The shipboard organic geochemistry program for Leg 178 included (1) real-time monitoring of volatile hydrocarbons as required by ODP safety regulations, (2) measurement of inorganic carbon content of the sediments, and (3) elemental analyses of total carbon, nitrogen, and sulfur.
For safety and pollution prevention, concentrations and distribution of light hydrocarbon gases, mainly methane (C1), ethane (C2), and propane (C3), were monitored upon retrieval of each core. The standard method used was headspace sampling as described by Kvenvolden and McDonald (1986). While the core was still on deck, a 5-cm3 sediment sample was collected using a calibrated cork borer. The sample was placed in a 21.5-cm3 glass serum vial that was sealed with a septum and metal crimp cap. When consolidated or lithified samples were found, chips of material were placed in the vial and sealed. Before gas analyses, the vial was heated to 60ºC for 30 min. A 5-cm3 volume of the headspace gas was extracted from each vial using a standard glass syringe. Vacutainer samples were collected from gas pockets by penetrating the liner using a syringe connected to a penetration tool. The constituents of the gas were analyzed using a Hewlett Packard (HP) 5890 II gas chromatograph equipped with a flame ionization detector (FID) and a stainless-steel column (2.4 m × 3.2 mm) packed with HayeSep S (80-100 mesh). When heavier hydrocarbons (C3+) were detected, the sample was analyzed using a natural gas analyzer (NGA). The NGA is used to quantify C1-C6 and also nitrogen, oxygen, and carbon dioxide. The NGA system is connected to an HP 5890 II gas chromatograph equipped with an FID, a thermal conductivity detector (TCD), and a DB-1 capillary column (60 m × 0.32 mm). Helium was used as the carrier gas, and HP Chemstation software was used for data acquisition and processing. Chromatographic response was calibrated against standards, and the result is reported as parts per million (ppm).
Inorganic carbon was determined using a Coulometrics 5011 Carbon Dioxide Coulometer equipped with a System 140 carbonate carbon analyzer. A 10- to 20-mg aliquot of freeze-dried ground sediment was weighed and reacted with 2 M HCl to liberate CO2. The CO2 was titrated electrochemically, and the change in light transmittance monitored by a photodetection cell. Calcium carbonate content (wt%) was calculated from the inorganic carbon content using the following equation and assuming that all the CO2 evolved was derived from dissolution of calcium carbonate:
Total carbon, nitrogen, and sulfur were analyzed using a Carlo Erba 1500 CNS Analyzer. A 10- to 20-mg aliquot of dried ground sediment was combusted at 1000ºC in a stream of oxygen. Nitrogen oxides were reduced to N2, and the mixture of N2, CO2, and SO2 gases was separated by gas chromatography and measured using a TCD. All measurements were calibrated against standards. The amount of total organic carbon (TOC) is calculated as the difference between total carbon (TC) and inorganic carbon (IC):
The type of organic matter in a selected group of samples was characterized by pyrolysis using a Delsi Rock-Eval II system. This method is based on a whole-rock pyrolysis technique designed to identify the type and maturity of organic matter and to detect the petroleum potential of sediments (Espitalié et al., 1986). The Rock-Eval system is controlled by a temperature program that first releases volatile hydrocarbons (S1) at 300ºC for 3 min. Hydrocarbons are then released via thermal cracking of kerogen (S2) as the temperature steadily increases to 550ºC at 25ºC/ min. 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 during the S2 program provides Tmax, a parameter used to assess the maturity of the organic matter. Between 300ºC and 390ºC of the stepped pyrolysis, CO2 released from the thermal degradation of organic matter (S3) is trapped and measured by a TCD in milligrams per gram dry sediment. Rock-Eval II parameters help to characterize organic matter through the following indices: hydrogen index (HI; S2/TOC x 100), oxygen index (OI; S3/TOC x 100), S2/S3 ratio, and production index (PI; S1/(S1+S2). Interpretation of Rock-Eval data is questionable for samples containing <0.5% TOC.
Freeze-dried sediment samples (~2 g) were extracted ultrasonically in hexane for 20 min to extract long-chain hydrocarbons. The sediment-solvent mixture was filtered through a 0.45-µm filter using a Millipore filter flask connected to a vacuum pump. Three replicate extracts were combined and evaporated under a stream of helium. The combined extract was redissolved in a few drops of hexane and analyzed using an HP 5890 II Plus gas chromatograph equipped with an HP Ultra 1 capillary column (50 m x 0.2 mm). The temperature program was 50ºC (1 min), 10ºC/min to 200ºC, 4ºC/min to 320ºC (20 min); the injector temperature was 290ºC, and the detector temperature was 300ºC. Chromatograms were evaluated on the basis of alkane standards available on board.