ORGANIC GEOCHEMISTRY

The shipboard organic geochemistry program for Leg 184 included (1) real-time monitoring of volatile hydrocarbons (HC) 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 and Geofina pyrolysis; and (5) measurement of total chlorin concentration by high-performance liquid chromatography (HPLC) employing fluorescence detection. All methods and instruments used during Leg 184 are described in detail by Emeis and Kvenvolden (1986), Kvenvolden and McDonald (1986), Harris and Maxwell (1995), and Shipboard Scientific Party, 1995.

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; also see "Headspace Sampling"). Immediately after core retrieval on deck, a sediment sample of ~5 cm³ 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. 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 discovered in the core liner, 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 an HP5890 II gas chromatograph equipped with a 1-cm³ sample loop, 8 × 1/8 stainless-steel column packed with HayeSep 5, and a flame ionization detector (FID). The C₁/C₂ ratio obtained is particularly important for indicating potential petroleum occurrences; organic-rich sediments commonly have a ratio of >1000, whereas values <200 may indicate potential petroleum generation related to depth and temperature (cf. Stein et al., 1995).

When heavier HC (C₃₊) were detected, the sample was also analyzed by a natural gas analyzer (NGA) as were selected samples from the southern basin site (Site 1143). The NGA is used to quantify C₁ to C₆ and also nitrogen, oxygen, carbon dioxide, and H₂S (>40 ppmv). Helium was used as a carrier gas. Data acquisition and processing were performed by an HP Chemstation. Chromatographic response was calibrated using commercial standards (Scotty II Analyzed Gases, Scott Specialty Gas Co.) and the results reported in parts per million, by volume (ppmv).

Headspace sampling is an important, but not thoroughly consistent, procedure for shipboard analysis. 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 may be employed, to hard carbonate-rich sediment, from which sample pieces must be cut. Mud generated by the drill bit should be avoided. Coherent cylindrical samples taken by the cork borer generally yielded higher C₁, C₂, and C₃ values per gram of sediment than did discrete pieces of harder sediment. However, this may also reflect the inherent lithologic capacity of the sediment to retain hydrocarbons. 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 C_1-3 absolute abundances, but the C₁/C₂ ratios appear unaffected. This may, therefore, be a viable method for verifying anomalous C₁/C₂ values of sediments (e.g., see "Organic Geochemistry" in the "Site 1148" chapter).

Inorganic carbon was determined using a Coulometrics 5011 carbon dioxide coulometer equipped with a System 140 carbonate analyzer. A total of ~10-12 mg of freeze-dried ground sediment was reacted with 2M HCl to liberate CO₂. The change in light transmittance monitored by a photo detection cell controlled the CO₂ titration. The percentage of carbonate was calculated from the inorganic carbon content, assuming that all the CO₂ evolved was derived from dissolution of carbonate by the following equation:

The amount of carbonate is expressed as weight percent, and no corrections were made for other carbonate minerals.

Total carbon (TC), nitrogen, and sulfur were determined using a Carlo Erba 1500 CNS analyzer, using 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. 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 from both OM and (clay) minerals. 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 ratio can be used to identify the source of the organic matter (fresh marine C/N 6 to 8, degraded marine 8 to 20, and terrestrial C/N >25). However, extensive diagenesis and burial can also increase the value of marine OM to 15 or greater (Meyers, 1994).

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 and degree of maturity of OM and to evaluate the petroleum potential of sediments (Espitalié et al., 1986) as well as 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 dry sediment. The temperature at which the kerogen yields the maximum amount of HC (top of the S₂ peak) provides the parameter T_max, 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 × 100), oxygen index (OI = S₃/TOC × 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 (T_max 420°-450°C). Values of >0.5 may indicate proximity of migrated HC or trapped petroleum. Interpretation of Rock-Eval OI data is considered to be compromised for samples containing >10% carbonate. Rock-Eval OI values are also unreliable for young and immature OM (<1 Ma or T_max <400°C). Samples with <0.5% TOC may not give reliable results because the S₁, S₂, S₃, and S₄ signals are very small.

Approximately 200 mg (dry weight) of homogenized freeze-dried sediment from each interval was extracted by triple solvent ice-cold sonication (15 min) and centrifugation (3000 rpm; 6 min). The extracts were evaporated to dryness under oxygen-free nitrogen. Volumetric solutions in methanol were prepared and analyzed using an HPLC system consisting of a Waters 501 solvent pump fitted with a Rheodyne 7725 injection valve (20-µL injection loop; flow rate 1 mL/ min) with a Waters 470 tunable scanning fluorescence detector (_ex = 407 nm; _em = 662 nm), eluting with methanol (Harris and Maxwell, 1995). The system was operated in the off-column mode, each sample injected twice, and the injection loop flushed with methanol between analyses. Data collection and processing was by an HP Chemstation data program.