The shipboard organic geochemistry program at Site 1171 included studies of volatile hydrocarbons, total organic and inorganic carbon, total nitrogen, total sulfur, and hydrogen and oxygen indexes. Rock-Eval pyrolysis and gas chromatography were performed (see "Organic Geochemistry" in the "Explanatory Notes" chapter) on headspace residues sampled at one per core. The CNS analysis and carbon coulometry (see "Organic Geochemistry" in the "Explanatory Notes" chapter) were performed on samples taken from Sections 1, 3, and 5 in each core.
Carbonate (CaCO3) content for the strata sampled at Site 1171 ranges from 0 to >95 wt% (Fig. F30; Table T17). In general, the carbonate distribution exhibits a two-tiered profile. Sediments from ~270 to 955 mbsf commonly contain <10 wt% CaCO3 (with a distinct excursion of ~60 wt% at ~415 mbsf and a broader zone from ~500 to 680 mbsf with values up to >20 wt%), whereas the carbonate content abruptly increases to mostly >90 wt% above 270 mbsf. Within the upper high-carbonate content strata, carbonate content values are consistently ~90 wt% from ~200 to 270 mbsf. From ~50 to 200 mbsf, carbonate content is mostly >90 wt%, with a slight decrease to ~85 wt% at ~108 mbsf. From ~50 mbsf, carbonate content values range from ~85 to 95 wt%.
The total organic carbon (TOC) content for most intervals at Site 1171 is <1 wt% (Fig. F30; Table T17). Note that the TOC content values determined by Rock-Eval pyrolysis and CNS analysis mostly provide similar TOC content profiles, although the absolute values differ for each method. In the upper 100 mbsf, however, the CNS results determined by difference indicate TOC content values up to >1 wt%, whereas Rock-Eval pyrolysis results indicate very little organic carbon content in this part of the section. Total nitrogen content ranges from 0 to 0.16 wt% (Fig. F30; Table T17), with the highest values occurring below ~800 mbsf; nitrogen content covaries with the TOC content (Fig. F30). From 955 to ~760 mbsf, total sulfur content displays an overall decrease from 2 to 0.1 wt% (Fig. F31; Table T17). From ~760 to 500 mbsf, the total sulfur profile varies from 0.1 to ~1 wt%. At 500 mbsf, total sulfur content increases to >2 wt%. From ~370 to 500 mbsf, sulfur content remains generally high, ranging from 0.6 to >3 wt%. Above ~370 mbsf, total sulfur values gradually decrease to values ranging from 0 to 1.2 wt%. No reliable total sulfur data were obtained above ~270 mbsf. We calculated the C/S ratios for strata below ~270 mbsf 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 fresh water settings (Berner and Raiswell, 1984). The implications of these ratios are discussed below.
Organic matter type was assessed using Rock-Eval pyrolysis and CNS analyses. A discussion of this methodology is included in "Organic Geochemistry" in the "Site 1168" chapter and "Organic Geochemistry" in the "Site 1170" chapter. The HI values from Rock-Eval pyrolysis range from 0 to 1000 mg of hydrocarbon per gram of TOC at Site 1171 (Fig. F30; Table T18). The highest HI values are at ~520 and 790 mbsf. Oxygen index (OI) values vary between 0 and 7000 mg of CO2 per gram of TOC. The Tmax values obtained from Rock-Eval pyrolysis range from 301° to 597°C (Fig. F32), although the most reliable Tmax values cluster between 420° and 430°C. The Tmax provides an estimate of organic matter thermal maturity, with values of <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 of >465°C are indicative of thermogenic gas zones (Espitalié et al., 1977; Peters, 1986). Note that some of the Rock-Eval pyrolysis data (particularly the low HI values) below ~850 mbsf may not be reliable because of instrument malfunction during analysis of those samples.
The extremely low TOC content values obtained from Rock-Eval pyrolysis in the upper ~270 mbsf at Site 1171 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 and valid for interpretation. However, total nitrogen contents above ~270 mbsf in the core are very low, so the C/N ratios must also be considered with care. The generally high OI values through much of the core are attributed to the thermal degradation of calcium carbonate during pyrolysis and are not considered in this interpretation.
The high carbonate content of Oligocene through Holocene sediments at Site 1171 primarily reflects dominance of calcareous nannofossils; foraminifers are secondary in importance (see "Biostratigraphy"). This observation is similar to those made for Sites 1168, 1169, and 1170. The overall upward increase in carbonate content through the sequence is again (as at previous sites) a direct consequence of a change from shallow marine to pelagic open-ocean conditions. Here, the transition from carbonate-poor to carbonate-rich sediments is abrupt at ~270 mbsf. This observation is similar to and coeval with the distribution of carbonate observed at Site 1170, and it implies either a rapid change in depositional environments or the presence of an unconformity and condensed section in the stratigraphic record.
As with Sites 1168 and 1170, two modes of carbonate and total organic carbon preservation are apparent in sediments from Site 1171 (Fig. F30). 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. Upper Paleocene through lower Oligocene sediments with relatively elevated TOC and low CaCO3 contents exist between ~270 mbsf and the base of Hole 1171AD. Here, mostly common to abundant ichnofossils (see "Lithostratigraphy") and Th/U ratios mostly >2 (see "Downhole Measurements") indicate dysoxic to oxic conditions, although short episodes of anoxia may have affected the seafloor (discussed immediately below). Through this interval, as at Sites 1168 and 1170, most of the high TOC-low CaCO3 sediments may represent enhanced burial efficiency of organic matter.
The organic matter type encountered provides some insight into depositional processes. Three intervals in HI values are observable on Figure F30. Paleocene through lowermost Eocene sediments (from ~780 mbsf to the base of the hole) show alternations between horizons containing marine and terrestrial organic matter, all overlain by an extremely high HI horizon. The C/N ratios, however, indicate dominantly marine organic matter preservation. In this case, the HI values appear to have more faithfully recorded organic matter type since abundant terrestrial palynomorphs have been described from the base of the interval (see "Biostratigraphy"). As described previously, these sediments contain generally low CaCO3 content and some of the highest TOC content values from the site. The total sulfur content through the interval is also amongst the highest observed at Site 1171, although it decreases upward through the interval; the C/S ratios indicate deposition under marine conditions. The Th/U values <2 (see "Downhole Measurements") suggest an episode of anoxia at the seafloor at ~920 mbsf. This observation is supported by the pervasively laminated sediments visible below ~910 mbsf in the core (see "Lithostratigraphy"). Interestingly, this anoxic horizon contains terrestrially derived organic matter. Apparently, large quantities of terrestrial organic matter were, at times, deposited on the seafloor. This organic matter provided a sufficient substrate for bacterial sulfate reduction, pore- and/or bottom-water oxygen consumption, and pyrite formation as organic matter was remineralized.
A second interval (lower to middle Eocene) with distinct organic matter type exists from ~500 to 780 mbsf at Site 1171. Here, the organic matter is either mixed terrestrial-marine or marine-oxidized marine (an observation supported by the C/N ratios), although it is more marine in character and the amplitude of organic matter type variation is less pronounced than in the underlying interval. Total sulfur content here is lower than in the underlying sediment, whereas carbonate content is generally higher and displays a pronounced increase from ~500 to 680 mbsf. The C/S ratios suggest deposition under alternating marine to brackish conditions from ~660 to 780 mbsf and under marine conditions from ~500 to 660 mbsf. Using the method of Adams and Weaver (1958), the Th/U values indicate anoxic conditions exist from ~570 to 580 mbsf.
The overall trend from mixed terrestrial-marine organic matter in the lower interval to more marine organic matter facies in the overlying interval suggests an overall relative sea-level rise. Within the interval from ~500 to 680 mbsf, calcareous nannofossils are abundant to common (see "Biostratigraphy"), whereas below, the nannofossil content is mostly rare with occasional intervals showing common to abundant nannofossils. These paleontological observations are compatible with an overall rise in sea level through the lower middle Eocene section of the core and can account for the carbonate content within these sediments. The record of brackish pore-water conditions (from the C/S ratios) may be difficult to resolve within this framework, however. Perhaps, overall conditions in the seaway were subject to a relative deepening, but fluctuations in climate (wet-dry cycles) were sufficient to cause episodes of relative "freshening" of the water column and/or sediment pore waters. This hypothesis may explain the mixed terrestrial-marine organic matter signature in the lowermost portions of the core and is supported by the overall paucity of calcareous nannofossils and foraminifers through this time. Alternatively, a diminished capacity for pyrite formation because of undefined limitations in iron or sulfate availability may have lead to a brackish water signature in the C/S values, which actually represent periods of decreased pyrite formation. In either case, more analyses will be needed to understand the significance of the geochemical record from this interval.
The second organic matter type interval is overlain by the highest HI interval at the site (>1000 mg of hydrocarbon per gram of TOC at ~520 mbsf). Here, density and P-wave velocity (and other downhole logging and physical properties) (see "Downhole Measurements") display an abrupt downhole increase, although no abrupt change in lithologic character was noted (see "Lithostratigraphy"). Interestingly, a hiatus representing up to 2 m.y. of missing section was identified between ~515 and 560 mbsf (see "Biostratigraphy"). In addition, the Th/U ratios <2 (see "Downhole Measurements") suggest anoxic seafloor conditions during deposition of the interval from ~460 to 520 mbsf. These characteristics are highly suggestive of a middle Eocene disconformity overlain by a marine flooding surface and condensed section.
A third interval in organic matter type is visible from ~270 to 500 mbsf at Site 1171. Through this middle Eocene to lower Oligocene interval, the organic matter is best characterized as marine to oxidized marine (an observation supported by the C/N ratios), although it is more marine in character than organic matter in the second interval. The total sulfur values include the highest values recorded at this site, and the C/S ratios indicate deposition under fully marine conditions. These trends are remarkably similar to those observed at Site 1170 and are highly suggestive of regional scale changes in seafloor redox conditions. Here, three subintervals in organic matter type are recorded. These subintervals are defined by zones of relatively high HI values (labile marine organic matter) separated by discrete horizons of relatively lower HI values (oxidized marine). The lowermost subinterval (from ~420 to 480 mbsf) contains some of the highest TOC and total sulfur content from the hole and includes Th/U values suggesting anoxic seafloor conditions; this subinterval corresponds to lithostratigraphic Subunit VA (see "Lithostratigraphy"). Barren to abundant calcareous nannofossils are reported through this interval (see "Biostratigraphy"). This subinterval is overlain by a discrete peak in carbonate content visible on the carbonate profile at ~415 mbsf. This carbonate peak exists in a similar stratigraphic position to a discrete limestone at Site 1170 and may represent a regionally correlative upper middle Eocene marker bed.
In the uppermost HI subinterval (from ~330 to 270 mbsf and corresponding to lithostratigraphic Unit IV; see "Lithostratigraphy"), the C/S ratios indicate an episode of late Eocene brackish water conditions. Admittedly, the relative availability of reduced iron or sulfate may have played a role in the C/S signature recorded here. However, this brackish water signature was recorded in the upper Eocene at Sites 1168 and 1170, suggesting that the interpretation may be correct. This change from marine to brackish conditions may easily be described by regional shoreline progradation. This regional-scale base-level fall interpretation is, however, not compatible with our interpretation of an overall relative sea-level rise during this time. Therefore, the increase in the C/S ratios may represent a regional change in seawater salinity (see also "Palynology"). Here, increases in carbonate content are visible and calcareous nannofossil assemblages are common to abundant (see "Biostratigraphy"). In coeval strata at Site 1170, relatively carbonate-rich horizons within a mostly carbonate-poor marine section were considered to suggest higher-frequency fluctuations in organic matter type than those resolved by our analyses.
Above ~270 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 (as observed at Sites 1168, 1169, and 1170). The extremely low TOC content values and low dinoflagellate cyst content (see "Biostratigraphy") through the Oligocene to Holocene portion of the core may record settling of organic matter through a well-mixed water column and/or to a well-oxygenated seafloor (as observed at Sites 1168, 1169, and 1170). 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 are the elevated total organic carbon values (>1 wt%) above ~110 mbsf on the CNS profile, an observation also made for a similar interval at Site 1170.
The similarity between geochemical parameters from Sites 1171 and 1170 is significant because it suggests regional-scale changes in seafloor and water-column conditions. Both sections record a mostly shallow and dysoxic Eocene seafloor. At Site 1170, the average Th/U ratios are lower than those obtained from coeval strata at Site 1171. This observation suggests that the seafloor oxygenation state within the Australo-Antarctic Gulf at Site 1170 was, in general, less well ventilated than the seafloor at Site 1171. In addition, however, the Th/U values are more variable, and evidence for late Paleocene and middle and late Eocene anoxic episodes are most pronounced at Site 1171, perhaps associated with deposition influenced by the Paleogene Pacific Ocean. These anoxic episodes may represent impingement of an open Pacific Ocean oxygen minimum zone on the shallow to bathyal marine setting of Site 1171 Eocene-Oligocene sediments. A stepwise change in Eocene organic matter facies from mixed terrestrial marine to dominantly marine is suggestive of an overall relative sea-level rise through this time. This overall rise culminated in an abrupt change to deep well-oxygenated conditions in the early Oligocene.
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 F33 (also see Table T19). Within the upper ~490 m of the cored sediment section, methane only occurred in minor concentrations (2-393 ppmv). However, equipment failure likely led to erroneous values from ~320 to 490 mbsf; 320 mbsf (middle Eocene) likely represents the approximate onset of methanogenesis in the section based on the pore-water sulfate profile (see "Inorganic Geochemistry"). A peak in methane concentration at ~490 mbsf is followed by an overall decrease in values to ~580 mbsf. Below 580 mbsf, methane concentrations exhibit a gradual increase to values >50,000 ppmv at ~650 mbsf and then steadily decrease to ~2100 ppmv at a depth of ~720 mbsf. From here, methane concentrations again exhibit a gradual increase to high values for Hole 1171D of >63,000 ppmv at ~860 mbsf. Methane concentrations range from ~5000 to 26,000 ppmv from here to the base of Hole 1171D. The ratio of methane vs. ethane plus propane (C1/C2+C3) shows maximum values from ~490 to 700 mbsf and decrease gradually to ~11 at ~940 mbsf. Percent wetness is below 15% through the core, thus falling below the range of values typical for economically viable gas reservoirs.
The low gas content of the uppermost 480 or 320 mbsf (measured and assumed, respectively) of Site 1171 is nearly identical to those in the stratigraphically equivalent headspace-gas profiles generated for Sites 1168, 1169, and 1170. At those sites, the low gas content was suggested to be a function of very little organic matter as a source of gas and pore-water profiles with sulfate-reduction processes limiting the onset of methanogenesis.
Below ~480 (320) mbsf at Site 1171, methane content increases to a broad multipeaked zone of high concentrations through to the base of the hole. The C1/C2+C3 ratios from ~480 to 700 mbsf suggest a biogenic origin. From ~700 to 900 mbsf, the C1/C2+C3 ratios indicate a mixed biogenic/thermogenic origin for the gases, whereas below 900 mbsf the gases have been generated by thermogenic processes. Headspace samples analyzed on the natural gas analyzer from ~520 mbsf to the base of the hole include butane (C4), pentane (C5), and hexane (C6), likely 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 west Tasmanian continental slope, which is consistent with the Paleocene to middle Eocene age (see "Biostratigraphy") of the thermogenic gas-containing strata at Site 1171.
The Tmax values obtained from Rock-Eval pyrolysis are interpreted to represent mostly immature organic matter (Fig. F32). In the upper ~460 mbsf, the Tmax values fluctuate through the maximum possible range observed at Site 1171. This wide range is attributed to extremely low TOC content values and the mixed marine-oxidized character of organic matter, which can generate an erroneously large range in Tmax values. Below 460 mbsf, the Tmax values display a sloping linear trend from ~420° to 430°C with depth. Within this trend are numerous elevated values, which were obtained from samples exhibiting double S2 peaks on the Rock-Eval pyrograms. Double S2 peaks have been attributed to the presence of bitumen (Clementz, 1979). The likely presence of bitumen and thermogenic gases suggests that the lower portions of Hole 1171 have been subject to some thermal maturation. We consider the sloping trend as probably most valid because the values were obtained from horizons containing at least 0.5 wt% TOC and do not display the double S2 peak characteristic. The Tmax values approaching 435°C at the base of Hole 1171D are indicative of the top of the "oil window." The depths in the core at which maturation is postulated to have occurred are as shallow as at Sites 1168 and 1170, again suggesting an alternate heat source to burial maturation.