Core sections destined for gas analysis were treated using a procedure modified from Kvenvolden and Redden (1980). A 6.6-cm-diameter, 5-cm-long section of whole-round core was cut from the recovered core as it lay on the catwalk core cradle within 5 to 10 min of core retrieval. The section was extruded into a 500-mL metal can equipped with two septa. The can containing the sample was filled with degassed water to the rim, and then 100 mL of water was removed. About 2 to 3 g of sodium azide was added as a bactericide. The sample with water was sealed in the can, leaving a 100-mL headspace. The headspace was purged with helium at a rate greater than 500 mL/min for a period of about 5 min; some cans were purged for longer periods of up to 10 min. The samples were frozen in the upside-down position and shipped to our shore-based laboratory for hydrocarbon gas analyses.
In the laboratory, the frozen samples were allowed to thaw until they reached ~20ºC. They were then placed into a high-speed shaker and shaken for 5 min. The hydrocarbon gases were partitioned into the headspace and analyzed. With a syringe, about 10 mL of gas was removed from the headspace for analysis. A similar volume of helium was injected to replace the volume of gas removed.
A Shimadzu GC-14A gas chromatograph equipped with a Chemipack C-18, 6 ft × 1/8 in 80/100 mesh stainless steel column was used to measure hydrocarbon gases from C1 to C7. The GC-14A is configured with a 1-mL, valve-actuated, sample loop for injection and a flame ionization detector (FID) for gas detection. Samples were introduced by syringe at atmospheric pressure, and a minimum of 10 mL of gas was used to flush the injection loop. Run conditions were 35ºC for 1.5 min, increasing at a rate of 20º/min to 150ºC and held at this temperature. Helium was used as the carrier gas at a constant flow rate of 3 kg/cm2. FID temperature was held at 150ºC. The gas chromatograph was calibrated using hydrocarbon gas standards of known concentrations.
Results are reported relative to the volume of wet sediment from which the gases were extracted (i.e., microliters of gas per liter of wet sediment [µL/L]). These units are unconventional, but they are convenient for expressing gas concentrations that result from this procedure and for comparison with previously reported results, for example, Kvenvolden et al. (1990).