The shipboard organic geochemistry program for Leg 188 included four routine sets of analyses: (1) headspace analysis for volatile hydrocarbons as required by ODP safety regulations; (2) measurement of inorganic carbon content of the sediments; (3) elemental analyses of total carbon, total nitrogen, and total sulfur content of sediment; and (4) characterization of organic matter by Rock-Eval pyrolysis. Procedures and instruments used during Leg 188 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 the Leg 150, 155, and 164 Initial Reports volumes (Shipboard Scientific Party, 1994, 1995, and 1996, respectively).
The composition of gases evolved from the sediment was determined at least once for every core. Visible void spaces in the core liner were sampled by syringe and analyzed for gas composition. The primary headspace sampling technique analyzed the gas given off by a known quantity of sediment after heating in an airtight vial.
Two gas chromatography (GC) systems were used for the gas analysis: a Hewlett-Packard 5890 Series II GC and a Hewlett-Packard 5890A natural gas analyzer. The Series II GC determines concentrations 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. For both systems, the chromatographic response was calibrated to standard gas mixtures.
Inorganic carbon (IC) content of sediment samples was determined by coulometry. Carbonate content of sediment (in weight percent) was calculated from IC content by assuming that all carbonate is present as calcium carbonate (CaCO3):
Total carbon, nitrogen, and sulfur contents of sediment samples were determined with a Carlo Erba NCS analyzer. Total organic carbon (TOC) content was calculated as the difference between total carbon (TC) and IC:
Type and quality of organic matter in sediment were evaluated by Rock-Eval pyrolysis. In this procedure, the volatile hydrocarbon content (in milligrams per gram) released by heating at 300°C for 3 min is called S1. The hydrocarbons produced by pyrolysis as the temperature is increased from 300° to 600°C at a heating rate of 25°C/min is called S2. CO2 (in milligrams per gram) generated between 300° and 390°C is called S3. The nominal temperature of the maximum rate of hydrocarbon yield during S2 analysis is Tmax. TOC is calculated from S1, S2, and S3 and from the oxidation of the remaining carbon in the sediment sample. The carbon-normalized hydrogen index (HI; in milligrams of hydrocarbon per gram of carbon) and the oxygen index (OI; in milligrams of carbon dioxide per gram of carbon) were calculated from the pyrolysis values: