The shipboard organic geochemistry program for Leg 181 included (1) real-time monitoring of volatile hydrocarbons as required by ODP safety regulations, (2) measurement of inorganic (carbonate) content of the sediments, (3) elemental analyses of total nitrogen, sulfur, and carbon, and (4) characterization of organic matter by Rock-Eval pyrolysis (Espitalié et al., 1986). All methods followed and instruments used during Leg 181 are described in detail by Emeis and Kvenvolden (1986), Kvenvolden and McDonald (1986), and in the "Explanatory Notes" chapter of Leg 156 (Shipboard Scientific Party, 1995).

Volatile Hydrocarbons

For safety and pollution prevention, concentrations and distribution of light hydrocarbon gases, mainly methane (C1), ethane (C2+), and propane (C3), were monitored for each core following the standard headspace sampling method described by Kvenvolden and McDonald (1986). A 5-cm3 sediment sample was collected with a calibrated borer tool while the core was still on deck and placed in a 21.5-cm3 glass serum vial. The sample was sealed with a septum and metal crimp cap. When consolidated or lithified samples were encountered, chips of material were placed in the vial and sealed. Before gas analyses, the vial was heated to 60° C for 30 min. A 5-cm3 subsample of the headspace gas was then extracted from each vial using a standard glass syringe for gas chromatography (GC) analysis. When gas was encountered, vacutainer samples were collected by penetrating the core liner using a syringe connected to a penetration tool.

The constituents of the gas were analyzed with a HP5890 II gas chromatograph equipped with a flame ionization detector (FID). When heavier hydrocarbons (C3+) were suspected or detected, the sample was analyzed by a natural gas analyzer (NGA). The NGA is used to quantify C1 to C6 and also nitrogen, oxygen, and carbon dioxide. Helium was used as a carrier gas. Data acquisition and processing were performed by a HP Chemstation. Chromatographic response was calibrated against authentic standards and the results reported as parts per million (ppm).

Inorganic Carbon

Inorganic carbon was determined using a Coulometrics 5011 Carbon Dioxide Coulometer equipped with a System 140 carbonate carbon analyzer. A sample of 10-12 mg of freeze-dried ground sediment was weighed and then reacted with 2M HCl to liberate CO2. The CO2, which formed a titratable acid with a blue monoethanolamine indicator, was electrochemically titrated. The percentage of calcium carbonate was calculated from the inorganic carbon content, assuming that all the CO2 evolved was derived from dissolution of calcium carbonate by the following equation:

CaCO3 [wt%] = IC [wt%] (inorganic carbon) × 8.33 (1)

The amount of carbonate is expressed as weight percent (wt%), assuming all the carbonate was present as calcite. No corrections were made for other carbonate minerals.

Elemental Analysis

Total carbon, nitrogen, and sulfur were analyzed using a Carlo Erba 1500 CNS Analyzer. An aliquot of 12 to 15 mg of freeze-dried ground sediment with a V2O5 catalyst was combusted at 1000° C 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. Detection of elements was performed by a thermal conductivity detector (TCD). All measurements were calibrated by comparison to natural standards. The amount of total organic carbon (TOC) was calculated as the difference between total carbon (TC) and inorganic carbon (IC), i.e:

TOC [wt%] = TC [wt%] - IC [wt%] (2)

Organic Matter Characterization and Maturity Determination

The type of organic matter in a selected group of samples was characterized by pyrolysis using a Delsi Nermag Rock-Eval II system. This method is based on a whole-rock pyrolysis technique designed to identify the type and the degree of maturity of organic matter and to detect the petroleum potential of sediments (Espitalié et al., 1986). The Rock-Eval system includes a temperature program that first releases volatile hydrocarbons (S1) at 300° C for 3 min. Hydrocarbons are then released via thermal cracking of kerogen (S2) as the temperature is increased to 550° C at 25° C min-1. S1 and S2 hydrocarbons are measured by a flame ionization detector (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 the parameter Tmax, which is used to assess the maturity of the organic matter. Between 300° C and 390° C of the stepped pyrolysis, CO2 released from the thermal degradation of organic matter (S3) is trapped, measured by a TCD and reported in milligrams per gram dry sediment. Rock-Eval II parameters help in characterizing organic matter by allowing the following indices to be calculated: Hydrogen Index (HI; S2/TOC × 100), Oxygen Index (OI; S3/TOC × 100), and S2/S3 value. Interpretation of Rock-Eval data is considered to be compromised for samples containing less than 0.5% TOC.