Shipboard organic geochemical studies of cores from Holes 1244A-1244C and 1244E included monitoring of hydrocarbon gases, carbonate carbon (IC), organic carbon, total sulfur (TS) and total nitrogen (TN) content and Rock-Eval pyrolysis characterization of organic matter. Procedures are summarized in "Organic Geochemistry" in the "Explanatory Notes" chapter.
Hydrocarbon gas measurements using the headspace method are listed in Table T6. Results are reported in parts per million by volume (ppmv) of methane (C1), ethane (C2), ethylene (C2=), and propane (C3) in the air headspace of a 21-cm3 vial containing a nominal 5-cm3 sediment sample. Results are plotted as ppmv of hydrocarbon gas component vs. depth in Figure F21. Methane contents increase rapidly from just above background levels, 9 ppmv at 0.05 mbsf, to ~40,000 ppmv at 14.1 mbsf and generally remain in the range of 20,000-40,000 ppmv to the base of the cored section at 333 mbsf. Dissolved sulfate is essentially depleted in the pore fluids in the uppermost 7 mbsf (see "Sulfate, Methane, and the Sulfate/Methane Interface" in "Interstitial Water Geochemistry"), and this depth effectively divides the zones of methanotrophy and methanogenesis. Trace amounts of ethane and propane are present below 7 mbsf but are not consistently present in the gas until depths of 130 and 260 mbsf, respectively. The unsaturated compound ethylene is sporadically present throughout the cored section (Table T6).
The composition of gas from voids or expansion gaps in the core liner is shown in Table T7. The void gas (vacutainer [VAC]) samples are relatively pure methane, generally with minimal air contamination, as shown by the ppmv values that approach or, in two cases, exceed 100%. Void gas samples reflect the composition of gas in the subsurface but not the amount. The subsurface gas content is probably proportional to the general abundance and internal pressure of core voids, but it is difficult to quantify.
Gas composition, as expressed by the C1/C2 value of headspace gas, is plotted vs. depth in Figure F22 and vs. sediment temperature in Figure F23. The sediment temperature was estimated assuming the measured geothermal gradient of 61°C/km (see "Downhole Tools and Pressure Coring"). The trend of C1/C2 values shows a general exponential increase in ethane content relative to methane with increasing depth. The C1/C2 values for the void gas samples show some consistent offsets in the trend that may be related to presence of methane hydrate in the cores. Specifically, there are apparent step offsets to lower C1/C2 values at depths of ~40 and 130 mbsf, possibly indicating the top and base of the zone of gas hydrate presence, respectively.
When the C1/C2 ratio of the void gas is plotted vs. temperature (Fig. F23), the C1/C2 trend falls within the normal range associated with in situ low-temperature ethane generation but approaches the "anomalous" field that indicates possible migrated thermogenic hydrocarbons (Pimmel and Claypool, 2001). Headspace methane values above ~10,000 ppmv are residual concentrations, representing only the gas retained by sediments after outgassing has taken place during retrieval of the core to the surface (Kvenvolden and Lorenson, 2000; Paull and Ussler, 2001). Moreover, there is variability in the results of the headspace technique resulting from nonuniform sample size. Accordingly, the methane ppmv in air concentrations were converted to an equivalent methane concentration in IW by the procedure outlined in "Organic Geochemistry" in the "Explanatory Notes" chapter. Gas concentrations expressed in millimoles of methane per liter of water are shown in Table T8 and plotted in Figure F24. Also shown in Figure F24 is the dissolved sulfate concentration ("Sulfate, Methane, and the Sulfate/Methane Interface" in "Interstitial Water Geochemistry"). Below ~7 mbsf, the residual dissolved CH4 content in the IW increases at about the depth in which sulfate concentration approaches zero.
Gas hydrate pieces and gas hydrate-cemented sediments were physically recovered from Sections 204-1244C-8H-1 and 10H-2 and 204-1244E-7H-6 and 12H-1. Evidence from electrical logs (see "Downhole Logging") and dissolved chloride profiles (see "Major and Minor Element Distributions" in "Interstitial Water Geochemistry") indicates that gas hydrates are definitely present in the subsurface at depths below ~40 mbsf and are possibly present at depths below ~32 mbsf. Gas hydrate-bearing intervals exist down to the base of the GHSZ, which is at ~127 mbsf at Site 1244. The composition of gas given off by decomposed hydrate pieces (Table T9) shows enrichment in ethane relative to the void gas samples analyzed at comparable depth intervals. The gas hydrate pieces from Hole 1244C were analyzed after storage at -80°C for 3 or 4 days, but the gas hydrate pieces from Hole 1244E were analyzed immediately. The general trend of the C1/C2 ratio of the void gas (Fig. F22) is offset in the direction of ethane depletion over the general interval where gas hydrate is present. A similar trend was observed in void gas samples from gas hydrate-bearing sediments at Blake Ridge during ODP Leg 164. Whether or not these shifts in the C1/C2 ratio are valid indicators of the presence of gas hydrate will be evaluated in cores from other sites during Leg 204.
In addition to the gas samples obtained from the decomposition of gas hydrate pieces recovered in the core, the gas composition of samples taken during controlled degassing of the PCS (for details on the PCS, see "Pressure Core Sampler" in "Downhole Tools and Pressure Coring") was also analyzed. The results are listed in Table T10 and shown in Figure F22. The gas from PCS experiments is enriched in air but generally has the same C1/C2 composition as the void gas samples from adjacent cores.
A total of 34 sediment samples (one per core in Holes 1244A-1244C, except for PCS cores) were analyzed for IC, total carbon (TC), organic carbon (OC) (by difference), TN, and TS. The results are reported in Table T11. IC content (plotted against depth of burial in Fig. F25) is relatively low (0.04-0.75 wt%). When calculated as CaCO3, the IC from sediments in Hole 1244C varies from 0.33 to 6.25 wt% (Fig. F25), reflecting primary changes in biogenic and authigenic carbonate. The sediments below 250 mbsf contain relatively low amounts of IC, reflecting a major change to lithostratigraphic Unit III. (see "Lithostratigraphic Unit III" in "Lithostratigraphic Units" in "Lithostratigraphy").
OC content of sampled intervals (Table T11; Fig. F25) is relatively high in comparison to other marine sediments, ranging from 0.87 to 1.83 wt% (average = 1.27 wt%). The analyzed sample with the highest OC content is at a depth of 28.26 mbsf. The C/N ratios are <10. Based on the C/N ratios, organic matter in the sediment is mainly marine in origin.
TN in the sediments varies between 0.13 and 0.22 wt% (Table T11; Fig. F25). The nitrogen data show no apparent trends vs. either depth or OC content. The sediment samples have TS contents ranging from 0.09 to 1.84 wt% (Table T11; Fig. F25). There is no apparent relationship between sulfur and OC. The sediments contain less sulfur relative to carbon in the depth interval from 18.76 to 237.99 mbsf. TS content in the sediment increases in cores below 238 mbsf.
Fourteen samples from Hole 1244C were characterized by Rock-Eval pyrolysis (Table T12). Samples with >1 wt% OC were selected for analysis. All samples have significant levels of pyrolyzable organic matter contents, with S2 yields ranging from 1.4 to 3.1 mg of hydrocarbons per gram (HC/g) of sediment. The low Tmax values and the well-defined S2 peak shapes in the pyrograms (not shown) indicate that samples contain mostly thermally immature primary organic matter. The measured S1 (0.2-0.6 mg/g) and production index (S1/[S1 + S2]) values (0.12-0.18) are relatively high for thermally immature sediments, suggesting possible low levels of staining by migrated hydrocarbons. The hydrogen index values range from 100 to 200 mg HC/g C but are not diagnostic for organic matter type at this level of maturity.