Site 1246 is located on the eastern flank of Hydrate Ridge and was cored to sample the Horizon B/B´ reflector pair in a position above the BSR. These reflectors were originally thought to be carbonate layers along permeable horizons. The shipboard organic geochemistry program at Site 1246 included analyses of hydrocarbon gases, carbonate and organic carbon (OC), total sulfur (TS), and total nitrogen (TN) content. A description of the methods used for these analyses is summarized under "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 F12. Methane content increases rapidly from levels of 3 to 18 ppmv in the shallowest samples at 0-3 mbsf, to ~4,900 ppmv at a depth of 7.7 mbsf, and to levels of 10,000-60,000 ppmv at greater depths. This sharp increase in headspace methane occurs at the approximate depth of the SMI, as seen at Sites 1244, 1251, and elsewhere. The vial headspace ppmv C1 concentration was recalculated to express the millimolar methane concentrations remaining in the cores (see "Organic Geochemistry" in the "Explanatory Notes" chapter). These estimates of absolute gas concentration are meaningful only in shallow zones where sediments are undersaturated or slightly supersaturated with respect to dissolved methane at surface conditions. The calculated dissolved CH4 concentration is shown in Figure F13, along with the sulfate depletion profile (see "Sulfate, Methane, and the Sulfate/Methane Interface" in "Interstitial Water Geochemistry"). Although the sample spacing at Site 1246 is not optimal for purposes of defining the depth of the SMI, the general pattern is consistent with this horizon being at a depth of ~8 mbsf.
C2 content is maintained at low levels (1-2 ppmv or below detection) in headspace analyses of cores from Site 1246 in the depth interval from the seafloor to ~105 mbsf. C2= is present at trace levels (0.3-2 ppmv) throughout the depth interval cored, whereas C3 traces are present in headspace gas only for samples below depths of 112 mbsf (Table T4; Fig. F12).
The composition of gas samples from voids or expansion gaps in the core liner are listed in Table T5 and plotted in Figure F14. The void gas ("vacutainer") samples are relatively pure methane, generally with minimal air contamination. The methane content in the core voids from Hole 1246B is generally >925,000 ppmv (>92.5% by volume). The ethane content of void gas shows a more stepwise increase pattern with depth at Site 1246. Ethane increases from 9 to 37 ppmv in the interval from 35 to 102.2 mbsf. In the next sample at 110.2 mbsf and deeper, the ethane content increases abruptly to a range of 350-640 ppmv. This increase in the relative ethane content is also apparent in the headspace gas analyses over the depth interval from 102.7 to 107.2 mbsf. Propane is generally absent from void gases within the depth interval where gas hydrates are stable. One sample at 24.7 mbsf has 7 ppmv of propane, but this sample is probably above the depth where gas hydrate is present (Table T5; Fig. F14). The absence of propane in void gas samples over the depth interval 36-110 mbsf is consistent with the gas being derived from decomposition of Structure I methane hydrate, which can contain ethane but excludes propane. Although the increase in ethane content beneath the GHSZ could be partly a result of the release of ethane from decomposed gas hydrate, the increase in propane and isobutane is more likely the result of migrated hydrocarbons.
Gas composition expressed as C1/C2 of headspace and void gas is plotted vs. depth in Figure F15. The C1/C2 ratios for both headspace and void gas do not show any systematic decrease other than the marked offset discussed above, which is a result of an increase in ethane content in sediments beneath the gas hydrate occurrence zone. This change in the gas composition occurs at a core depth of ~105 mbsf and correlates with the deepest chloride anomaly (104.87 mbsf) at Site 1246 (see "Chloride Concentration and the Presence of Gas Hydrate," in "Interstitial Water Geochemistry"). The base of the GHSZ, as inferred from gas and pore water composition, is shallower than the depth of the BSR calculated from seismic data (114 mbsf). The discrepancy is even greater for depth of the base GHSZ (149 mbsf) estimated from the measured temperature gradient (0.049°C/m) and the methane-seawater pressure-temperature equilibrium.
A single gas hydrate sample from Section 204-1246C-12H-4 (at 105 mbsf) was analyzed for hydrocarbon composition, with the results given in Table T6. The gas from the analyzed sample was ~92% methane, with 5% air contamination. Interestingly, the gas from this hydrate sample contained 8.3 ppmv of propane, a component that should be excluded from Structure I hydrate, along with 17.1 ppmv of ethane. It is not known if this trace of propane was actually present in the gas hydrate or if it was present in some of the sediment associated with the gas hydrate. The C1/C2 ratio of the gas from the gas hydrate is about the same as in void gas samples from this depth at Site 1246 (Fig. F15).
A total of 16 sediment samples (one per core) were analyzed for carbonate carbon (IC), total carbon (TC), OC (by difference), TN, and TS. The results are listed in Table T7. Carbonate carbon content (Fig. F16) varies from 0.14 to 0.81 wt% at Site 1246. When calculated as CaCO3, the carbonate contents measured in the Hole 1246B sediments vary from 1.14 to 6.75 wt% (Fig. F16). The analyzed samples near the proposed depths of the Horizon B/B´ reflectors do not appear to be enriched in calcium carbonate.
OC content varies from 0.37 to 1.68 wt% (average = 1.20 wt%) (Table T7; Fig. F16). The highest OC content measured is 1.68 wt% at a depth of 185.95 mbsf. Although nitrogen contents are low (0.06-0.20 wt%), the C/N ratio is generally <10, which is consistent with a dominantly marine source of organic matter. Sediment samples have TS contents ranging from 0.11 to 1.25 wt% (Table T7), with no distinct relation to OC content.
The results of Rock-Eval pyrolysis of selected samples are given in Table T8. 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.