, Mg²⁺, Ca²⁺, and K⁺ were determined with a Dionex DX-120 ion chromatograph. Cl^– was measured by silver nitrate titration method using a Metrohm 665 titrator. Na⁺ values reported in site reports were determined by charge balance calculations using the following equation:

Na⁺ = 2SO₄^2– + Cl^– + Alk – K⁺ – 2Mg²⁺ – 2Ca²⁺,

where Alk is alkalinity. Ion charges of minor dissolved species are neglected in this calculation. Chemical data for interstitial water are reported in molar concentration units.

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. For calibration of the ion chromatographic method, IAPSO solutions, diluted with 18 MΩ deionized water to 0%, 10%, 20%, 40%, 60%, 80%, and 100%, were used at the beginning of each batch (i.e., site) run. Elemental analysis by ICP-AES was modified from Murray et al. (2000) as outlined in Shipboard Scientific Party (2003). Specifically, a “master standard” was prepared by blending 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 MΩ 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% HNO₃/1 L) to generate “working standard” solutions. Calibrations were made for each of the chemical constituents measured by 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 run. The precision of the ICP-AES measurements was assessed by multiple analyses of the IAPSO standard and a “drift solution,” which was made by diluting the master standard with NaCl/HNO₃ matrix and 18 MΩ deionized water. Analytical precision for the measured constituents in the interstitial water is reported in Table T2.

Sedimentary inorganic and organic carbon, nitrogen, and sulfur concentrations

Inorganic carbon concentrations were determined using a Coulometrics 5011 carbon dioxide coulometer equipped with a System 140 carbonate analyzer at a frequency of two per 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 content (in weight percent) was calculated from the inorganic carbon (IC) content with the assumption that all inorganic carbon is present as calcium carbonate:

CaCO₃ (wt%) = IC (wt%) × 8.33.

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 1σ standard deviation of the mean of multiple determinations of a standard) for weight percent IC was ±1.3% (for Sites U1302–U1307).

Total carbon (TC), total nitrogen (TN), and total sulfur were determined using a Carlo Erba 1500 CNS analyzer on a subset of the samples used for inorganic carbon determinations. Aliquots of 10 mg of freeze-dried, ground sediment with ~10 mg V₂O₅ oxidant were combusted at 1000°C in a stream of oxygen. Nitrogen oxide was reduced to N₂, and the mixture of N₂, CO₂, H₂O, and SO₂ gases was separated by a GC and detection performed by a TCD. All measurements were calibrated by comparison to pure sulfanilamide as standard. Analytical precision for TC and TN was 4.4% and 6.7%, respectively (for Sites U1302–U1307). Contents of total organic carbon (TOC) (in weight percent) were calculated as the difference between total carbon and inorganic carbon:

TOC (wt%) = TC (wt%) – IC (wt%).

The carbon/nitrogen atomic ratios of organic materials were calculated from TOC and total nitrogen concentrations to identify sources of organic matter (i.e., marine or terrestrial). No pyrolysis analyses were performed during Expedition 303.

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