Shipboard organic geochemical studies of cores from Holes 1165B and 1165C included monitoring of hydrocarbon gases, carbonate and organic carbon, total sulfur and total nitrogen content, and Rock-Eval pyrolysis characterization of organic matter. Procedures are summarized in "Organic Geochemistry" in the "Explanatory Notes" chapter.
Hydrocarbon gas measurements by the headspace method are reported in Table T8. Results are reported in parts per million by volume (ppmv) of methane (C1), ethane (C2), and propane (C3) in the air headspace of a 20-cm3 vial containing a nominal 5-cm3 sediment sample. Results are plotted as parts per million by volume of hydrocarbon gas component vs. depth in Figure F49. Methane content increases slowly from background levels (4 ppmv) to 400 ppmv in the cored interval between 30 and 150 mbsf and increases rapidly (400-20,000 ppmv) over the interval from 150 to 230 mbsf. Methane levels remain within a relatively uniform range (20,000-40,000 ppmv) down to 700 mbsf and then increase again to the 40,000-80,000 ppmv range in the deepest part of Hole 1165C (700-980 mbsf). The linear dissolved sulfate gradient above 130 mbsf projects to 0 mM concentration at a depth of 147 mbsf (see "Inorganic Geochemistry"), where the sulfate-free sediments inevitably result in the onset of methanogenesis and the presence of abundant methane in deeper cores (Claypool and Kaplan, 1974). It is unusual to have small but significant levels of headspace methane (50-400 ppmv) present within the sediment depth interval in which dissolved sulfate is present (seafloor to 147 mbsf). Usually methane is maintained at background levels by microbial methane oxidation within the zone of sulfate reduction. Such methane oxidation may not be occurring in Site 1165 sediments.
Ethane and propane make consistent downhole appearances below 150 and 820 mbsf, respectively. Gas composition as expressed by the C1/C2 value of headspace gas is plotted vs. depth in Figure F50 and vs. estimated sediment temperature (assuming an average geothermal gradient of 48°C/km) in Figure F51. The C1/C2 value shows the usual exponential increase in ethane content relative to methane with increasing depth. When C1/C2 is plotted vs. temperature (Fig. F51), the C1/C2 trend falls within the normal range associated with in situ low-temperature ethane generation and well to the right of the "Anomalous" field that would indicate migrating thermogenic hydrocarbons (JOIDES Safety Manual, 1992).
Headspace methane values above ~10,000 ppmv are residual concentrations, representing only that gas retained by sediments after outgassing has taken place during core retrieval to the surface (Kvenvolden and Lorenson, 2000). Moreover, there is some variability in the results of the headspace technique due to nonuniform sample size. Additionally, when cores become lithified it is more difficult to obtain reproducible sample volumes. Typically, indurated core pieces are broken into fragments <1 cm and the headspace vial is filled about one-half full. To investigate the possible effect of variable sample size on headspace gas concentration, all of the analyzed sample-filled headspace vials were weighed to an accuracy of ~0.1 g on a mechanical triple-beam laboratory balance. Sample net weights reported in Table T9 are after subtracting the average tare weight (16.6 g) of an empty—but labeled, crimped, and septum capped—headspace vial. Sample weights were converted to sample volume using a bulk density vs. depth relationship (see "Physical Properties"). Calculated sample volumes were used to estimate effective headspace volumes for each analysis and are presented in Table T9. Note that sample volumes varied between 2.8 and 7.9 cm3. Kvenvolden et al. (1989) give the equations for converting from parts per million by volume in headspace concentration units to absolute volume of gas per volume of sediment using sample volumes. The resulting methane concentrations, expressed in microliters of CH4 per liter of wet sediment, are listed in Table T9. Further evaluation of the quantity of methane present in cores is possible if the methane concentration is expressed relative to the volume of pore water that originally contained most of the gas. An average porosity vs. depth function (see "Physical Properties") was used to convert from a volume of sediment to volume of water basis. Gas concentrations expressed in millimoles of methane per liter of water (mM), are also shown in Table T9 and plotted in Figure F52. Also shown on Figure F52 is the solubility of methane in seawater at 0°C and 1 atmosphere pressure (2.3 mM). Below ~175 mbsf, the residual dissolved CH4 content in pore water is supersaturated with respect to surface solubility but unsaturated relative to estimated subsurface methane solubility.
Site survey data show a bottom-simulating reflector (BSR) on a seismic line located ~62 nmi from Site 1165. Although there is no BSR at Site 1165, with the appearance of methane in the cores, a protocol was established for examining and possibly sampling any methane hydrate occurrences. No evidence for gas hydrate was present in the cores. Moreover, there was insufficient gas to cause voids or gas pockets within the core liner. It is likely that the pore waters at Site 1165 lacked the minimum dissolved methane concentration required for the stabilization of methane hydrate. Under the pressure and temperature conditions at Site 1165 (see "In Situ Temperatures"), the minimum dissolved methane concentration required for the stabilization of methane hydrate is ~80-90 mM near the seafloor, rising rapidly to ~180-200 mM at the base of the methane hydrate stability zone. The depth of the theoretical base of methane hydrate stability at Site 1165 is ~460 mbsf.
A total of 172 sediment samples (one to two samples per core) were analyzed for carbonate carbon, and 47 selected (darker colored) samples from Holes 1165B and 1165C were analyzed for total carbon, OC (by difference), total nitrogen (TN), and total sulfur (TS). The results are reported in Table T10. Inorganic carbon (IC) content is plotted against depth of burial in Figure F53A. Carbonate content is uniformly very low (0.01-0.1 wt% IC), with the exception of scattered beds at the base of the section with between 0.1 and 1 wt% IC and 12 samples throughout the cored section with higher carbonate content (1-7.5 wt% IC).
OC content of sampled intervals (Table T10; Fig. F53B) ranges from 0 to 1.97 wt%, with most samples between 0.1 and 0.5 wt%. The highest OC contents (1.81 and 1.97 wt%) are in the depth interval from 110 to 112 mbsf. Only one other sample, at 823 mbsf, exceeds the 1 wt% carbon level (1.31 wt% OC).
TN content is generally between 0 and 0.13 wt% but with values as low as 0 wt% (Table T10). The TN data show no apparent trends vs. either depth or OC content. TS contents are similarly scattered, with many analyzed samples showing sulfur below detectable limits (Table T10). Whereas marine sediments generally show a consistent relationship between TS and OC content (TS = 0.36 × OC) (Goldhaber and Kaplan, 1974), samples from Site 1165 tend to have more variability. Site 1165 sediments generally contain excess sulfur relative to carbon in the interval from 0 to 350 mbsf and are more sulfur deficient in the lower part.
Eleven samples from Holes 1165B and 1165C were characterized by Rock-Eval pyrolysis (Table T11). Samples with >0.5 wt% OC were selected for analysis. All samples have low pyrolyzable carbon contents, with S2 yields ranging from 0.1 to 0.7 mg of hydrocarbon per gram of sediment. The Tmax values and the broad S2 peak shapes in the pyrograms (not shown) indicate that most samples contain mixtures of small amounts of primary marine organic matter with variable amounts of possibly recycled and degraded thermally mature organic matter. The shallowest samples, at 27.9 and 76.7 mbsf, have the highest Tmax values (479° and 540°C, respectively), indicating the highest proportions of recycled organic matter. The two samples (110.2 and 111.7 mbsf) with the highest OC content (1.81 and 1.97 wt%) have broad pyrograms and Tmax values of 434° and 439°C. If the primary organic matter in these samples is thermally immature (Tmax < 420°C), then a significant proportion (10%-20%) of recycled, high-maturity (Tmax > 500°C) organic matter could be present. Alternatively, the two organic-rich samples from the 110-112 mbsf interval could also contain even larger proportions (50%?) of less mature (Tmax = 460°C) recycled organic matter.