Site 1245 is located on the western flank of Hydrate Ridge. The shipboard organic geochemistry program at Site 1245 included analyses of hydrocarbon gases, carbonate and organic carbon, and total sulfur and nitrogen content. Descriptions of the methods used for these analyses are summarized in "Organic Geochemistry" in the "Explanatory Notes" chapter.
Concentrations of methane (C1), ethane (C2), ethylene (C2=), and propane (C3) were measured using the headspace technique. Sediments were sampled in every section of the first core and with decreasing frequency in subsequent cores, to a minimum of one sample per available core. The results are reported in parts per million by volume (ppmv) in Table T7 and plotted against depth in Figure F29. Methane content in Hole 1245B varies from 11 to 42 ppmv in the upper 6 mbsf. It rapidly increases to 1,321 ppmv at 7.5 mbsf and 9,964 ppmv at 8.5 mbsf and remains in the range of 10,000-60,000 ppmv to the base of the cored section. In addition to the relative concentration of methane in the headspace vial, the C1 values are expressed in millimoles per liter (mM) of pore water in Table T7 and illustrated in Figure F30. Based on the methane concentration profile, the onset of methanogenesis occurs at a depth of ~8 mbsf in Holes 1245B and 1245D. Dissolved sulfate is also essentially depleted in the pore water in the uppermost 8 mbsf (see "Sulfate, Methane, and the Sulfate/Methane Interface" in "Interstitial Water Geochemistry").
Small amounts of ethane (0.5-18.2 ppmv) are detected in the headspace vials above 130 mbsf. Ethane contents increase between 130 and 200 mbsf, ranging from 70 to a maximum value of 557 ppmv at 142 mbsf. Propane is also abundant in this interval, ranging from 76 to 430 ppmv. Enrichment of ethane and propane gases in this interval probably reflects the presence of migrated thermogenic hydrocarbons (Fig. F29). Ethylene is present sporadically throughout the cored section (0.5-1.4 ppmv) (Table T7) and is probably related to biological activity in the shallow sediments.
The compositions of light hydrocarbon and nonhydrocarbon gases from voids or expansion gaps in the core liner are shown in Table T8 and illustrated in Figure F31. Methane contents in the voids sampled from Hole 1245B are generally >900,000 ppmv (>90% by volume) (Fig. F31), except for a few samples contaminated with air. The depth interval between 130 and 200 mbsf is characterized by a thousand-fold increase in the amount of C2-C5 hydrocarbons present (Fig. F31). A similar trend is also apparent in the headspace gas analysis. The presence of these thermogenic hydrocarbons between depths of 130 and 200 mbsf is due to the cores intersecting a zone of migrated thermogenic hydrocarbons, presumably originating from deeper in the accretionary complex. Horizon A or other permeable layers may act as a migration conduit for these hydrocarbons. Beneath the zone of maximum wet gas hydrocarbon enrichment at ~150 mbsf, the level of C2-C5 hydrocarbons in the cores gradually decreases and reaches "normal" background levels below depths of ~300 mbsf.
Gas composition as expressed by the C1/C2 ratio of headspace and void gas is plotted vs. depth in Figure F32. The C1/C2 ratios for either sample type show little systematic change above the BSR. The C1/C2 ratios of the void gas samples show a shift to lower values at 50-80 mbsf, indicating a relative enrichment in ethane of the void gas. This shift is possibly related to gas hydrate decomposition during core recovery. Below the depth where the buildup of methane enables gas hydrates to form, the void gas samples contain, mainly, gas released from gas hydrate dissociation during core recovery. The decrease of the C1/C2 ratio resulting from the presence of migrated thermogenic wet gases is distinctive at depths between 130 and 200 mbsf. An anomalous decrease of the C1/C2 ratio is also distinctive between the depths that correlate with the seismic features identified as the BSR and Horizon A.
The C1/C2 ratio is plotted vs. estimated sediment temperature in Figure F33. The C1/C2 plots show evidence for migrated hydrocarbons in the depth interval from ~130 to 300 mbsf (temperatures of 12°-17°C), which results in ratios that are lower than expected for the prevailing sediment temperature. Sediment temperatures were estimated from a seafloor temperature of 4.5°C and a geothermal gradient of 52°C/km (see "Downhole Tools and Pressure Coring").
Gas hydrate pieces and gas hydrate-bearing sediments were recovered from cores on the catwalk and analyzed. Four gas samples from decomposed pieces of gas hydrate were analyzed from Samples 204-1245B-6H-5, 103 cm; 9H-CC; 204-1245C-7H-5, 40 cm; and 13H-4 56-74 cm (Table T9). The concentration of methane varies from 227,000 to 945,000 ppmv (22.7%-94.5% by volume) as a result of air contamination during sampling. The C1/C2 ratios of the gas from the dissociated gas hydrate from Samples 204-1245B-9H-CC and 204-1245C-13H-4, 56-76 cm, show enrichment of ethane in the gas hydrate relative to the adjacent void gas samples (Fig. F32). However, ethane was not enriched in gas from Sample 204-1245B-6H-5, 103 cm, and was below detection levels in the gas hydrate sample from Sample 204-1245C-7H-5, 40 cm.
Five deployments of the pressure core sampler (PCS) successfully retrieved full (1 m long) cores from depths of 17.0-291.2 mbsf. The composition of gas samples obtained during controlled PCS degassing experiments are listed in Table T10. One PCS sample from Core 204-1245C-16P provided a pressure curve and gas content that confirms the subsurface presence of methane hydrate (see "Downhole Tools and Pressure Coring"). Based on the volume-averaged composition, the C1/C2 ratios for gas in the PCS cores fall within the trend defined by vacutainer (VAC) void gas samples (Fig. F32), except for the sample recovered from 17.0 mbsf.
A total of 27 sediment samples (one per core except for special tool cores) were analyzed for carbonate carbon (IC), total carbon (TC), OC, total nitrogen (TN), and total sulfur (TS). The results are listed in Table T11 and plotted in Figure F34. IC content varies from 0.14 to 2.19 wt%. Concentration of IC is relatively high at 520.68 mbsf, where biogenic calcareous material is present in high amounts (see "Lithostratigraphy"). When calculated as CaCO3, the carbonate contents of the sediments vary from 1.19 to 25.48 wt% (Fig. F34).
OC content varies from 0.71 to 1.46 wt% (average = 1.11 wt%) (Table T11; Fig. F34). The C/N ratio is <10, suggesting that marine organic matter is dominant in the sediment. C/N ratios are mainly controlled by variation in the OC content of the sediments, with nitrogen being relatively uniform throughout the section at levels of 0.11-0.22 wt% (Table T11; Fig. F34). The nitrogen data show no apparent trends vs. either depth or OC content. The TS contents vary from 0.12 to 1.23 wt% (Table T11), with a consistent relationship between TS and OC content (Fig. F34).
The results of Rock-Eval pyrolysis of selected samples are given in Table T12. 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.