The shipboard organic geochemistry program for Leg 189 included three routine sets of analyses. First, headspace (and vacutainer where appropriate) analysis for volatile hydrocarbons (and other gases) were performed as required by ODP safety regulations. Second, elemental analyses of total carbon, total nitrogen, and total sulfur content of sediment samples (and calculation of total organic carbon) were determined. Third, organic matter type and maturity was characterized using C/N ratios and Rock-Eval pyrolysis. Most of the procedures and instruments used during Leg 189 are described by Emeis and Kvenvolden (1986) and generally are the same as those used during most recent ODP legs. Brief comments on routine sampling and deviations from standard practice are noted below; more detailed notes are presented in the "Explanatory Notes" chapters of ODP Initial Reports Volumes 150, 156, 164, and 181 (Shipboard Scientific Party, 1994, 1995, 1996, 1998).
Sediment gas composition was determined at least once from every core. The primary sampling technique was headspace sampling. For the headspace method, a cork borer was used to obtain a measured volume of sediment from the top of one section of each core immediately after on-deck retrieval. The sediment, with a typical volume of ~5 cm3, 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 encountered, rock chips were placed in the vial and sealed. Before gas analysis, the vial was heated at 60°C for 30 min. A 5-cm3 volume of the headspace gas was extracted from each vial using a standard glass syringe. Gaseous voids visible through the core liner were sampled using a syringe and vacutainer.
Two gas chromatography (GC) systems were used for gas analysis: a Hewlett-Packard 5890 Series II GC and a Hewlett-Packard 5890A natural gas analyzer. The Series II GC determines concentration of C1 (methane), C2 (ethane), and C3 (propane) hydrocarbons with a flame ionization detector (FID). The natural gas analyzer measures concentrations of C1 through C7 hydrocarbons with an FID as well as N2, O2, and CO2 with a thermal conductivity detector (TCD). For both systems, the chromatographic response was calibrated to standard gas mixtures.
Total carbon, nitrogen, and sulfur contents of sediment samples were determined with a Carlo Erba Model NA1500 CNS analyzer. Approximately 6 mg of freeze-dried, ground sediment was combusted in oxygen at 1000°C. In this process, helium acts as a carrier gas, the oxygen is removed, and the combustion gases are reduced and separated by GC and quantified with a TCD. Total organic carbon (TOC) content was calculated as the difference between total carbon (TC) and the inorganic (carbonate) carbon (IC) value generated by carbonate coulometry (i.e., TOC = TC - IC).
The origin of sedimentary organic matter can be characterized using organic carbon/nitrogen (C/N) values generated by the CNS analyzer. The average C/N ratio of marine zoo- and phytoplankton is between 5 and 8, whereas terrestrially derived organic matter has ratios between 25 and 35 (e.g., Emerson and Hedges, 1988; Meyers, 1994).
Sedimentary organic matter type and quality were further evaluated by Rock-Eval pyrolysis of hydrocarbons (Espitalié et al., 1986). In this procedure, volatile hydrocarbon content (in milligrams per gram of sample) released by heating at 300°C for 3 min is measured by the FID and labeled as an S1 peak on the Rock-Eval pyrogram. Hydrocarbon quantity (in milligrams of hydrocarbon per gram of sediment) produced by pyrolysis as the temperature is increased from 300° to 600°C at a heating rate of 25°C/min is also measured by the FID and is called the S2 peak. The nominal temperature at which the maximum rate of hydrocarbon yield is attained during S2 analysis is the Tmax value, which roughly equates to thermal maturity. CO2 (in milligrams per gram), generated between 300° and 390°C, is measured using a TCD and is called the S3 peak. TOC is calculated from S1, S2, and S3 and from the oxidation of the remaining carbon in the sediment sample measured by a second TCD. The carbon-normalized hydrogen index (HI) (in milligrams of hydrocarbon per gram of carbon) and oxygen index (OI) (in milligrams of carbon dioxide per gram of carbon) are calculated from pyrolysis values by HI = (100 × S2)/TOC, and OI = (100 × S3)TOC.