INTERSTITIAL WATER CHEMISTRY
AND HEADSPACE GAS

The shipboard inorganic and organic geochemistry program for Leg 185 focused on Site 1149 because Site 801 was reoccupied predominantly for igneous rock drilling. In Hole 801C, the water sampling temperature probe (WSTP) was run to sample borehole water on three different occasions, as described in "Microbiology". A single pushcore (designated as Hole 801D) taken with the RCB at Site 801 was analyzed for interstitial waters. At Site 1149 the combined inorganic and organic program included (1) real-time monitoring of carbon dioxide and volatile hydrocarbons (HC), as required by ODP safety regulations, (2) measurement of inorganic carbon and carbonate content of the sediments, (3) elemental analyses of total carbon, nitrogen, and sulfur, and (4) a comprehensive interstitial water protocol.

For safety and pollution prevention, concentrations and ratios of light HC gases, mainly methane (C₁), ethane (C₂), and propane (C₃), were monitored for each core following the standard headspace sampling method described by Kvenvolden and McDonald (1986). Immediately after core retrieval on deck, a sediment sample of ~5 cm³ was collected using a calibrated borer tool, placed in a 21.5-cm³ glass serum vial, and sealed immediately with a septum and metal crimp cap. When consolidated or lithified samples were encountered, chips of material were placed in the vial and sealed. Before gas analyses, the vial was heated at 70°C for a minimum of 20 min. A 5-cm³ subsample of the headspace gas was extracted from each vial using a 5-cm³ glass gas syringe for gas chromatography (GC) analysis. When gas pockets and expansion voids were encountered, gas samples were collected by penetrating the liner using a 50-cm³ plastic syringe with a plastic three-way valve connected to a steel penetration tool. Gas HC constituents were analyzed using a HP6890 gas chromatograph equipped with a flame ionization detector.

Inorganic carbon was determined at a frequency of one sample per core (and occasionally more frequently) using a Coulometrics 5011 carbon dioxide coulometer equipped with a System 140 carbonate carbon analyzer. Ten to 12 mg of freeze-dried ground sediment was reacted with 2 M HCl acid to liberate CO₂. The CO₂ was titrated and the change in light transmittance monitored by a photo detection cell. The weight percentage of calcium carbonate was calculated from the inorganic carbon content assuming that all the CO₂ evolved was derived from dissolution of calcium carbonate by the following equation:

The amount of carbonate is expressed as weight percent, assuming all the carbonate was present as calcite. No corrections were made for other carbonate minerals.

Total carbon, nitrogen, and sulfur were determined using a Carlo Erba 1500 CNS analyzer. An aliquot of 5-15 mg freeze-dried, crushed sediment with ~10 mg V₂O₅ oxidant were combusted at 1000°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 and detection performed by a thermal conductivity detector. The H₂ value is not useful because it represents hydrogen from both organic matter and minerals (clay). All measurements were calibrated by comparison to pure sulfanilamide as standard. The amount of total organic carbon was calculated as the difference between total carbon and inorganic carbon, as follows:

Interstitial waters were extracted from 10- to 15-cm-long whole-round sections cut immediately after core retrieval on deck. At Site 1149, samples were gathered at a frequency of one per section of core in the upper 60 mbsf, one per core from 60 to 100 mbsf, and approximately every second core until interstitial water could no longer be extracted. These samples were coordinated closely with those taken by the microbiologists. After extruding the sediment from the core liner, the surface of each whole round was carefully scraped with a spatula to remove potential contamination. Interstitial waters were extracted by placing the sediment in a titanium squeezer and applying pressures up to 40,000 lb (~4150 psi) using a Carver hydraulic press. Water samples were collected into acid-cleaned plastic syringes and filtered through sterile 0.45-µm Gelman polysulfone disposable filters. Samples for shipboard analyses were stored in plastic vials. Aliquots for shore-based analyses were stored in heat-sealed acid-washed plastic tubes and/or glass vials, as requested by the given investigators.

Interstitial-water analyses followed the procedures outlined by Gieskes et al. (1991). Salinity was measured with a Goldberg optical handheld refractometer. The pH and alkalinity were measured by Gran titration with a Brinkmann pH electrode and a Metrohm autotitrator. The Cl^- concentration was measured by titration. Concentrations of H₄SiO₄, PO₄^3-, and NH₄⁺ were measured by spectrophotometric methods with a Milton Roy Spectronic 301 spectrophotometer. Concentrations of K⁺, Mg²⁺, Ca²⁺, and SO₄^2- were measured by ion chromatography using a Dionex DX-100 instrument. Concentrations of Li⁺, Sr²⁺, Fe³⁺, Mn²⁺, and Rb⁺ were measured by flame atomic absorption spectrophotometry using a Varian SpectrAA-20.

Analytical precision was determined by replicate analyses of natural samples as well as of calibration standards reanalyzed as unknowns. Values of precision, expressed as percent of the measured value, are as follows for the respective constituents: alkalinity, <1.5%; Cl^-, 0.4%; Ca²⁺, <1%; Mg²⁺, 0.5%; Si⁺ and NH₄⁺, ~5%; K⁺, <3%; SO₄^2-, <4%; and Na⁺, <5%.