The shipboard organic geochemistry program for Leg 205 included (1) real-time monitoring of volatile hydrocarbon gases; (2) measurement of the inorganic carbon concentration to determine the amount of carbonate in the sediments; (3) elemental analyses of total organic carbon, total sulfur, and total nitrogen; and (4) characterization of organic matter. All instruments and methods used during Leg 205 are described below. Additional details are summarized in Pimmel and Claypool (2001) as well as Emeis and Kvenvolden (1986). These analyses were performed as part of the routine shipboard safety requirements. The sediment samples needed were usually taken from the end of a core section close to the interstitial water sample, whenever possible.
During Leg 205, the composition and concentration of hydrocarbons and other gases in the sediments were analyzed, usually at intervals of one per core. Two different methods, headspace and vacutainer, were used.
In the headspace method, gases released from the sediments after core recovery were analyzed by gas chromatography (GC). Immediately after retrieval on deck, a calibrated cork borer was used to obtain a defined volume of sediment from the core. Usually an ~5-cm3 sediment sample was placed in a 20-cm3 glass vial that was sealed with a Teflon septum and a metal crimp cap. When consolidated or lithified sediments were encountered, chips of this material were placed in the glass vial and sealed. The vial was then heated for 30 min at a constant temperature of 60°C. A 5-cm3 volume of the headspace gas in the vial was extracted through the septum with a standard glass syringe for analysis by GC. The vacutainer method of gas collection was used where gas pockets or expansion voids were visible in the core when it was still in the core liner. A special piercing tool was used to penetrate the core liner; an attached 50-mL syringe equipped with a small three-way stopcock valve was employed to sample the gas.
Headspace and vacutainer gas samples were analyzed using the GC3, a Hewlett Packard 6890 gas chromatograph equipped with a stainless steel column, packed with HayeSep R porous polymer (80/100 mesh), and a flame ionization detector (FID). The GC3 measured the concentrations of methane, ethane, ethene, propane, and propene. Either the headspace syringe or the vacutainer was directly injected into the GC using a 0.25-cm3 sample loop. Helium was used as the carrier gas at a flow rate of 30 mL/min, increased to 80 mL/min after 3 min, and the GC oven was initially held at 100°C for 5.5 min and then heated up to 140°C. When higher concentrations of propane and higher hydrocarbons were suspected, gas samples were analyzed by the Natural Gas Analyzer (NGA), which measures hydrocarbons from methane to decane as well as N2, O2, CO2, H2S, and CS2. The NGA is a Hewlett Packard 6890 flame ionization multivalve, multicolumn gas chromatograph equipped with both a thermal conductivity detector (TCD) and an FID. Four columns were used sequentially to provide rapid partitioning and measurement of the gases. The sample was injected onto the entire set of columns using three sample loops. Helium was used as the carrier gas.
The data from both methods were collected and evaluated with a Hewlett Packard Chemstation data acquisition and analysis system. Chromatographic responses were calibrated against preanalyzed standards. On both GCs, the detection limit for hydrocarbons was 1-3 ppmv and the relative measurement accuracy was 3%-8%.
Three 2-cm3 sediment samples were routinely taken from each core for analysis of total carbon (TC), carbonate carbon (IC), total organic carbon (TOC), total sulfur (TS), and total nitrogen (TN).
Carbonate carbon concentrations of the samples were measured using a Coulometrics 5030 carbonate-carbon analyzer. An ~10-mg sample of freeze-dried, ground material was acidified with 2-N HCl in a glass tube. The liberated CO2 was carried through a scrubber to remove SO2 and then into the coulometer cell, where it was absorbed and reacted with monoethanolamine, a colorimetric indicator, causing the blue color to fade. The color change in the solution was measured by a photodetector cell. A program using Labview software read the titration results and provided the concentrations for IC and CaCO3 (in weight percent). The measurement accuracy is ~1%, and the detection limit is ~0.5 wt%.
TC, TS, and TN were determined using a Carlo Erba NA 1500 CHNS analyzer. About 5 mg of freeze-dried, ground sediment or rock was mixed with V2O5 and combusted in a reactor at 1000°C in an oxygen atmosphere. The combustion products SO2, CO2, and NO2 are carried by helium gas through a column packed with WO3 and Cu at 1000°C to reduce NO2 to N2. The gases were then separated by a GC (Poropak Q/S 50/80 mesh) and quantified with a TCD. TOC contents were calculated as the difference between TC and IC. For carbon, the measurement precision is ~0.06 wt% (accuracy is ~2%) and the detection limit is ~0.3 wt%, whereas nitrogen and sulfur measurements show low accuracy of ~20%, and for both, the detection limits are ~0.02 wt%.
The type of organic matter was evaluated in a selected set of samples (based on the C1/C2 hydrocarbon ratio) by pyrolysis using a Delsi Nermag Rock-Eval II Plus TOC system. The method is based on a whole-rock pyrolysis technique that is designed to identify the type and maturity of organic matter and to detect the petroleum potential of the sediments (Tissot and Welte, 1984; Espitalié et al., 1986). The Rock-Eval system involves a temperature program that first releases volatile hydrocarbons (S1) at 300°C for 3 min. Hydrocarbons are then released by thermal cracking of kerogen (S2) as the temperature is increased from 300° to 550°C at 25°C/min. S1 and S2 hydrocarbons are measured by an FID. The temperature at which the kerogen releases the maximum amount of S2 hydrocarbons provides Tmax, a parameter used to assess the maturity of organic matter. Between 300° and 390°C of the stepped pyrolysis, CO2 that was released from the thermal cracking of organic matter (S3) is trapped and measured by a TCD. For the characterization of organic matter, the following parameters can be calculated: