GEOCHEMISTRY

The shipboard geochemistry program for Leg 186 included (1) real-time monitoring of volatile hydrocarbons for safety and pollution prevention as required by ODP regulations; (2) measurement of inorganic carbon and carbonate content of sediments; (3) elemental analyses of total nitrogen, hydrogen, sulfur, and carbon; and (4) measurement in interstitial waters of salinity, alkalinity, pH, and concentration of major dissolved ionic species (i.e., chloride, sulfate, ammonium, sodium, potassium, magnesium, calcium, lithium, and strontium). All methods and instruments used during Leg 186 are described in detail by Emeis and Kvenvolden (1986), Kvenvolden and McDonald (1986), and in the "Explanatory Notes" chapter of the Leg 156 Initial Reports volume (Shipboard Scientific Party, 1995).

The main focus of shipboard organic geochemical analyses is to monitor abundances of the light hydrocarbons methane (C₁), ethane (C₂), ethylene (C₂₌), propane (C₃), and propylene (C₃₌) to assess possible hazardous hydrocarbon accumulations at depth. These accumulations can potentially pollute the environment and/or put the ship in risk. The assessment of potential hydrocarbon accumulations is based on the C₁/C₂ and C₁/C₃ values. As a general rule, values less than 200 for C₁/C₂ and 2000 for C₁/C₃ justify caution. These values, however, are also related to depth and temperature (Stein et al., 1995; JOIDES Journal, 1992) and should be used only as a guideline. Any decision regarding stopping the drilling operation should be determined based on those ratios, in conjunction with deviations from usual trends of these values with depth, coupled with significantly higher abundances of heavier hydrocarbons.

During Leg 186, concentrations of light hydrocarbon gases were monitored for each core following the standard headspace sampling method described by Kvenvolden and McDonald (1986). Briefly, immediately after the core was retrieved and cut into 150-cm sections, a No. 6 cork borer was used to obtain a sediment sample from the top of one of the sections. This ~5-cm³ sediment sample was placed in a 21.5-cm³ glass serum vial and sealed 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 analysis, sediment samples were heated in an oven at 60°C for 20 min. A gas-tight syringe and needle was then used to extract approximately 5.0 mL of headspace gas. When gas pockets were encountered, free-gas samples were collected by penetrating the liner with a syringe connected to a penetration tool.

Constituents of the headspace and free-gas samples were routinely analyzed using a Hewlett-Packard (HP) 6890 gas chromatograph (GC) equipped with a 0.25-cm³ sample loop, an 8 × -inch stainless steel column packed with HaySep R (80/100 mesh), and a flame ionization detector (FID). The carrier gas was helium with a flow of 30 mL/min. The initial temperature of the GC oven was held at 120°C. Data acquisition and processing were performed by HP Chemstation version 5.04. Free-gas samples were also analyzed with a natural gas analyzer (NGA) to quantify light hydrocarbons (methane to hexane) and nonhydrocarbon gases including nitrogen, oxygen, and carbon dioxide. The NGA system consists of a Hewlett-Packard 6890 GC equipped with two different columns and detectors. Hydrocarbons were analyzed with a 60 m × 0.32 mm SGE 50QC3 BP-1 capillary column and an FID. The initial temperature of the GC oven was held constant at 40°C for 10 min and then increased to 100°C at 10°C/min. Helium was used as the carrier gas at a flow of 1.5 mL/min. Nonhydrocarbon gases were analyzed isothermally (150°C) using a sequence of packed columns: a 6-in stainless steel HaySep R (80/100 mesh) column connected to a 3-ft molecular sieve 13 × (60/80 mesh) column, and a 6-in stainless steel Porpak T (50/80 mesh) column. A thermal conductivity detector (TCD) was used for detection. Helium was the carrier gas. Data acquisition and processing were performed by HP Chemstation software. Chromatographic response was calibrated against authentic standards and the results reported as parts per million (ppm).

Inorganic carbon was determined at a frequency of three samples per core using a Coulometrics 5011 carbon dioxide coulometer equipped with a System 140 carbonate carbon analyzer. Aliquots of about 10 mg of freeze-dried, ground sediment were weighed and reacted with 2N HCl to liberate CO₂. The evolved CO₂ was coulorometrically titrated using monoethanolamine. Pure calcite standards were used for quantification. Carbonate content (in weight percent, wt%) was calculated from the inorganic carbon (IC) content assuming that all the carbonate was present as calcite using the following equation:

CaCO₃ (wt%) = IC (wt%) × 8.332. (2)

Total carbon (TC), hydrogen, nitrogen, and sulfur were analyzed using a Carlo Erba 1500 CNS Analyzer at a frequency of one sample per core. About 10 mg of freeze-dried, ground sediment was weighed and combusted with a V₂O₅ catalyst at 1000°C in a stream of oxygen. Nitrogen oxides were reduced to N₂ and the mixture of evolved gases was separated by gas chromatography. Detection of separated gases was performed by TCD, using sulfanilamide as a calibration standard. The amount of total organic carbon (TOC) was calculated as the difference between TC and IC as follows:

TOC (wt%) = TC - IC. (3)

Atomic C/N values were calculated from TOC and total nitrogen concentrations.

Interstitial water samples were extracted from 5- to 10-cm-long whole-round sections cut immediately after core retrieval on deck. During Leg 186, samples were gathered at a frequency of one per core for the first 30 m, and one every three cores until IW could no longer be extracted. After extruding the sediment from the core liner, the surface of each whole-round section was carefully scraped with a spatula to remove potential contamination. IW samples were extracted by placing the sediment in a titanium squeezer and applying pressures as high as 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 and those for shore-based analyses were stored in heat-sealed acid-washed plastic tubes and/or glass vials.

Analyses of IW 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 NH₄⁺ were measured by spectrophotometric methods with a Milton Roy Spectronic 301 spectrophotometer. Concentrations of K⁺, Mg²⁺, Na⁺, Ca²⁺, and SO₄^2- were measured by ion chromatography using a Dionex DX-120 instrument. Concentrations of Li⁺ and Sr²⁺ were measured by flame atomic absorption spectrophotometry using a Varian SpectrAA-20.

Analytical precision was determined by replicate analyses of natural samples and by reanalyzing standards 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%; NH₄⁺, ~5%; K⁺, <3%; SO₄^2-, <4%; and Na⁺, <5%.