The shipboard organic geochemistry program at Site 1168 included studies of volatile hydrocarbons, total organic and inorganic carbon, total nitrogen, total sulfur, and hydrogen and oxygen indexes. Rock-Eval pyrolysis, CNS analysis, gas chromatography, and carbon coulometry were performed (see "Organic Geochemistry" in the "Explanatory Notes" chapter).
Carbonate (CaCO3) content for the strata sampled at Site 1168 ranges from 0 to 90 wt% (Fig. F27; Table T16). In general, the profile exhibits an overall increasing upsection trend. Sediments from 878 to ~700 mbsf commonly contain <10 wt% CaCO3, except for several distinct excursions with values of up to 38 wt%. Carbonate content steadily increases between ~700 and 550 mbsf, ranging from ~10 to 40 wt%. From ~550 to 450 mbsf, a broad declining trend in percent carbonate was measured, with values ranging from approximately <10 to 50 wt%. Above this broad decrease is a narrow zone of increasing carbonate content (~20-50 wt%) from 450 to ~370 mbsf, which is overlain by a narrow decrease to ~310 mbsf (carbonate content as low as ~17 wt%). Carbonate content steadily increases from the top of this horizon to ~250 mbsf, where values of ~90 wt% CaCO3 are attained; one region of depressed values exists at ~200 mbsf. From here, carbonate content remains at ~90 wt% until ~40 mbsf, above which the values decrease to ~80 wt% (with a value as low as 50 wt%) to the seafloor.
The total organic carbon content for most intervals at Site 1168 is <1 wt%, although deposits below ~700 mbsf contain up to 5 wt% TOC (Fig. F27; Table T16). Note that TOC values determined by Rock-Eval pyrolysis and CNS analysis provide similar TOC profiles (Fig. F27; Tables T16, T17). Total nitrogen content ranges from 0 to 0.18 wt% (Table T16), with the highest values being between 700 mbsf and the base of Hole 1168A; nitrogen content covaries with TOC content (Fig. F27). From 878 to ~540 mbsf, total sulfur content is generally high, ranging from near 0 to >2 wt% (Fig. F28; Table T16). Above ~540 mbsf, the total sulfur profile shows an overall gradual decrease upsection with occasional values >1 wt% dispersed through the profile. Sulfur standards were inconsistent from ~320 to 680 mbsf. Nonetheless, we calculated C/S ratios assuming that all of the sulfur exists as pyrite within the sediments. C/S values <2 are generally considered representative of marine environments, whereas C/S values >5 indicate freshwater-influenced environments (Berner and Raiswell, 1984).
Organic matter type was assessed using Rock-Eval pyrolysis and CNS analyses. In general, high HI (>~200) and low oxygen index (OI) (<~150) values from Rock-Eval pyrolysis are indicative of marine (Type II) or lacustrine (Type I) organic matter, although lacustrine organic matter generally shows higher HI and lower OI values than marine counterparts (Espitalie et al., 1977; Peters, 1986). In addition, C/N ratios of ~5-8 are generally considered to indicate marine organic matter (Bordovskiy, 1965; Emerson and Hedges, 1988). In contrast, terrestrially derived organic matter from higher plants (Type III) exhibits relatively low HI (<~150), relatively high OI values, and C/N ratios of ~25-35. Oxidized Type I and II organic matter may show HI and OI values similar to those obtained from Type III organic matter, and consistently low HI and OI values are characteristic of Type IV (highly oxidized) organic matter.
Hydrogen index values from Rock-Eval pyrolysis range from 0 to >600 mg of hydrocarbon per gram of TOC at Site 1168 (Fig. F27; Table T17). The highest HI values are in sediments above 300 mbsf, whereas below 300 mbsf only discrete horizons contain values exceeding 150. Oxygen index values vary between 21 and 5250 mg of CO2 per gram of TOC. The Tmax values obtained from Rock-Eval pyrolysis range from 350° to 590°C (Fig. F29), although the most reliable Tmax values cluster between 400° and 430°C. Tmax values provide an estimate of organic matter thermal maturity, with values <435°C being indicative of immaturity relative to petroleum generation. The "oil window" is generally considered to range between Tmax values of 435°-465°C, whereas values >465°C are indicative of thermogenic gas zones (Espitalie et al., 1977; Peters, 1986).
The extremely low TOC values obtained from Rock-Eval pyrolysis in the upper 300 m of sediment at Site 1168 must be considered as lowering the reliability of the HI and OI values in this interval. We performed duplicate and triplicate analyses on many of the samples through this section of the core to validate the results. We found a close correspondence between HI values and C/N ratios, which suggests that our results are consistent and valid for interpretation. However, total nitrogen contents through the core are generally low, so C/N ratios must also be considered with care. The generally high OI values through much of the core are here attributed to the thermal degradation of calcium carbonate during pyrolysis and are not considered in this interpretation.
The high carbonate content of sediments at Site 1168 mainly reflects the dominance of calcareous nannoplankton and foraminifers (see "Biostratigraphy"). The overall upward increase in carbonate content through the cored interval is a direct consequence of a change from brackish/shallow-marine to pelagic open-ocean conditions. Within this depositional setting, an overall upward decrease in clastic sediment content was observed (see "Lithostratigraphy"), indicating an upward decrease in clastic sediment dilution. The implications to interpretations of depositional environment and sequence stratigraphy of the broad carbonate decline observed from ~530 to 320 mbsf are unclear. However, this zone corresponds to a distinct maximum in Ca2+ obtained from sediment pore waters, suggesting this zone may have been subject to preferential dissolution (see "Inorganic Geochemistry").
A plot of carbonate content vs. TOC content (Fig. F30) shows that a wide range of percent carbonate values are present in sediments with low TOC values. However, the highest TOC values are only in those sediments with low carbonate content. Sediments with high TOC and low CaCO3 are between ~700 and 878 mbsf, where clastic content is highest. This observation suggests that organic matter content in Hole 1168A is a function of preferential preservation (enhanced burial rate?) or enhanced terrestrial organic matter delivery associated with higher clastic sediment input, or both.
The type of organic matter encountered provides some insight into depositional processes at Site 1168. The basal portion of Hole 1168A from ~700 to 878 mbsf contains strong geochemical evidence for terrestrial organic matter influence on deposition at the site. As described above, this interval contains the lowest percent carbonate and highest TOC values, likely as a function of clastic input. Here, HI values of mostly <150 and C/N ratios >20 indicate terrestrial organic matter preservation, and total sulfur values and C/S ratios suggest dominantly brackish water conditions. Marine incursions into this relatively nearshore setting are indicated by fluctuations to higher HI, lower C/N, and relatively high total sulfur values at discrete horizons through this interval. A shift from terrestrial to marine-influenced conditions is indicated by organic carbon and carbonate preservation between ~700 and 750 mbsf at Site 1168. Here total sulfur and C/S values suggest brackish to marine water conditions. From ~700 to 500 mbsf, TOC content is still relatively high (0.5-1 wt%) but displays an overall decline that is maintained throughout the rest of the core. In this interval, HI values continue to suggest preservation of mostly Type III (terrestrial) organic matter; however, the elevated OI values may represent oxidized organic matter deposition. The C/N values through this interval are closer to marine values (0-15), although the signal is somewhat elevated compared with a typical marine signature. These parameters suggest mixed inputs of terrestrial or oxidized marine with marine organic matter to the seafloor. The percent carbonate begins a steady upward increase through this interval, which generally continues through the rest of the section. The C/S values suggest dominantly marine depositional conditions, with excursions into the brackish water realm. From ~450 to 350 mbsf, carbonate increases to >40 wt%, whereas TOC values decrease significantly to below 1. The C/S ratios indicate marine depositional conditions, with low values attributable to extremely low total sulfur values for those horizons. Discrete horizons containing relatively higher HI values and low C/N ratios indicate marine organic matter preservation, whereas lower HI values likely record seafloor-redox conditions as discussed below.
Above ~300 mbsf, the extremely high carbonate content (up to 95 wt%) represents either a decrease in clastic dilution or enhanced carbonate preservation and may indicate enhanced biogenic productivity. The extremely low TOC values through this interval likely record the settling of organic matter through a well-mixed water column and/or to a well-oxygenated seafloor. HI values vary widely, and the C/N ratios show a wide range of values from 0 to ~35. We suggest that the variations in organic matter type record variations in seafloor redox conditions, although we cannot discount the possibility of limited terrestrial input to the system. In this conceptual model, intervals containing high HI and low C/N values likely record preservation of residual marine organic matter perhaps associated with carbonate producers, or dinocysts (see "Biostratigraphy"). Relatively high C/N and low HI intervals could represent oxidized marine organic matter in which N- and H-bearing functional groups have been cleaved to produce a more refractory, carbon-rich residuum. The high C/N and low HI units could also represent a total oxidation of labile marine organic matter and subsequent preservation of minor quantities of refractory terrestrial residuum.
The Tmax values obtained from Rock-Eval pyrolysis are interpreted to represent mostly immature organic matter (Fig. F29). In the upper half of the core, Tmax values fluctuate through the maximum possible range. This wide range is attributed to extremely low TOC values and the mixed marine to oxidized marine character of organic matter, which can generate an erroneously large range in Tmax values. In the lower half of the core, by contrast, Tmax values gradually smooth out and exhibit a sloping linear trend from ~410° to 435°C with depth. We consider these values as valid because they were obtained from horizons containing at least 0.5 wt% TOC composed of Type I organic matter; note, however, that Type I organic matter can generate erroneously low Tmax values.
Concentrations of volatile hydrocarbon gases were measured from every core using the standard Ocean Drilling Program (ODP) headspace-sampling technique and gas chromatographic analysis. Profiles of methane content (Fig. F31; Table T18), and various methane, ethane, and propane ratios, are presented in Figure F31. Within the upper ~230 m of the cored sediment section, methane is present only in minor concentrations (2-716 ppmv) except near the surface, where methane content increases to 2400 ppmv (Fig. F31; Table T18). From 350 to ~500 mbsf, methane concentrations show a pronounced peak to ~52,000 ppmv. Below 500 mbsf, methane concentrations decrease relatively, although they remain high (up to >40,000 ppmv) and display a wide range. The ratios of methane vs. ethane plus propane (C1/C2+C3), in contrast, show maximum values at the sediment surface (~8000) while decreasing gradually to 300-500 at ~500-700 mbsf and further to <100 at the base of the hole (Fig. F31). The percent wetness stays below 5% within the entire core, thus falling below the range of values typical for economically viable gas reservoirs (Fig. F31).
The mostly low gas content of the uppermost 230 m of Site 1168 is likely a function of two characteristics of the sediment. First, the sediments contain very little organic matter as a source of natural gas. Second, pore-water profiles show that appreciable SO42- exists to ~230 mbsf; thus, the sulfate reduction process may be limiting the onset of methanogenesis in this interval (see "Inorganic Geochemistry"). Below ~230 mbsf, methane content increases to a broad multipeaked zone of high concentrations between ~350 and 450 mbsf. The C1/C2+C3 ratios indicate that this gas has a biogenic origin. The presence of this peak immediately beneath the zone of sulfate reduction is characteristic of microbially driven diagenetic depth zonation (Claypool and Kaplan, 1974). However, the presence of elevated methane contents within strata generally characterized by TOC values of <0.5 wt% suggests that these lithologies may not be the sole gas source (i.e., the gas has migrated from elsewhere). In this case, the most likely source for the elevated biogenic methane content is the high TOC terrestrial organic matter (gas prone)-containing strata in the older parts of the section. Obviously, if any methane migrates higher in the section (above ~250 mbsf), it is likely oxidized in the sulfate reduction zone. However, this process does not describe the peak in methane content observed at Site 1168, if gas migration has occurred. Therefore, a mechanism must be invoked to trap gas in the strata. No obvious lithostratigraphic traps were recognizable (see "Lithostratigraphy"). In fact, porosity measurements actually indicate an increase above the high methane content horizon (see "Physical Properties"). Interestingly, a zone of soft-sediment deformation was observed at ~325 mbsf (see "Lithostratigraphy"), and degassing intervals were observed during headspace sampling apparently centered on discrete particles at ~330-345 mbsf. These observations were considered as circumstantial evidence for methane clathrates; the bottom of the gas hydrate stability zone was calculated at ~300 mbsf (see "Inorganic Geochemistry"). A resistivity "spike" at ~250 mbsf (see "Downhole Measurements") may also be suggestive of gas hydrates at an interval above the strata with relatively high methane content. If clathrates exist in the subsurface at Hole 1168A, they could prevent methane from migrating higher in the sedimentary sequence.
Beneath the zone of relatively elevated biogenic methane content, methane concentrations and the C1/C2+C3 values show an overall decrease. The C1/C2+C3 ratios approach the thermogenic range near the base of Hole 1168A. A limited number of gas samples were taken with a vacutainer in horizons where gas pockets were visible through the core liner. Vacutainer and headspace samples analyzed on the natural gas analyzer, from horizons between ~480 and 878 mbsf, include some butane (C4) and pentane (C5), suggesting a thermogenic origin for at least some of the gas phase (Hunt, 1996). These measurements are consistent with observations of thermogenic gas in shallow areas on the western Tasmania continental slope within ~30 km of Site 1168 (Hinz et al., 1986). In their study, Hinz et al. (1986) postulated an early to middle Eocene source rock, which may exist beneath the upper Eocene sediment at Site 1168.
Tmax values approaching 435°C are indicative of the top of the "oil window." Of further interest to the thermal maturity of strata is the observation of double S2 peaks on Rock-Eval pyrograms in cores between ~750 and 830 mbsf from Hole 1168A. Double S2 peaks have been attributed to in situ organic matter and bitumen existing as a product of hydrocarbon generation from some of the in situ material (Clementz, 1979). Fluorescence was observed in core material treated with acetone between ~700 and 878 mbsf, suggesting the presence of liquid hydrocarbons (Shipboard Scientific Party, 1995), although the fluorescence may have been derived from acetone soluble portions of bitumen. The presence of bitumen and thermogenic gases suggests that the lowermost portions of Hole 1168A have been subject to some degree of thermal maturation. The depths in the core at which this postulated maturation has occurred are anomalously low, suggesting the possibility of an alternate heat source to burial maturation.