ORGANIC GEOCHEMISTRY

The shipboard organic geochemistry program during Leg 207 included

  1. Core-by-core monitoring of volatile hydrocarbons 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 bulk organic matter by Rock-Eval pyrolysis; and
  5. Studies of solvent-extractable organic matter components.

The laboratory procedures and equipment employed during Leg 207 were adapted from those described in the "Explanatory Notes" chapters of recent ODP legs, with supplements from the technical guides for shipboard organic geochemistry (Emeis and Kvenvolden, 1986; Kvenvolden and McDonald, 1986; Pimmel and Claypool, 2001).

Interstitial Gas Analyses

Concentrations of the light hydrocarbon gases methane, ethane, propane, and propene were monitored for safety and pollution prevention. The C1/C2 ratio is particularly useful for indicating possibly overpressured conditions; biogenic gases commonly have ratios >1000, whereas values <200 may indicate potential petroleum generation related to increasing depth and temperature (cf. Stein et al., 1995).

Our scientific objectives during Leg 207 included exploration of possible microbial activity in several sequences. It was predicted that organic-rich black shales in the OAEs would support elevated populations of prokaryotes, as has been demonstrated in Pleistocene sapropels (Coolen et al., 2002). A special analytical scheme was carried out on board the JOIDES Resolution to characterize interstitial gases originating from microbial activity.

Routine Headspace Gas Analysis of Volatile Hydrocarbons

Sampling of headspace gases in each core followed the standard procedure described by Kvenvolden and McDonald (1986). Immediately after core retrieval on deck, a ~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 laboratory 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 70C for 30 min. A 5-cm3 subsample of the headspace gas was extracted from each vial using a glass gas syringe and analyzed by gas chromatography.

Volatile hydrocarbons were analyzed using a Hewlett-Packard (HP) 6890 gas chromatograph (GC) equipped with sample loop, 2.4 m x 3.2 mm (8 ft x 1/8 in) stainless steel column packed with HaySep R porous polymer packing (80/100 mesh), and flame ionization detector (FID). The GC oven was programmed at 100C for 5 min, then to increase to 140C at 50C/min, and finally to remain at 140C for another 4 min. Helium was used as a carrier gas. For data acquisition and processing, HP Chemstation software was used. 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 [L/L]).

Headspace Gas Analyses of Microbial Volatile Hydrocarbons

Duplicate 4-cm3 samples of sediment were collected using a cutoff 5-cm3 syringe or borer tool and placed in a 21.5-cm3 headspace vial immediately after core retrieval on deck. The samples were taken immediately to the microbiology laboratory where ~5 mL of 1-N NaOH was added. The vials were then sealed with Teflon/silicone septa and metal crimp caps. The two samples were shaken gently for ~1 hr. One sample (A) was reserved for shore-based control studies. Headspace gas from vial B was analyzed ~36 hr after recovery to determine interstitial volatile hydrocarbons (C1–C4). A 4-mL aliquot of headspace gas was removed with a glass gas-tight valve syringe, compressed to 2.5 cm3, and the pressure in the syringe barrel was reduced to atmospheric by briefly opening the valve. The chromatographic conditions used were as described in "Routine Headspace Gas Analysis of Volatile Hydrocarbons".

The remaining headspace content of vial B was then transferred to a clean, evacuated headspace gas vial containing ~5 mL of water using a double-ended needle. This sample was reserved for future isotopic analysis. Headspace gas contents of vial B will be measured again after ~2 months to measure any additional desorbed volatile hydrocarbons.

Natural Gas Analyses

Headspace samples of interstitial gases were also analyzed with the natural gas analyzer (NGA) when high concentrations of C2+ hydrocarbons or nonhydrocarbon gases such as H2S or CO2 were anticipated. The NGA system consists of a HP 6890 GC equipped with multiport valves that access different column and detector combinations. Both FID and thermal conductivity detectors (TCD) are employed; helium was used as a carrier gas. Detectable gases are N2, O2, CO2, H2S, CS2, and hydrocarbons in the range of C1–C10, which are detected by applying a system of four columns for sequential partitioning. First, a 60 m x 0.32 mm capillary column coated with a 1-m film of DB-1 (J&W, Inc.) and the FID were employed for separation and detection of C1–C7 hydrocarbons isothermally at 50C (15 min). The multicolumn system also contains a 1.8-m (6 ft) stainless steel column packed with HaySep R porous polymer (80/100 mesh) (acid washed). This column was held at 80C, and used for the separation of air and methane from O2, N2, CO2, C2, C3, ethylene, and propylene as well as H2S, if present in high concentrations. Then, a nonheated combination of a 15.2-cm (6 in) stainless steel column packed with Poropak T (50/80 mesh) in line with a 0.9-m (3 ft) column packed with molecular sieve 13x (60/80 mesh) was used for O2, N2, and C1 analysis. All nonhydrocarbon gases were analyzed isothermally using the sequence of packed columns, as stated above, and the TCD. For data acquisition and evaluation, an HP ChemStation computer system was used. Chromatographic responses are calibrated against preanalyzed standards; gas contents are reported in parts per million by volume (ppmv).

Carbonate Analysis

The weight percentage of inorganic carbon was determined by titration using a Coulometrics 5011 CO2 analyzer equipped with a System 140 carbonate analyzer. A 20-mg freeze-dried ground sediment sample was reacted with 2-N HCl to liberate CO2. The gas is cleaned of SO2 and transferred into a vial filled with a blue-colored aqueous proprietary solution using CO2-free air as a carrier gas. An acidic reaction of the CO2 gas with this solution induces a color change. A base is added stepwise until the now-clear liquid returns to its original tint. The CO2 titration process is controlled by a photodetection cell, which constantly monitors the change in light transmittance of the solution. The volume of basic titrant is proportional to the inorganic carbon content of the sample. The percentage of carbonate is calculated from the inorganic carbon using the following equation:

CaCO3 (wt%) = IC (wt%) x 8.33.

This method assumes that all of the CO2 evolved was derived from dissolution of calcium carbonate. No corrections were made for other carbonate minerals.

Elemental Analysis

Total carbon, nitrogen, and sulfur were determined using an autosampler-equipped Carlo Erba NA 1500 CHNS analyzer, which combusts sediment samples in tin cups with an oxidant at 1000C in a stream of oxygen. The analytical procedure employs a sample of 5- to 15-mg freeze-dried, crushed sediment with ~10 mg V2O5 acting as an oxidation catalyst. Helium was used as a carrier gas. Nitrogen oxides were reduced to N2, and the mixture of N2, CO2, H2O, and SO2 gases was separated by gas chromatography. Detection of individual components was performed with a TCD. The hydrogen value is not useful because it represents both hydrogen derived from organic matter and that produced from water bound to clay minerals. All measurements were calibrated by comparison to a pure sulfanilamide standard. The amount of total organic carbon (TOC) was calculated as the difference between total carbon (TC) and inorganic carbon (determined from coulometry),

TOC (wt%) = TC (wt%) – IC (wt%).

In addition to the TOC concentration, elemental analysis yields the C/N atomic ratio, which can be used to help identify sources of organic matter (fresh marine C/N = 6–8; degraded marine C/N = 8–20; continental C/N = >20).

Organic Matter Characterization and Maturity Determination

The type of organic matter was characterized by programmed pyrolysis using a Delsi Nermag Rock-Eval II system. This method is based on a whole-rock pyrolysis technique designed to identify the type of organic matter and its maturity and to evaluate the petroleum potential of sediments (Espitali et al., 1986). Although the Rock-Eval II system can provide an independent measure of TOC, the TOC module did not function during Leg 207 and TOC values determined as described in "Elemental Analysis" were used instead.

The Rock-Eval system includes a temperature program that first releases volatile hydrocarbons from organic matter by heating at 300C for 3 min (S1). Hydrocarbons are then released via thermal cracking of kerogen (S2) as the temperature is increased to 550C at 25C/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 hydrocarbons (top of the S2 peak) provides the parameter Tmax, which is used to assess the thermal maturity of the organic matter. Between 300 and 390C of the programmed pyrolysis, CO2 released from the thermal degradation of organic matter (S3) is trapped and subsequently measured by TCD and reported in milligrams per gram dry sediment. Rock-Eval analysis provides the calculated parameters hydrogen index (HI), oxygen index (OI), and the S2/S3 ratio:

HI = S2/TOC x 100 and
OI = S3/TOC x 100.

In general, high hydrogen index values (>400) indicate large proportions of well-preserved algal and microbial organic matter, whereas high oxygen index values (>100) are an indicator of continental organic matter or of immature organic matter of all sources.

The production index (PI) is defined as

PI = S1/(S1 + S2).

This value is usually <0.2 in immature rocks; values of 0.3–0.4 are typical for samples in the petroleum window (Tmax = 420–450C). Values of >0.5 may indicate the proximity of migrated hydrocarbons or trapped petroleum.

Interpretation of Rock-Eval oxygen index data may be compromised for samples containing >10 wt% carbonate, and the values themselves are also unreliable for young and immature organic matter (<1 Ma or Tmax = < 400C) (Peters, 1986). Furthermore, samples with <0.5 wt% TOC do not give reliable results because of the small size of the S1, S2, and S3 signals.

Solvent Extractable Components: Higher Hydrocarbons and Alkenones

Solvent-extractable organic constituents were examined to assess the biomarker contents of organic matter in Cretaceous black shales. The focus of these analyses was to determine the relative compositions of the hydrocarbons and related components in selected sediment sequences. Measurement of absolute concentrations of individual constituents was not attempted. These data are reported in Forster et al., this volume.

The extraction procedure consisted of ultrasonic extraction of 1–4 g of sediment for 30 min using 8 mL of CH2Cl2. The extract was transferred to a vial and reduced to dryness under N2. It was transferred in hexane to a silica column and successively eluted with 4 mL hexane (aliphatics), 4 mL 1:1 hexane:dichloromethane (aromatics), and 4 mL dichloromethane (polar hydrocarbons). The hexane eluate is predominantly aliphatic hydrocarbons and monoaromatic hydrocarbons. The CH2Cl2 eluate contains aromatic hydrocarbons and ketones, including alkenones. Each eluant was taken to near dryness under N2 and transferred using 50–100 L of hexane to a vial for analysis by gas chromatography–mass selective detector (GC-MSD). Extraction of 1 g of Colorado oil shale was used to verify the extraction and separation procedures.

The GC-MSD consists of an HP 6890 GC with an HP-5973 MSD and an HP 7683 automatic liquid sampler. The GC is equipped with an electronically program-controlled split-splitless injector and an HP capillary column (5% phenyl methyl siloxane; 30 m x 0.25 m) programmed from 70 to 130C at 20C/min, then at 4C/min to 320C, and held at 320C for 20 min. Helium was used as the carrier gas. The transfer line was set at 280C and the source of the MSD at 230C. The MSD scanned from 27 to 800 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 literature values.

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