Shipboard organic geochemistry during Leg 183 included the following analyses: analyses of the light hydrocarbon gases in the sediment, determination of carbonate carbon concentrations, and elemental analyses of carbon, hydrogen, nitrogen and sulphur.
The primary focus of shipboard organic geochemical analyses is to provide, on the basis of observations and expertise, advice concerning the probable risks of an uncontrolled release of petroleum hydrocarbons (i.e., gas and/or oil). Imminent risks to the ship and/or the environment must be assessed, and the ODP operations manager and co-chief scientists must be advised in time so that drilling can be terminated and the hole plugged and abandoned before penetration of a hazardous depth interval.
As hydrocarbon generation is a natural and inevitable result of the maturation of buried organic matter, the objective from the safety standpoint is to distinguish possible hazardous accumulations of hydrocarbons against the background of the normal increase in hydrocarbon content with depth. The degree of organic matter maturation and its ability to generate hydrocarbons may be estimated from the content of C2+ hydrocarbons (C2, ethane; C2=, ethylene; C3, propane; C3=, propylene) relative to methane (C1). The C1/C2+ ratio changes from >1000 for immature to <100 for mature organic matter. Plots of log C1/C2 ratios vs. depth give downhole trends. Deviations from these trends toward a significantly higher contribution of heavier hydrocarbons justifies caution. As a general guideline, values <200 for C1/C2 and 2000 for C1/C3 justify caution.
During Leg 183, concentrations of the light hydrocarbon gases, C1 through C3, were monitored in each sediment core. Gases were extracted using the headspace technique for bulk sediments. Immediately after the core was retrieved and cut into 150-cm sections, a No. 6 cork borer was used to obtain a measurable volume of sediment from the top of one of the sections, usually midcore. This ~5-cm3 sediment sample was placed in a glass vial, sealed, and heated in an oven at 60ēC for 30 min before gas analysis. A gas-tight syringe and needle was then used to extract ~5.0 mL of headspace gas.
Headspace gas was analyzed using a Hewlett Packard 5890 Series II gas chromatograph (GC) equipped with a 2.44 m × 3.18 mm stainless steel column, packed with HaySep S (100/120) mesh, and a flame ionization detector. The headspace syringe was directly connected to the GC via a 1.0-cm3 sample loop. A Hewlett Packard GC ChemStation (Rev A.05.04) was used for data collection and evaluation. Calibration was performed using Scotty II and IV analyzed gases.
Carbonate carbon concentrations were determined using a Coulometrics CO2 coulometer. Sediments were generally analyzed at a frequency of one sample per core.
The coulometric cell is filled with a partially aqueous proprietary solution containing monoethanolamine (ME) and a colorimetric indicator. A platinum cathode (negative electrode) and a silver anode (positive electrode) are placed in the cell, and the cell assembly is positioned between a light source and a photodetector (measuring the intensity of radiant energy) in the coulometer.
The samples (~10 mg of freeze-dried, ground sediment) are acidified with 2N HCl in a heated vial to generate forms of inorganic carbon as carbon dioxide. Carbon dioxide free air carries the CO2 through a scrubbing system into the coulometer for detection. In the coulometer cell, the CO2 is quantitatively absorbed, reacting with ME to form a titratable acid that causes the color to fade. The photodetector monitors the change in the solution's color as percent transmittance. As the percent transmittance increases, the titration current is automatically activated to generate a base at a rate proportional to the percent transmittance. When the solution returns to its original color, the current stops.
The weight percentage of carbonate is calculated from the inorganic carbon (IC) content, with the assumption that all inorganic carbon is present as calcium carbonate (CaCO3):
CaCO3 = IC ˇ 8.332. (11)Calibration was performed using pure calcium carbonate as a standard.
Total carbon, hydrogen, nitrogen, and sulfur were determined using a Carlo Erba 1500 NCS Analyzer. Sediments were generally analyzed at a frequency of one sample per one or two cores and samples of basaltic basement that were analyzed by XRF were also analyzed by NCS (see "Igneous Petrology").
Approximately 5 mg of freeze-dried, ground sediment and vanadium pentoxide (V2O5) was combusted at 1000ēC in a stream of oxygen. Using helium as a carrier gas, the oxygen was removed and the combustion products were reduced. The reduced gases were separated by gas chromatography (a 2-m packed column) and quantified with a thermal conductivity detector. The EAGER 200 software program was used for data collection and analyses.
Sulfanilamide (16.27% N, 41.84% C, 18.62% S, and 4.86% H) was used as the primary standard and the National Institute of Standards and Technology's Estuarine Sediment (0.18% N, 1.79% C, and 0.96% S), which is more representative of marine particulate matter, as the secondary standard. Reagents for the elemental analyzer included the oxidation catalyst (tungstic anhydride granules), a desiccant (magnesium perchlorate), and reduced copper wire.
The organic carbon (OC) was then calculated from the total carbon (TC) found from the NCS Analyzer and the inorganic carbon (IC) found from the coulometer:
OC = TC - IC. (12)