Concentrations of volatile hydrocarbons, inorganic (carbonate) carbon, and organic carbon, and the type of organic matter were determined during Leg 175. These analyses were carried out as part of the routine shipboard safety requirements and to provide preliminary information for shore-based organic geochemical research.
Samples of low molecular weight hydrocarbons and other gases were obtained by two different methods. The routine headspace procedure (Kvenvolden and McDonald, 1986) involved placing ~5 cm3 of sediment from each core into a 21.5-cm3 glass serum vial. The vial was sealed with a septum and a metal crimp cap and 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 (Kvenvolden and McDonald, 1986).
Headspace and gas-pocket samples were both routinely analyzed using a Hewlett Packard 5890 II Plus gas chromatograph (GC) equipped with a 2.4 m × 3.2 mm stainless steel column packed with HaySep S (80-100 mesh) and a flame ionization detector (FID). This instrument quickly measures the concentrations of methane (C1), ethane (C2), ethene (C2=), propane (C3), and propene (C3=). The gas syringe was directly connected to the gas chromatograph via a 1-cm3 sample loop. Helium was used as the carrier gas, and the GC oven was held at 90°C. Data were collected and evaluated with a Hewlett-Packard 3365 Chemstation data-handling program. Calibrations were done using Scotty IV analyzed gases, and gas concentrations were measured in parts per million.
When high concentrations of C2+ hydrocarbons or of nonhydrocarbon gases, such as H2S or CO2, were anticipated, gas samples were analyzed with the natural gas analyzer (NGA). The NGA system consists of a Hewlett-Packard 5890 II Plus GC equipped with multiport valves that accesses two different column and detector combinations. Hydrocarbons from methane to hexane were measured with a 60 m x 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 using a sequence of packed columns—a 15-cm HaySep R column connected to a 1-m molecular sieve column and a 2-m Poropak T column—with a thermal conductivity detector (TCD). Helium was the carrier gas in both systems, and a Hewlett Packard Chemstation data system was used.
Three 5-cm3 sediment samples were routinely selected from each core for analysis of total carbon (TC), carbonate carbon, total organic carbon (TOC), nitrogen, and sulfur. Additional samples were taken from intervals of special interest, particularly dark-colored and presumably organic matter-rich sediments, and were similarly analyzed.
Carbonate carbon concentrations of the samples were determined using a Coulometrics 5011 carbonate carbon analyzer. A sample of about 15 mg of freeze-dried, ground sediment was reacted with 2N HCl. The liberated CO2 forms a titratable acid with a blue monoethanolamine indicator solution that causes the color to fade. The change in color is measured by a photodetector cell coupled to a platinum anode that produces base electrically, which, in turn, returns the indicator to its original color. Carbonate contents are expressed as weight percent CaCO3, assuming that all the carbonate was present as calcite or aragonite.
Concentrations of total carbon, nitrogen, and sulfur were determined using a Carlo Erba 1500 carbon-nitrogen-sulfur analyzer. Approximately 5 mg of freeze-dried, ground sediment was combusted at 1000°C in a stream of oxygen. Nitrogen oxides were reduced to nitrogen (N2), and the mixture of CO2, N2, and sulfur dioxide (SO2) was separated by gas chromatography and quantified with a TCD. TOC concentrations were determined as the difference between carbonate carbon and TC concentrations. Comparison of concentrations obtained by the difference between total and carbonate carbon and by direct measurement of organic carbon has shown that the shipboard procedure is quite accurate for sediments containing >0.1 wt% TOC (Meyers and Silliman, 1996). Organic matter atomic carbon/ nitrogen ratios were calculated from TOC concentrations and total nitrogen concentrations.
The type of organic matter in organic carbon-rich sediments was evaluated by pyrolysis using the Delsi-Nermag Rock-Eval II pyrolysis system. This uses a whole-rock pyrolysis technique to identify the type and maturity of organic matter and to determine the petroleum potential of the sediments (Espitalié et al., 1986). The Rock-Eval system involves a temperature program that first releases volatile hydrocarbons (S1) at 300°C for 3 min and then releases hydrocarbons from thermal cracking of kerogen (S2) as the temperature increases from 300° to 600°C at 25°C/min. S1 and S2 hydrocarbons are measured by an FID and reported in milligrams per gram of sediment. The temperature at which the kerogen yields the maximum amount of hydrocarbons during the S2 program provides Tmax, a parameter that indicates the maturity of the organic matter. Between 300° and 390°C of the pyrolysis program, CO2 released from the thermal degradation of organic matter (S3) is trapped and measured by a TCD in milligrams per gram of sediment. Rock-Eval II parameters characterize organic matter by allowing the following indices to be calculated: hydrogen index (HI), (100 × S2)/TOC; oxygen index (OI), (100 × S3)/TOC; S2 /S3 ratio; production index (PI), S1 /(S1 + S2); and petroleum potential (PC; pyrolyzable carbon), 0.083 (S1 + S2). Interpretation of Rock-Eval data is considered to be unreliable for samples containing <0.5 wt% TOC (Peters, 1986). A Hewlett-Packard 3365 Chemstation computer data-analysis system was used to integrate and store the results obtained from the Rock-Eval analyses.