Site 1249 is located near the summit of Hydrate Ridge. The shipboard organic geochemistry program at Site 1249 consisted of analyses of hydrocarbon and nonhydrocarbon gases from headspace, voids, dissociated gas hydrates, and samples from the PCS degassing experiments. 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) in the cores were measured using the headspace technique. The results are reported in Table T6 and plotted as parts per million by volume (ppmv) of gas component vs. depth in Figure F16. Methane contents are very high near the seafloor, with as much as 31,000 ppmv at 0.6 mbsf. Headspace samples generally contain <100,000 ppmv of methane because of degassing during core retrieval. However, at Site 1249 several samples (Sections 204-1249B-2A-2, 204-1249C-3H-1, 3H-2, 4H-1, and 204-1249F-6X-1) contain extremely high amounts (220,000-560,000 ppmv) of methane. These large quantities of gas could only be caused by the presence of gas from dissociated hydrate in the headspace samples. The concentration of ethane is also very high in these samples, ranging from several hundred to 1426 ppmv. Ethylene is sporadically present at trace levels (0.4-0.7 ppmv). Propane is present in relatively high concentrations (tens of ppmv), even in the upper part of the analyzed section (Table T6; Fig. F16). Enrichment of ethane and propane at shallow depths probably reflects the presence of migrated thermogenic hydrocarbons. The high concentrations of methane in the sediments and the presence of gas hydrate at the seafloor are consistent with the observations of active venting of gas at this location.
The composition of gas samples from voids or expansion gaps in the core liner are listed in Table T7. The void gas (vacutainer [VAC]) samples are relatively pure methane, generally with minimal air contamination. Contents of methane in the voids are generally >900,000 ppmv (>90% by volume), unless diluted by air (Fig. F17). Ethane contents of core void gas samples decrease with depth from ~1,300-2,000 ppmv near the seafloor to 48 ppmv near the base of the cored sequence. Concentrations of propane in void gas samples are also relatively high, ranging from 5 to 79 ppmv. Iso- and normal butanes are present below 20 mbsf down to the base of the cored sequence, and their concentrations are also significant considering the shallow depth of the sediments (Table T7; Fig. F17).
These trends are evidence against a diffuse upward migration of thermogenic hydrocarbon gases through the cored section. Instead, the gas probably migrates along focused conduits to relatively permeable zones near the seafloor. The formation and decomposition of gas hydrate may also have some influence on the observed gas compositions.
Gas composition expressed as the C1/C2 ratio of headspace and void gas is plotted vs. depth in Figure F18. Several values of C1/C2 for headspace gas fall within the trend line of the void gas profile, probably because these samples contained gas hydrate. C1/C2 ratios for both headspace and void gas show a systematic decrease from the seafloor to 40 mbsf and then a gradual increase down to base of the cored section. The minimum in the C1/C2 trend at 30-40 mbsf suggests proximity to the permeable zone at this depth.
When C1/C2 ratios of the void gas are plotted vs. temperature (Fig. F19), the C1/C2 trend falls within the "anomalous" field from the seafloor down to 40-50 mbsf and shifts back to the normal range at greater depths.
Gas hydrates and gas hydrate-bearing sediments were recovered from cores on the catwalk. A total of 16 hydrate samples were decomposed in syringes, and the gas hydrate-bound gases were analyzed (Table T8). The concentration of methane varies from 169,544 to 983,641 ppmv as a result of air contamination during sampling. Four samples of gas hydrate-bound gas contain only methane and ethane, consistent with that gas being derived from Structure I gas hydrate. However, the other samples contain considerable amounts of propane and butanes, suggesting the possibility of Structure II gas hydrate. The C1/C2 ratio also reflects the enrichment of ethane in the gas hydrate-bound gas. Dissociated gas hydrate samples with relatively high C1/C2 ratios may reflect the influence of gas from surrounding sediments (Table T8; Fig. F18).
Three deployments of the PCS successfully retrieved full (1 m) cores from depths of 14.0-71.9 mbsf (Cores 204-1249C-6P and 204-1249F-4P and 14P). The composition of gas samples obtained during controlled PCS degassing experiments are listed in Table T9. All three PCS samples show pressure curves and gas contents that confirm the subsurface occurrence of methane hydrate (see "Downhole Tools and Pressure Coring"). Based on the volume-averaged composition, the C1/C2 ratios for gas from the PCS cores fall on the VAC/void gas trend (Fig. F19).