The shipboard organic geochemistry program at Site 1170 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 values for the strata sampled at Site 1170 range from 0 to >95 wt% (Fig. F25; Table T17). In general, the carbonate distribution exhibits a two-tiered profile; sediments from 776 to ~470 mbsf commonly contain <2 wt% CaCO3 (except for a distinct excursion with a value of ~90 wt% at ~590 mbsf), whereas values of CaCO3 content abruptly increase to mostly >80 wt% above 470 mbsf. Within the upper high carbonate content strata, CaCO3 content values fluctuate between 75 and 90 wt% from ~460 to 370 mbsf. From ~370 to 100 mbsf, carbonate content is mostly >90 wt%. From here, carbonate content values exhibit an overall decrease (ranging from ~<60 to 85 wt%) to ~20 mbsf; above 20 mbsf, values increase to ~80-90 wt%.
The total organic carbon (TOC) content for most intervals at Site 1170 is <1 wt%, although deposits below ~600 mbsf contain up to 2 wt% TOC (Fig. F25; Tables T17, T18). Note that TOC content values determined by Rock-Eval pyrolysis and CNS analysis provide similar TOC content profiles, although the absolute values differ for each method. Total nitrogen content ranges from 0 to 0.17 wt% (Fig. F25; Table T17), with the highest values occurring below ~600 mbsf; nitrogen content covaries with TOC content (Fig. F25). From ~754 to 476 mbsf, total sulfur content ranges from 0.25 to ~2.9 wt% (Fig. F26; Table T17). Above ~476 mbsf, the total sulfur profile shows an abrupt decrease, although no reliable data were obtained above ~425 mbsf. We calculated C/S ratios for the deepest strata at the site assuming that all of the sulfur exists as pyrite. The C/S values <2 are generally considered representative of marine environments, whereas the C/S values >5 indicate relatively freshwater environments (Berner and Raiswell, 1984). See "Discussion" for the implications of these ratios.
Organic matter type was assessed using Rock-Eval pyrolysis and CNS analyses. In general, high hydrogen index (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 (Espitalié et al., 1977; Peters, 1986), whereas C/N ratios of ~5-8 are generally considered to indicate marine organic matter (Bordovskiy, 1965; Emerson and Hedges, 1988). The H/C ratios provide information similar to that provided by the Rock-Eval pyrolysis HI; H/C values >1 in this study indicate the presence of lipid-rich organic matter, likely of marine affinity. In contrast, terrestrially derived organic matter (Type III) exhibits relatively low HI and high OI values, C/N ratios of ~25-35, and relatively low H/C ratios. Oxidized Types I and II organic matter may show HI and OI values similar to Type III organic matter; very low HI and OI values are characteristic of Type IV highly oxidized organic matter.
The HI values from Rock-Eval pyrolysis range from 0 to 800 mg of hydrocarbon per gram of TOC at Site 1170 (Fig. F25; Table T18). The highest HI values are in sediments above 100 mbsf and from ~350 to 435 mbsf; horizons containing values >150 also exist between ~530 and 670 mbsf. The OI values vary between 0 and >10000 mg of CO2 per gram of TOC. The Tmax values obtained from Rock-Eval pyrolysis range from 300° to 590°C (Fig. F27), although the most reliable Tmax values cluster between 420° and 435°C. The Tmax provides an estimate of organic matter thermal maturity, with values <435°C indicative of immaturity relative to petroleum generation. The "oil window" is generally considered to range between Tmax values of 435° and 465°C, whereas values >465°C are indicative of thermogenic gas zones (Espitalié et al., 1977; Peters, 1986).
The extremely low TOC content values obtained from Rock-Eval pyrolysis in the upper part of the sediment (to ~480 mbsf) at Site 1170 lower the reliability of HI and OI values in this interval. We performed replicate analyses on many of the samples to validate the results. The HI values and C/N ratios (discussed immediately below) exhibit similar trends, which suggests that most of our results are consistent. However, total nitrogen contents above ~480 mbsf in the core are very 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 the Oligocene through Holocene sediments at Site 1170 mainly reflects dominance of calcareous nannofossils (see "Biostratigraphy"); this observation is similar to those made for Sites 1168 and 1169. The overall upward increase in carbonate content through the core is a direct consequence of a change from shallow marine to pelagic open-ocean conditions (also see "Biostratigraphy"). However, although the transition from carbonate-poor to carbonate-rich sediments is gradual at Site 1168, the change is abrupt at Site 1170. This observation implies either a rapid change in depositional environments or, more likely, the presence of a hiatus(es) or condensed section(s) in the stratigraphic record here.
Two general modes of carbonate and TOC preservation are observable in the sediment from Site 1170 (Fig. F25). High carbonate content values generally correspond to sediments with low TOC content values, whereas low carbonate-content sediment contains a range of TOC content values including the highest values observed at the site. Middle Eocene sediments with high TOC and low CaCO3 contents exist between ~460 mbsf and the base of Hole 1170A. Here, common to abundant ichnofossils (see "Lithostratigraphy") and Th/U ratios >2 (see "Downhole Measurements") suggest a dysoxic to oxic seafloor, so it is unlikely that anoxic conditions can be invoked to describe the higher sedimentary organic carbon content in this interval. At Site 1168, high TOC-low CaCO3 sediments were suggested to represent preferential preservation (enhanced burial rate?) of organic matter, or enhanced organic matter delivery associated with higher clastic sediment flux (or both); these processes were most likely active in the middle Eocene at Site 1170.
The organic matter type encountered provides some insight into depositional processes. Sediments from ~690 to 776 mbsf (lithostratigraphic Subunit VB) contain a terrestrial component of preserved organic matter (HI values <130 and C/N ratios >20). As described above, this interval contains generally low CaCO3 content and the highest TOC values from the site. These attributes likely were formed by enhanced clastic and terrestrial organic matter delivery to the site. Zones of increased carbonate content (up to 9 wt%) observable from ~730 mbsf to the base of the hole appear to contain some marine organic matter, and therefore, may represent episodes of marine-dominated sedimentation with lower terrestrial organic matter inputs to the seafloor. These characteristics are compatible with observations of dinoflagellate cyst assemblages dominated by Enneadocysta partridgei, a dinoflagellate apparently often associated with nannofossils (see "Biostratigraphy"). Of note are the lithologic descriptions of carbonate-rich horizons through the interval from ~710 to 740 mbsf (see "Lithostratigraphy"). Total sulfur values and C/S ratios suggest dominantly marine conditions through this interval at Site 1170 (Fig. F26).
A shift from mixed terrestrial-marine to marine-influenced conditions is indicated by organic matter (type) and carbonate preservation between ~650 and 690 mbsf at Site 1170, as well as palynological studies (see "Biostratigraphy"). Here, total sulfur and C/S values again suggest deposition under marine conditions. Note that TOC content decreases through this marine organic matter interval, supporting the suggestion that terrestrial organic matter delivery dominated total organic matter content in these sediments (including the underlying strata from 690 to 776 mbsf); E. partridgei again dominates the dinoflagellate cyst assemblage here (see "Biostratigraphy"). From ~650 to 620 mbsf, TOC content increases to >1 wt%, corresponding to a shift to lower HI values indicating increased terrestrial organic matter delivery to the seafloor. From ~620 to 585 mbsf, TOC decreases to <0.5 wt%. Here, abundant glauconite was observed during lithologic descriptions of Site 1170 (see "Lithostratigraphy"). Through this interval, a prominent limestone bed is observable on the carbonate content profile, and at least one horizon of increased carbonate content was noted in the lithologic descriptions between ~630 and 640 mbsf (see "Lithostratigraphy"); the dinoflagellate assemblage in each of these horizons is again dominated by E. partridgei (see "Biostratigraphy"). The presence of these relatively carbonate-rich horizons within a mostly carbonate-poor marine section (below ~460 mbsf at Site 1170) suggests the possibility of higher-frequency marine to terrestrially influenced fluctuations than those resolved by our analyses.
From ~585 to 460 mbsf, carbonate content displays a slight overall increase and marked fluctuations between ~0.5 and 5 wt% CaCO3. These variations are likely attributable to variations in calcareous nannofossil content noted through this interval (see "Lithostratigraphy"). The TOC values decline to ~0.5 wt% and are dominated by marine organic matter preservation. A horizon from ~585 to 530 mbsf displays the strongest marine signature based on organic matter type (highest HI values), the highest total sulfur content, and C/S ratios indicative of marine depositional conditions. This horizon overlies the prominent limestone bed at ~585 mbsf. Interestingly, the dinoflagellate cyst assemblage for this interval is dominated by deflandreoids, a group associated with the presence of diatoms (see "Biostratigraphy"). Marine organic matter preservation is interrupted by an episode of reduced HI values from ~500 to 530 mbsf, suggestive of terrestrial organic matter preservation. This suggestion is not supported by C/N and H/C ratios, however. Again, abundant glauconite was noted in the lithologic descriptions (see "Lithostratigraphy") in this zone of apparent terrestrial organic matter preservation. The interval from ~500 to 520 mbsf also exhibits a relative increase in carbonate content (to >10 wt% CaCO3), C/S ratios, E. partridgei content, and an overall decrease in total sulfur content, although sulfur values increase to >1.5 wt% at the top of the interval. A return to dominantly marine organic matter preservation is observed from ~500 to 460 mbsf. Here, the C/S values increase toward the brackish salinity realm. These observations suggest rapidly changing environmental conditions during deposition and early diagenesis of this upper Eocene interval.
Above ~460 mbsf, the extremely high carbonate contents (up to 95 wt%) represent either a reduction in clastic input or enhanced carbonate preservation and may indicate enhanced biogenic productivity. The extremely low TOC content through this portion of the core may record settling of organic matter through a well-mixed water column and/or to a well-oxygenated seafloor. The HI values vary widely, and the C/N ratios show a wide range of values from 0 to >40. The apparent variations in organic matter type may record variations in seafloor redox conditions, although we cannot discount the possibility of limited terrestrial input to the system. Of note in this regard are the elevated TOC values (>1 wt%) at ~53 mbsf on the Rock-Eval pyrolysis profile and at ~72 mbsf on the CNS profile. Here, pyritized vitrain (i.e., "woody" material) was observed in the core; this observation is supported by a Type III organic matter characterization by Rock-Eval pyrolysis. Intervals containing high HI and low C/N values may record preservation of residual marine organic matter perhaps associated with carbonate producers (see "Biostratigraphy"). Relatively high C/N and low HI intervals may represent oxidized marine organic matter in which N- and H-bearing functional groups have been cleaved to produce a more refractory, carbon-rich residuum. Alternatively, the high C/N and low HI units may represent 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. F27). In the upper half of the core, Tmax values fluctuate through the maximum possible range observed at Site 1170. This wide range is attributed to extremely low TOC content and the mixed marine-oxidized character of organic matter, which can generate an erroneously large range in Tmax values. In the lower half of the core, by contrast, the Tmax values gradually smooth out and exhibit a sloping linear trend from ~420° 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 ODP headspace-sampling technique and gas chromatographic analysis. Profiles of methane content and various methane and heavier volatile hydrocarbon ratios are presented in Figure F28 (also see Table T19). Within the upper ~480 m of the cored sediment section, methane only occurred in minor concentrations (0 to 1027 ppmv). From 480 to ~545 mbsf, methane concentrations increased to a pronounced peak of ~32,200 ppmv. Below 545 mbsf, methane concentrations exhibited a pronounced decrease to values of <1000 at ~570 mbsf, and then they steadily increased to a high value of ~66,500 ppmv at a depth of ~700 mbsf. Relatively high methane concentrations (>20,000 ppmv) were observed from here to the base of Hole 1170D. The ratio of methane vs. ethane and propane (C1/C2+C3) showed maximum values above ~500 mbsf and decreased gradually to <30 at ~725 mbsf. The percent wetness is below 4% through the core, thus falling below the range of values typical for economically viable gas reservoirs.
The low gas content of the uppermost 480 m of Site 1170 is nearly identical to those in the stratigraphically equivalent headspace-gas profiles generated for Sites 1168 and 1169. At those sites, the low gas content was suggested to be a function of two characteristics of the sediment that also partially explain the gas profiles at Site 1170. First, the sediments contain very little organic matter as a source of natural gas. Second, pore-water profiles show that appreciable SO42- exists in Oligocene and younger strata, so sulfate reduction processes may be limiting the onset of methanogenesis at these sites.
Below ~480 mbsf at Site 1170, methane content increases to a broad multipeaked zone of high concentrations through to the base of the hole. The C1/C2+C3 ratios above ~545 mbsf indicate that this gas has a biogenic origin. This abrupt change from low to high methane-containing sediment corresponds to a sharp gradient to zero pore-water sulfate (Fig. F28). These observations suggest the presence of a barrier to diffusional pore-water processes (see "Inorganic Geochemistry"). Lithologic descriptions indicate the presence of lithified glauconitic sands and an abrupt upward change from these "greensands" to limestone and chalk through this section of the core (see "Lithostratigraphy"); each of these lithologies could serve as a permeability barrier. The presence of small amounts of methane within the zone of relatively high pore-water sulfate concentrations (between ~425 and 480 mbsf) suggests that minor quantities of methane are migrating across this barrier.
Beneath the zone of relatively elevated biogenic methane content, methane concentrations and C1/C2+C3 values show an overall decrease, followed by an increase in methane content with correspondingly low C1/C2+C3 values. The C1/C2+C3 ratios approach the thermogenic range near the base of Hole 1170D. Headspace samples analyzed on the natural gas analyzer from ~670 mbsf to the base of the hole, include butane (C4), pentane (C5), and hexane (C6), indicative of a thermogenic origin for this gas (Hunt, 1996). Hinz et al. (1986) postulated an early to middle Eocene source rock for thermogenic gas observed on the western Tasmania continental slope, which is consistent with the middle Eocene age (see "Biostratigraphy") of the thermogenic gas-containing strata at Site 1170.
The Tmax values approaching 435°C at the base of Hole 1170D are indicative of the top of the "oil window." Of further import to the thermal maturity of the strata in Hole 1170D is the observation of double S2 peaks (attributed to the presence of bitumen [Clementz, 1979]) on most Rock-Eval pyrograms from ~530 mbsf to the base of the hole. Fluorescence observed in core material treated with acetone near the base of Hole 1170D suggests the presence of liquid hydrocarbons (Shipboard Scientific Party, 1995), although the fluorescence was more likely derived from acetone soluble portions of the bitumen. The likely presence of bitumen and thermogenic gases suggests that the lower portions of Hole 1170D have been subject to some thermal maturation. The core depths at which maturation is postulated to have occurred are low, suggesting an alternate heat source to burial maturation.