The shipboard organic geochemistry program for Leg 198 included (1) real-time monitoring of volatile hydrocarbons (HC) in headspace gases as required by ODP safety regulations; (2) measurement of inorganic carbon (IC) and carbonate content of the sediments; (3) elemental analyses of total carbon, nitrogen, and sulfur; (4) characterization of organic matter (OM) by Rock-Eval pyrolysis; and (5) examination of composition of solvent-extractable components.
The laboratory methodologies and instruments employed during Leg 198 follow those used in recent ODP legs, as described in the "Explanatory Notes" chapters and supplemented by the technical guides for shipboard organic geochemistry (Emeis and Kvenvolden, 1986; Kvenvolden and McDonald, 1986; Pimmel and Claypool, 2001).
Concentrations of light hydrocarbon gases methane (C1), ethane (C2), and propane (C3) were monitored for safety and pollution prevention. The C1/C2 ratio obtained is particularly important for indicating potential petroleum occurrences; sediments rich in organic carbon (Corg) commonly have a ratio of >1000, whereas values <200 may indicate potential petroleum generation related to increasing depth and temperature (cf. Stein et al., 1995).
Sampling of headspace gases in each core followed the standard procedure described by Kvenvolden and McDonald (1986). Immediately after core retrieval on deck, an ~5-cm3 sediment sample was collected using a borer tool, placed in a 21.5-cm3 glass serum vial, and sealed on deck or immediately in the lab with a septum and metal crimp cap. For consolidated or lithified samples, chips of material were placed in the vial and sealed. Prior to 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 and analyzed by gas chromatography (GC).
Gas HC constituents were analyzed using a HP5890 II gas chromatograph equipped with a sample loop, an 8 ft x 1/8 in stainless-steel column packed with HayeSep R, and a flame ionization detector (FID). Helium was used as a carrier gas. HP Chemstation software was used for data acquisition and processing. Chromatographic responses were calibrated using commercial standards (Scotty II Analyzed Gases, Scott Specialty Gas Co.) and the results reported in parts per million by volume (ppmv [µL/L]).
Headspace sampling is an important but rather inconsistent procedure for shipboard analysis of hydrocarbon gases. Sample size and nature can vary, depending on the condition of the core. This can range from soft, organic-rich clay, when the borer tool can be employed, to hard carbonate-rich sediment, from which sample pieces must be cut. Higher C1, C2, and C3 values per gram of sediment were generally obtained from coherent cylindrical samples taken by the cork borer than from discrete pieces of lithified sediment. This discrepancy may also reflect the inherent capacity of the sediment lithology to retain hydrocarbons, but it appears not to affect the C1/C2 ratio. It is clearly important to employ consistent sample-collection procedures for comparable downcore headspace hydrocarbon analysis. Additional gas may be released by further heating if necessary for replicate analyses of the same headspace sample. Reheated samples generally yield 10%-50% lower C1-C3 absolute abundances, but the C1/C2 ratios appear to be unaffected. Therefore, this may be a viable method for verifying anomalous C1/C2 values of sediments.
The percentage of IC was determined using a Coulometrics 5011 CO2 coulometer equipped with a System 140 carbonate analyzer. A total of ~10-12 mg of freeze-dried, ground sediment was reacted with 2 N HCl to liberate CO2. The change in light transmittance monitored by a photodetection cell controlled the CO2 titration. The percentage of carbonate was calculated from the IC content using the following equation:
This method assumes that all of the CO2 evolved was derived from dissolution of calcium carbonate. No corrections were made for other carbonate minerals.
Total carbon (TC), nitrogen, and sulfur were determined using a Carlo Erba 1500 CNS analyzer, which combusts sediment samples in tin cups with an oxidant (V2O5) 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 gas chromatography and detection performed by a thermal conductivity detector (TCD). The H2 value is not useful because it represents hydrogen derived from OM as well as water bound to clay minerals.
The analytical procedure employed a new combustion column for each sample batch. An aliquot of 5-15 mg freeze-dried, crushed sediment with ~10 mg V2O5 oxidant was combusted at 1000°C in a stream of oxygen. All measurements were calibrated by comparison to pure sulfanilamide as standard. The amount of total organic carbon (TOC) was calculated as the difference between TC and IC (determined from coulometry):
In addition to the carbon analytical data, the C/N atomic ratio can be used to identify the source of the organic matter (fresh marine C/N: 6 to 8, degraded marine C/N: 8 to 20, and terrestrial C/N: >25). However, extensive diagenesis and burial can also increase the C/N ratio of marine OM to 15 or greater (Meyers, 1994), and/or the C/N ratio can be lowered by oxidation of organic carbon and sorption of ammonia on clay minerals (e.g., Müller, 1977).
The type of organic matter was characterized by programmed pyrolysis using a Delsi Rock-Eval II system. This method is based on a whole-rock pyrolysis technique designed to identify the type of OM and its maturity and to evaluate the petroleum potential of sediments (Espitalié et al., 1986), while also providing a measure of TOC.
The Rock-Eval system includes a temperature program that first releases volatile hydrocarbons (S1) at 300°C for 3 min. Hydrocarbons are then released via thermal cracking of kerogen (S2) as the temperature is increased to 550°C at 25°C/min. The S1 and S2 hydrocarbons are measured by FID and reported in milligrams per gram of dry sediment. The temperature at which the kerogen yields the maximum amount of HC (top of the S2 peak) provides the parameter Tmax, used to assess the maturity of the OM. Between 300° and 390°C of the programmed pyrolysis, CO2 released from the thermal degradation of organic matter (S3) is trapped and subsequently measured by a TCD and reported in milligrams per gram dry sediment. Rock-Eval parameters facilitate characterization of OM by allowing the following indices to be calculated: hydrogen index (HI = S2/TOC x 100), oxygen index (OI = S3/TOC x 100), and S2/S3 ratio. In general, high OI values (>100) are an indicator of terrestrial OM or of immature OM of all sources. The production index is defined as S1/(S1+S2). This value is usually <0.2 in immature rocks; values of 0.3 to 0.4 are typical for samples in the petroleum window (Tmax = 420-450°C). Values of >0.5 may indicate the proximity of migrated HC or trapped petroleum. Interpretation of Rock-Eval OI data is compromised for samples containing >10 wt% carbonate, and the values themselves are also unreliable for young and immature OM (<1 Ma or Tmax < 400°C). Samples with <0.5 wt% TOC may not give reliable results because of the small size of the S1, S2, and S3 signals.
Solvent-extractable organic constituents were examined to assess the characteristics of organic matter in Cretaceous black shales, especially the origin of the organic matter. The focus of these analyses were qualitative—to recognize the identity of the hydrocarbons and other components present and their relative abundance. No quantitative determination of the concentrations of individual constituents was made.
Extraction procedures were as follows: ~1-4 g of sediment were extracted ultrasonically using CH2Cl2 (8 mL) for 30 min. The extract was transferred to a vial and reduced to dryness under N2. It was transferred in hexane to a silica column to remove polar constituents and recover hydrocarbon and ketone fractions by successive elution with hexane (4 mL) and CH2Cl2 (4 mL). Each eluant was taken to near dryness under N2 and transferred using hexane (50-100 µL) to a vial (with small-volume insert) for analysis by gas chromatography-mass selective detector (GC-MSD). An extraction of 1 g of Colorado oil shale was used to test these procedures. The hexane eluate is predominantly aliphatic hydrocarbons and less-polar aromatic hydrocarbons (e.g., monoaromatics). The CH2Cl2 eluate contains aromatic hydrocarbons and ketones, including alkenones.
The GC-MSD is a Hewlett-Packard 6973 system consisting of a HP 6890 GC with an MSD and a HP 7683 automatic liquid sampler (ALS). The GC is equipped with an electronic program controlled (EPC) split-splitless injector and a HP capillary column (5% phenyl methyl siloxane; 30 m x 0.25 µm) programmed from 40° to 130°C at 20°C/min, then at 4°C/min to 320°C, and held isothermally at 320°C for 20 min. He is used as the carrier gas. The transfer line is set at 280°C, and the source of the MSD is set at 230°C. The MSD scanned from 27 to 500 m/z. HP MS Chemstation software was used for data acquisition and processing. The identity of individual hydrocarbons was determined from their mass spectral characteristics and GC retention times by comparison with the literature.