Site 1248 is located within a prominent "bright spot" on deep-towed side-scan sonar seafloor reflectivity and a wipe-out zone on 3-D seismic data near the crest of southern Hydrate Ridge (see "Introduction"). The shipboard organic geochemistry program for Holes 1248B and 1248C included analyses of hydrocarbon gases, carbonate carbon (IC), organic carbon (OC), total sulfur (TS) and total nitrogen (TN) content, and Rock-Eval characterization. A description of the methods used 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 T5 and plotted as parts per million by volume (ppmv) of gas components vs. depth in Figure F16. Methane content is anomalously high near the seafloor, reaching 29,000-38,000 ppmv in the shallowest samples at 0.05-1.5 mbsf. The concentration of ethane is very high (46-63 ppmv) in the shallowest samples, decreases between ~40 and 100 mbsf, and increases to several hundreds of ppmv at depths below ~100 mbsf. Ethylene is present sporadically at trace levels (0.4-0.9 ppmv) throughout the depth interval cored. Propane is present in relatively high concentrations (tens of ppmv) in the upper section (0-120 mbsf) and is even more abundant (hundreds of ppmv) in headspace gas below 120 mbsf (Table T5; Fig. F16). The anomalously high concentrations of hydrocarbon gases near the seafloor indicate that Site 1248 is located in a highly advective fluid-flow setting. This interpretation is further supported by the presence of gas voids and gas hydrate in shallow sediments and pore water characteristics such as the absence of dissolved sulfate (see "Carbon Cycling" in "Interstitial Water Geochemistry").
Gas voids in cores are found as shallow as 1 mbsf. The composition of gas samples from voids or expansion gaps in the core liner are listed in Table T6. The void gas (vacutainer) 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). In the upper ~100 mbsf, ethane contents of void gas show a gradual decrease with depth, from ~1300-1700 ppmv near the seafloor to ~140 ppmv in the interval around 90 mbsf. Beneath 90-100 mbsf, the ethane contents increase abruptly to a range of 13,000-17,000 ppmv (Fig. F18). This order-of-magnitude increase in the relative ethane content below ~100 mbsf is also apparent in the headspace gas analyses. These trends are probably related to the migration of thermogenic hydrocarbons along relatively permeable zones near Horizon A (~128-132 mbsf) and may also be influenced by the formation and decomposition of gas hydrate (Fig. F17). The exact mechanism producing such relative ethane enrichment is still being investigated.
Propane contents in void gas range from 10.5 to 100.9 ppmv in the upper 100 mbsf and abruptly increase to >16,000 ppmv at ~135 mbsf. Isobutane and normal butane are present throughout the interval cored, and their concentrations also significantly increase below ~100 mbsf (Table T6; Fig. F17). Moreover, pentanes and hexanes, which are not present in the upper section, are present in the interval around ~135 mbsf (Table T6). The presence of C5+ hydrocarbons indicates the migration of thermogenic gases from greater depth. The prominent seismic reflection, Horizon A, which is composed of sand and silt (including volcanic ash components), may serve as a major migration pathway that facilitates fluid flow from the deeper subsurface into the shallow sediment.
Gas compositions expressed as the C1/C2 ratio of headspace and void gas are plotted vs. depth in Figure F19. The C1/C2 ratios for both headspace and void gas show a systematic increase from the seafloor to ~100 mbsf and then decrease to ~130 mbsf. This anomalous trend may reflect the mixing of gases from three hydrocarbon pools: (1) ethane-enriched mixed microbial and thermogenic gas near the seafloor, (2) ethane-depleted microbial gas at ~100 mbsf, and (3) ethane-enriched mixed microbial and thermogenic gas apparently migrating near Horizon A at ~130 mbsf. The observed C1/C2 gradient is inconsistent with predominantly vertical migration of ethane-enriched hydrocarbon gases to the seafloor through the cored interval. Instead, our interpretation suggests that the near-seafloor ethane-enriched gases may have migrated from the area of most active gas flux along shallow lateral stratigraphic or structural pathways. Alternatively, the observed trend of the C1/C2 ratios may reflect the processes of gas hydrate crystallization in the subsurface.
When C1/C2 ratios of the void gas are plotted vs. temperature (Fig. F20), the C1/C2 trend falls within the "anomalous" field, both near the seafloor and at depths greater than ~100 m. This suggests the possibility that migrated thermogenic hydrocarbons, probably associated with overpressured oil and gas reservoirs, may be present below the cored interval (Pimmel and Claypool, 2001). The C1/C2 ratio of void gases was found to be as low as 70 and that of headspace gases as low as 29 (i.e., well beyond the suggested limit of the anomalous field). Based on hydrocarbon monitoring data, Hole 1248C was abandoned at 149.0 mbsf to minimize the possible risk of blowouts and/or petroleum spills.
Gas hydrate veins, nodules, and gas hydrate-cemented sediments were recovered from cores on the catwalk. At Site 1248, 11 samples of gas hydrates were decomposed and the gas hydrate-bound gases were analyzed (Table T7). Most gases are relatively pure methane, with minimal air contamination. Two samples from the shallowest cores contain relatively high concentration of H2S (25 and 11,812 ppmv). However, these samples also contained a significant amount of surrounding sediment, which may have caused the presence of H2S. Similarly, propane and butanes measured in most gas hydrate samples are probably present in associated sediments.
Two clean (minimal contamination from sediment) pieces of gas hydrate were decomposed for geochemical analysis (Table T7). One sample contains only methane and ethane, suggesting that gas was derived from Structure I gas hydrate. The other sample contains high concentrations of propane (C3 > C2) and butanes, suggesting the possibility of Structure II gas hydrate. Although thermogenic Structure II gas hydrates are common in petroleum basins such as the Gulf of Mexico (Sassen et al., 2001) and the Caspian Sea (Ginsburg and Soloviev, 1998), only Structure I methane hydrates were found at Hydrate Ridge prior to ODP Leg 204 (Gutt et al., 1999).
The C1/C2 ratios for gas hydrate-bound gas show the same trend as void gases (Fig. F19). However, most gas hydrate samples are slightly enriched in ethane relative to void gases from adjacent depths.
A total of 13 sediment samples were analyzed for IC, total carbon (TC), OC (by difference), TN, and TS. The results are reported in Table T8. IC content, plotted against depth of burial in Figure F21, varies from 0.15 to 2.46 wt% (average = 0.66 wt%). The greatest concentration of IC at 1.16 mbsf correlates well with the carbonate-rich patch described in "Lithostratigraphy." The relatively high concentration of IC at the base of the cored section is related to the presence of calcareous fossils. When calculated as CaCO3, the carbonate contents in the sediments vary from 1.13 to 20.49 wt% (Fig. F21).
OC content of sampled intervals (Table T8; Fig. F21) ranges from 0.58 to 1.62 wt% (average = 1.10 wt%). The analyzed sample with the highest OC content is at 90.86 mbsf, where the concentration of IC is lowest in the section. The C/N ratios are <10. Based on the C/N ratios, OC in the sediments is predominantly of marine origin.
TN in the sediments varies consistently between 0.11 and 0.23 wt%. (Table T8; Fig. F21). The nitrogen data show no apparent trends vs. either depth or OC content. The sediment samples have TS contents ranging from 0.17 to 0.82 wt% (Table T8; Fig. F21) and show a consistent relationship between sulfur and OC.
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.