Site 1250 is located near the top of Hydrate Ridge and was cored to sample carbonate rocks near the Pinnacle. The shipboard organic geochemistry program at Site 1250 included analyses of hydrocarbon gases, carbonate and organic carbon, and total sulfur and total nitrogen content. A description of the methods used for these analyses is summarized in "Organic Geochemistry" in the "Explanatory Notes" chapter.
The levels of methane (C1), ethane (C2), ethylene (C2=), and propane (C3) remaining in cores were measured using the headspace technique. The results are reported in Table T4 and plotted as parts per million by volume (ppmv) of gas component vs. depth in Figure F19. Methane content is high (>12,000 ppmv of headspace) in the shallowest cores and remains relatively constant and high throughout the cored interval down to 140 mbsf. Gas in cores was not analyzed by the headspace technique for the deeper samples in Hole 1250F. At Site 1250, there is no shallow zone with low or zero methane content, suggesting that the sulfate/methane interface is at or very near the seafloor. This is confirmed by dissolved sulfate in the pore water, which is <1 mM in the first core just beneath the seafloor consistent with prior observations (see "Carbon Cycling" in "Interstitial Water Geochemistry") (Torres et al., 2002). The vial headspace ppmv C1 concentration was recalculated to express the millimolar methane concentrations remaining in the cores. Calculated dissolved C1 concentration is shown in Table T4 and, except for a few samples, is close to saturation levels for methane dissolved in seawater at surface conditions.
Headspace ethane (C2) content is 10 ppmv near the seafloor, decreases to 1 ppmv at a depth of ~65 mbsf, and is maintained at low levels (1-3 ppmv) in the depth interval from 65 to ~105 mbsf. Ethylene (C2=) is present at trace levels (0.5 ppmv) in only one sample above and in higher amounts (4-8 ppmv) below the BSR. Propane is present in headspace gas samples above but not below the BSR (Table T4; Fig. F19). Headspace gas analysis was not conducted in cores deeper than 140 mbsf (Hole 1250F), in order to concentrate efforts on analysis of void gas samples as discussed below.
The compositions of gas samples from voids or expansion gaps in the core liner are listed in Table T5 and plotted on Figure F20. The void gas (vacutainer [VAC]) samples are relatively pure methane, with variable air contamination. The methane content in the core voids from Hole 1250B is frequently >950,000 ppmv (>95% by volume). The ethane content of void gas samples is high (700-800 ppmv) near the seafloor, then decreases to levels of 20-40 ppmv in the depth interval 58-63 mbsf. Void gas samples in the shallow zone near the seafloor also contain C3-C5 hydrocarbons, and gas hydrate is present near the seafloor. There is apparent hydrocarbon enrichment in the sediments near the seafloor but not at depths of 50-60 mbsf, suggesting lateral migration of wet gas hydrocarbons that have come up from depth along some focused conduit. Ethane content is maintained at low levels (127 ppmv or less) in Site 1250 sediments down to a depth of ~100 mbsf, where ethane begins to increase again and peaks at values of 1260 ppmv at depths of 115 mbsf. This is also about the estimated depth of the BSR. At other sites drilled during Leg 204, the ethane content in core void gas increases just beneath the base of methane hydrate stability, rather than above it. It appears that at Site 1250 the BSR coincides with a zone of enrichment in C2+ hydrocarbons. Below depths of 115 mbsf, the ethane content decreases and remains at 200-400 ppmv levels to a depth of 150 mbsf, where another zone of C2+ hydrocarbon enrichment is present in Horizon A.
Gas composition expressed as C1/C2 of headspace, and void gas is plotted vs. depth in Figure F21. The trends noted above for ethane and C2+ hydrocarbons are magnified in the C1/C2 ratio plot. Distinct features of Figure F21 are three zones of C2+ hydrocarbon enrichment at the seafloor, at the depth of the BSR (114 mbsf), and at the depth of Horizon A (150 mbsf). These zones of anomalous heavy hydrocarbon enrichment may represent permeable pathways for lateral migration, with composition of sediment gas between these zones representing the more typical background of dominantly microbial methane.
At Site 1250, samples from near the seafloor (1.4-6.5 mbsf) and from depths of 81.5, 86.7, and 100.2 mbsf were analyzed for the composition of gas from dissociated hydrate, with the results given in Table T6. The gas from the analyzed samples ranged from 55% to 98% methane, with the balance being mainly air contamination. Some of the shallowest gas hydrate samples gave off minor H2S (400-1600 ppmv), and all samples analyzed produced CO2 (240-5300 ppmv). Gas from several samples contained traces of propane and, in one case, some isobutane. Sample 204-1250D-10H-3, 53-84 cm, produced more propane than ethane. The presence of gas components other than methane and ethane has important implications for hydrate structure. No procedures were used on the ship to evaluate hydrate structure. Many of the samples contained intermixed sediment, and it is not known if trace components could have come from dissolved gas in the sediment rather than from the hydrate itself. The C1/C2 ratios of gas hydrate samples are in the same range as void gas samples from the same depths at Site 1250 (Fig. F21), although two of the three deeper samples appear somewhat depleted in ethane.
Five deployments of the PCS successfully retrieved cores from depths of 35-120 mbsf (Cores 204-1250C-9P; 204-1250D-5P, 13P, and 18P; and 204-1250F-4P). The composition of gas samples obtained during controlled PCS degassing experiments is listed in Table T7. Based on the volume-averaged composition, the C1/C2 ratios of gas from the PCS cores fall on the VAC/void gas trend (Fig. F21).
A total of 13 sediment samples were analyzed for carbonate carbon (IC), total carbon (TC), organic carbon (OC) (by difference), total nitrogen (TN), and total sulfur (TS). The results are listed in Table T8. IC (Fig. F22) varies from 0.12 to 0.65 wt% at Site 1250. When calculated as CaCO3, the carbonate contents measured in the Hole 1250B sediments vary from 1.0 to 5.4 wt% (Fig. F22). The sediments at Site 1250 generally contain relatively low amounts of IC.
OC content varies from 1.03 to 1.64 wt% (average = 1.32 wt%) (Table T8; Fig. F22). TN contents range from 0.15 to 0.21 wt%, and the C/N ratios are all <10, consistent with a dominantly marine source of organic matter. Sediment samples have total sulfur contents ranging from 0.14 to 0.67 wt% (Table T7), with no distinct relation to OC content.
The results of Rock-Eval pyrolysis of selected samples are given in Table T9. This analysis was performed in part to evaluate the possible presence of migrated liquid hydrocarbons. Although the production index values seem moderately elevated (i.e., >0.1), they are fairly typical for continental margin sediments cored by ODP. There is no correlation between increased C2+ gas components and higher production index values and no definitive evidence for oil staining.