The shipboard organic geochemistry program for Leg 204 included four routine sets of analyses: (1) analysis for volatile hydrocarbons in cores as required by ODP safety regulations; (2) measurement of carbonate carbon (IC) content of the sediments; (3) elemental analyses of total carbon (TC), total nitrogen (TN), and total sulfur (TS) content of sediment; and (4) characterization of organic matter by Rock-Eval pyrolysis. Procedures and instruments used during Leg 204 are described by Pimmel and Claypool (2001) and are generally the same as those used during most recent ODP legs. Brief comments on routine sampling and deviations from standard practice are noted below.
For pollution prevention and safety, composition of gases in sediment typically was determined at least once every core. Two sampling methods were employed: headspace (HS) sampling and syringe or vacutainer (VAC) sampling. Headspace gas was collected after heating a nominal 5-cm3 plug of sediment at 70°C for 30 min in an airtight vial. VAC gas samples were collected from gas pockets, bubbles, or voids in the core by piercing the core liner and allowing gas to expand into a 60-cm3 syringe connected to the penetration tool.
Gas obtained by either method was analyzed by one of two gas chromatography systems: a Hewlett Packard 5890 Series II (GC3) or a Hewlett-Packard 5890A natural gas analyzer (NGA). The first system determines concentrations of C1-C3 hydrocarbons with a flame ionization detector (FID); the latter system measures concentrations of C1-C7 hydrocarbons with an FID as well as N2, O2, CO2, and H2S with a thermal conductivity detector (TCD). For both systems, chromatographic response was calibrated to six different gas standards, with variable quantities of low molecular weight hydrocarbons, N2, O2, CO2, H2S, Ar, and He.
In both the HS and VAC analyses, the gas composition is expressed as component parts per million by volume (ppmv) relative to the analyzed gas. To the extent that sampling procedures are uniform, the differences in the HS results reflect differences in the amount of gas remaining in the cores. Major variation in the C1 ppmv results for the VAC samples reflects dilution of the gas void samples with air. The HS results were recalculated to adjust for sample size variability and to express the result on a more meaningful concentration basis, at least for lower concentration ranges. HS samples were weighed, and net sample weights were calculated by subtracting an average vial weight (15.6 g). Wet bulk density of the corresponding depth in the core was used to convert sample weight to sample volume. Porosity at the depth of the sample was used to estimate the volume of water in the sample. Accordingly, the methane concentration can be expressed as millimoles per liter of pore water,
where density and fractional porosity are taken from average depth trends presented in the "Physical Properties" section of each site chapter.
IC content of sediment samples was determined by coulometry. Carbonate content of sediment (in weight percent) was calculated from IC by assuming all carbonate is present as calcium carbonate:
TC, TN, and TS contents of sediment samples were determined with a Carlo Erba NCS analyzer. TOC content was calculated as the difference between TC and IC:
The type and quantity of organic matter in sediment were evaluated by Rock-Eval pyrolysis. In this procedure, volatile hydrocarbon content (in milligrams per gram) released at 300°C for 3 min is S1. The hydrocarbon quantity (in milligrams per gram) released as the temperature is increased from 300° to 600°C at 2°C/min is S2. The nominal temperature of the maximum rate of hydrocarbon yield during S2 analysis is defined as Tmax. TOC content was independently measured, and the carbon-normalized hydrogen index (HI), in milligrams of hydrocarbon per gram of carbon (mg HC/g C), was calculated from the pyrolysis assay as