Site 1247 is located northwest of the southern summit of Hydrate Ridge. The shipboard organic geochemistry program at Site 1247 included analyses of hydrocarbon gases, carbonate and organic carbon, and total sulfur and total nitrogen contents. 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 for every core available using the headspace technique. The results are reported in parts per million by volume (ppmv) in Table T4 and illustrated as ppmv vs. depth in Figure F16. Methane content in Hole 1247B varies from 4 to 25 ppmv in the uppermost 6.6 mbsf. It increases to 749 ppmv at 9.6 mbsf and to 12,599 ppmv at 11.1 mbsf. It 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 T4. Based on the methane concentration profile, the onset of methanogenesis occurs at a depth of ~11 mbsf in Hole 1247B (Fig. F17). Dissolved sulfate in pore water is also essentially consumed by 11 mbsf (see "Sulfate, Methane, and the Sulfate/Methane Interface" in "Interstitial Water Geochemistry"). No ethane is detected within the upper 10 mbsf. The concentration of ethane is very low in the samples from 10 to 110 mbsf (0-6.8 ppmv). It increases rapidly to 41.0 ppmv at 130.3 mbsf and to a maximum value of 615.7 ppmv at 165.0 mbsf. Propane is also abundant in this interval (130.3-165.0 mbsf), ranging from 62.3 to 735.7 ppmv. Enrichment of heavy hydrocarbon gases in this interval suggests migration of wet hydrocarbon gases (Fig. F16). Ethylene is sporadically present throughout the cored section (0.7-3.6 ppmv) (Table T4). The relative richness of ethylene between 165.0 and 174.6 mbsf suggests that the ethylene in this interval may also be related to the migration of wet gases.
High-resolution sampling and analysis of gas voids in cores was carried out to define the gas hydrate occurrence zone (GHOZ) based on gas composition, which is listed in Table T5 and plotted in Figure F18. The contents of methane in the voids from Hole 1247B are generally >900,000 ppmv (>90% by volume) (Fig. F18), except for the samples contaminated with air. The ethane concentration is uniform in the upper 40 mbsf. The first shift in the C1/C2 ratio, indicating ethane enrichment perhaps resulting from dissociated gas hydrate, is detected at 44 mbsf. An increase of ethane concentration in void gases is also apparent at depths of ~50 and 80 mbsf. Another slight anomaly is present at ~115 mbsf. Hydrocarbon gases from these depths are characterized not only by an enrichment of ethane but also by a depletion of propane (Table T5). Relative enrichment of ethane and depletion of propane may be explained by significant contribution of gas from dissociated Structure I gas hydrate during core recovery. These changes in gas composition show good correlation with IR temperature data (see "Infrared Scanner" in "Physical Properties"). We can define the top of the GHOZ at 44 mbsf, based on the composition of void gas data. The base of the GHSZ/GHOZ is usually marked by an order-of-magnitude increase in ethane (e.g., Site 1251) perhaps as a result of its release from dissociated gas hydrates. However, at Site 1247, it is very difficult to define the base of the GHSZ/GHOZ because the depth interval between 120 and 220 mbsf is characterized by an anomalous increase in the amount of thermogenic wet gas hydrocarbon gases (Fig. F18). The increase in ethane and propane concentrations in this interval is also apparent in the headspace gas analysis. The thickness of the anomalous zone for heavy hydrocarbon gases is greater at Site 1247 than at Site 1245, and C4+ hydrocarbons are also more abundant. Based on this abundance of C4+ hydrocarbon gases, Site 1247 appears to be closer to the source of migrating hydrocarbons than Site 1245. Horizon A or another permeable layer below Horizon A may act as a migration conduit to facilitate the transport of thermogenic hydrocarbons from greater depths.
Gas composition as expressed by the C1/C2 ratio of headspace and void gases is plotted vs. depth in Figure F19. The C1/C2 ratio for void gas samples shows a shift to lower values that indicates the effect of gas from decomposed gas hydrate. A decrease of the C1/C2 ratio presumably from migrated thermogenic ethane is also distinctive at depths ranging from 130 to 220 mbsf.
The C1/C2 ratio is plotted vs. estimated sediment temperature in Figure F20. The C1/C2 vs. temperature plot shows evidence for migrated hydrocarbons in the depth interval from ~130 to 220 mbsf, as indicated by ratios that are too low for the prevailing sediment temperature.
Gas hydrate pieces and gas hydrate-bearing sediments were recovered at Site 1247 in cores sampled on the catwalk. Two samples were analyzed to determine the gas composition associated with the dissociation of a gas hydrate (piece recovered from Section 204-1247B-12H-4) (Table T6). The gas from this gas hydrate sample shows enrichment of ethane and depletion of propane relative to what is believed to be the composition of dissolved gas in the core. The composition of the gas derived from the dissociated gas hydrate is consistent with a Structure I methane hydrate. On the C1/C2 plot (Fig. F19), the hydrate-bound gas falls on the trend of void gas samples also believed to be derived from gas hydrates and is enriched in ethane compared to "baseline" C1/C2 ratios of dissolved gas.
Two deployments of the PCS successfully retrieved full (1 m long) cores from depths of 23.1 and 123.8 mbsf. The composition of gas samples obtained during controlled PCS degassing experiments is listed in Table T7. Core 204-1247B-16P (123.8 mbsf) shows sufficient gas content to confirm the subsurface presence of methane hydrate (see "Downhole Tools and Pressure Coring"). Based on the volume-averaged composition, the C1/C2 ratio of this sample is similar to the C1/C2 ratio in void gases from adjacent depths (Fig. F19).
A total of 21 sediment samples were analyzed for carbonate carbon (IC), total carbon, organic carbon (OC), total nitrogen, and total sulfur. The results are listed in Table T8 and plotted in Figure F21. IC varies from 0.27 to 1.57 wt%, with the maximum value at 110.46 mbsf. When calculated as CaCO3, the IC content of the sediment varies from 2.24 to 13.07 wt% (Fig. F21).
OC content varies from 0.68 to 1.48 wt% and averages 1.09 wt% (Table T8; Fig. F21). The C/N ratio is <10, suggesting that marine organic matter is dominant. Nitrogen in the sediments ranges from 0.10 to 0.20 wt% (Table T8; Fig. F21). The nitrogen data show no apparent trends vs. either depth or OC content. The total sulfur contents vary from 0.20 to 1.07 wt% (Table T8).
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 the Ocean Drilling Program (ODP). There is no correlation between increased C2+ gas components and higher production index values and no definitive evidence for oil staining.