Site 1251 is located in a slope basin in a contrasting environment to sites on or near Hydrate Ridge. The shipboard organic geochemistry program at Site 1251 included analyses of hydrocarbon gases, carbonate and organic carbon (OC), as well as total sulfur and total nitrogen content. 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) 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 component vs. depth in Figure F18. Methane content increases rapidly from levels of 48-178 ppmv in the shallowest samples at 0-3 mbsf to ~40,000-60,000 ppmv at ~6 mbsf and below. In addition to the ppmv concentrations of hydrocarbons in the vial headspace, the C1 values have been recalculated to express the millimoles of methane remaining in the cores per liter of pore water. These estimates of absolute gas concentration are meaningful only in shallow zones where sediments are undersaturated or slightly supersaturated with respect to dissolved methane. The calculated dissolved CH4 concentration is shown in Figure F19, along with the sulfate depletion profile illustrating the depth of the SMI. Sample depths in the upper part of Hole 1251B have been increased by an empirically determined constant amount (3.7 m) only for purposes of plotting in Figure F19. The methane content vs. depth curves for the upper part of Holes 1251B and 1251C coincide after the depths in Hole 1251B are adjusted downward. Apparently, the first core in Hole 1251B did not recover the SMI.
Ethane content is low (1-2 ppmv or below detection levels) in headspace analyses of cores from Site 1251 in the depth interval from the seafloor to 195 mbsf. Ethylene is sporadically present at trace levels (0.3-2 ppmv) throughout the depth interval cored, whereas propane traces are present in headspace gas only for samples below depths of 313 mbsf (Table T5; Fig. F18).
The compositions of gas samples from voids or expansion gaps in the core liner are listed in Table T6 and plotted in Figure F20. The void gas (vacutainer [VAC]) samples are relatively pure methane, generally with minimal air contamination. Contents of methane in the voids from Hole 1251B are generally >900,000 ppmv (>90% by volume), unless diluted by air. Ethane content of void gas shows a gradual increase with depth, from 3 ppmv near the seafloor to a range of 7-33 ppmv in the interval from 85 to 185 mbsf just above the BSR. Beneath the BSR, the ethane content increases abruptly to a range of 200-300 ppmv. This order-of-magnitude increase in the relative ethane content just below the BSR is also apparent in the headspace gas analyses. These trends may be related to the formation and decomposition of gas hydrate, but the exact mechanism producing this relative ethane enrichment is still being investigated. Propane contents range from 3 to 26 ppmv, with no apparent relationship to depth or the base of GHSZ (Table T6; Fig. F20).
Gas composition expressed as the C1/C2 ratio of headspace and void gas is plotted vs. depth in Figure F21. The C1/C2 ratios for both headspace and void gas do not show any systematic decrease, other than the marked offset discussed above due to an increase in ethane content in sediments beneath the BSR.
No gas hydrate pieces or gas hydrate-bearing sediments were physically recovered from cores on the catwalk although several Cl- and IR anomalies were detected (see "Physical Properties" and "Interstitial Water Geochemistry"). Eight deployments of the PCS successfully retrieved full (1 m) cores from depths of 20-291 mbsf. Two of these (Cores 204-1251D-6P and 204-1251B-12P) contained sufficient gas to confirm a subsurface presence of methane hydrate (see "Downhole Tools and Pressure Coring"). The compositions of gas samples obtained during controlled PCS degassing experiments are listed in Table T7. The C1/C2 ratios for gas from all but one of the PCS cores (based on volume-averaged composition) fall on the vacutainer/void gas trend (Fig. F20), confirming that gases exsolved in the core liner are a valid representation of subsurface hydrocarbon composition. Gas from PCS Core 204-1251D-10P at 76.4 mbsf is almost totally lacking in ethane and appears to be from sediment that is undersaturated with respect to methane hydrate. Gas from this sample may reflect ethane depletion in the pore water, which is a possible consequence of the ethane enrichment (theoretically) occurring in the gas hydrates.
A total of 39 sediment samples (one per core except for special tool cores) were analyzed for carbonate carbon (IC), total carbon (TC), OC (by difference), total nitrogen (TN), and total sulfur (TS). The results are listed in Table T8. IC, plotted against depth of burial in Figure F22, varies from 0.04 to 2.19 wt%. Concentrations of IC are relatively high at 255.78, 284.68, and 333.78 mbsf. When calculated as CaCO3, the IC in Hole 1251B sediments varies from 0.3 to 18.27 wt% (Fig. F22). Samples from depths of 255.8, 284.7, and 333.8 mbsf contain >15 wt% CaCO3. The upper two CaCO3-enriched samples are from a depth interval in Hole 1251B that contains up to 50% biogenic carbonate (see "Lithostratigraphy"). The lower sample is probably authigenic carbonate. The sediments below 350 mbsf generally contain relatively low amounts of carbonate carbon.
OC content varies from 0.58 to 3.06 wt% (average = 1.37 wt%) (Table T8; Fig. F22). The analyzed sample with the highest OC content is at a depth of 352.48 mbsf, where the C/N ratio approaches a value of 10, possibly resulting from the input of terrestrial organic matter. The C/N ratios of the other samples are generally <10, suggesting that input of marine organic matter was dominant during deposition. An exception is the sample at 304 mbsf, which also is relatively enriched in OC at 1.87 wt%. For the most part, C/N ratio varies according to the content of OC in the sediment because the concentration of TN is relatively uniform throughout the section. Total nitrogen in the sediments varies consistently between 0.11 and 0.22 wt% (Table T8; Fig. F22), with no apparent trends with either depth or OC content.
Sediment samples have total sulfur contents ranging from 0.17 to 1.77 wt% (Table T8), with the amount of sulfur roughly proportional to OC content. A plot of sulfur vs. carbon (not shown) for Site 1251 sediments has a linear relationship with a slope of 0.5. This type of plot has traditionally been used to estimate the fraction of deposited OC oxidized during sulfate reduction (Goldhaber and Kaplan, 1974; p. 601). Typical marine sediments have sulfur to OC ratios of 0.36, which suggest that ~20% of the deposited OC was oxidized during sulfate reduction. The ratio of 0.5 for Site 1251 sediments would suggest that >40% of the OC was oxidized during sulfate reduction (i.e., there is excess sulfur present for the amount of preserved OC). However, the assumptions underlying the use of the sulfur/organic carbon ratio do not include the possibility of sulfate reduction coupled with AMO. Probably the apparent excess of sulfur in Site 1251 sediments is due to the fact that a significant component of the sulfur is derived from sulfate reduced in connection with methane oxidation.
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.