The shipboard organic geochemistry analyses for Leg 182 included (1) real-time monitoring of volatile hydrocarbons for safety and pollution prevention; (2) inorganic carbon (IC) analyses to determine calcium carbonate content; (3) elemental analyses of total carbon (TC) and nitrogen; and (4) preliminary characterization of organic matter by Rock-Eval pyrolysis. All methods and instruments used during Leg 182 are described below; further details are available in Emeis and Kvenvolden (1986).
Concentrations of light hydrocarbon gases were monitored at intervals of one sample per core following the standard headspace sampling method described by Kvenvolden and McDonald (1986). Upon core retrieval a 5-cm3 sediment sample was collected and placed in a 21.5-cm3 glass serum vial that was sealed with a septum and metal crimp cap. For a consolidated or lithified sample, rock chips were placed in the vial and sealed. After heating to 60°C for 20 min, a 5-cm3 volume of headspace gas was extracted from the vial using a standard glass syringe. When gas pockets were observed, vacutainer samples were collected directly from the core while still in the core liner by penetrating the liner using a syringe connected to a penetration tool.
Constituents of the headspace and vacutainer gas samples were routinely analyzed using a Hewlett Packard 5890 II Plus gas chromatograph (GC) equipped with an 8 ft × 1/8 in stainless steel column packed with HaySep S (100-120 mesh) and a flame ionization detector (FID). Concentrations of methane, ethane, ethene, propane, and propene were obtained. The headspace syringe or vacutainer was directly connected to the GC with a 1.0-cm3 sample loop. The carrier gas was helium, and the GC oven was held at 90°C. Data were collected using a Hewlett-Packard 3365 Chemstation data handling program.
When high concentrations of C2+ hydrocarbon gases or of nonhydrocarbon gases such as H2S or CO2 were found, gas samples were analyzed using the natural gas analyzer (NGA). The NGA system consists of a Hewlett-Packard 5890 II Plus GC equipped with two different columns and detectors. Hydrocarbons from methane to hexane were analyzed using a 60 m × 0.32 mm DB-1 capillary column and FID. The GC oven was heated at 40°C for 10 min and then to 65°C at 10°C/min. Nonhydrocarbon gases were analyzed isothermally (150°C) 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, and detected using a thermal conductivity detector (TCD).
Three 5-cm3 sediment samples were routinely selected from each core for IC analysis. Inorganic carbon concentration was determined using a Coulometrics 5011 CO2 coulometer. An ~10- to 15-mg sample of freeze-dried, ground sediment was weighed and reacted with 2 N HCl. The liberated CO2 was titrated and the endpoint determined by a photodetector. Percentage of CaCO3, expressed as weight percent calcium carbonate, was calculated from the IC content, assuming that all evolved CO2 was derived from dissolution of CaCO3, by the equation
No correction was made for the presence of other carbonate minerals.
Total carbon, nitrogen, and sulfur concentrations were determined using a Carlo Erba 1500 CNS elemental analyzer. About 10 mg of freeze-dried ground sediment was weighed and combusted at 1000°C in a stream of oxygen. Nitrogen oxides were reduced to N2, and the mixture of CO2, N2, and SO2 gases was separated by gas chromatography followed by thermal conductivity detection. Organic carbon (TOC) concentration was calculated as the difference between TC and IC concentrations. Atomic C/N values were calculated from TOC and nitrogen concentrations.
The organic matter in selected organic carbon-rich sediment samples was characterized by pyrolysis using a Delsi Rock-Eval II system. This method is based on a whole-rock pyrolysis technique designed to characterize the type and maturity of the organic matter and to estimate the petroleum potential of the sediments (Espitalié et al., 1986). The Rock-Eval system incorporates a temperature program that initially expels volatile hydrocarbons (S1) as the sample is heated at 300°C for 3 min and then, as the temperature increases from 300° to 600°C at 25°C/min, releases the hydrocarbons (S2) resulting from thermal cracking of kerogen. S1 and S2 hydrocarbons are measured and reported in milligrams per gram of dry sediment. The temperature at which the kerogen yields the maximum amount of hydrocarbons during the S2 program provides Tmax, a parameter used to assess the maturity of the organic matter. Between 300°C and 390°C of the pyrolysis program, CO2 (S3) is released from the organic matter, trapped, measured by TCD, and reported in milligrams per gram of dry sediment. Rock-Eval II parameters are used to characterize organic matter by calculation of the following indices: hydrogen index (HI = S2/TOC × 100); oxygen index (OI = S3/TOC × 100); and S2/S3. Rock-Eval data are generally unreliable for samples containing <0.5% TOC.