31
Ma.).We recovered a range of biogenic and siliciclastic sediments from the uppermost Cretaceous to Quaternary (Fig. F16), including the interval of the sediment sequence recording the seaway opening south of Australia and the development of Southern Ocean circulation. The sedimentary sequences allow recognition of three distinct phases of sedimentation (a siliciclastic interval, a transitional unit, and a biogenic carbonate sequence) and three sedimentary provinces (the more restricted west Tasmanian margin [Site 1168], the transitional STR [Sites 1169, 1170, and 1171], and the more ventilated ETP [Site 1172]).
The siliciclastic sequence extends from the upper Maastrichtian (drilled on the ETP) to the upper Eocene and consists of shallow-water silty claystone and clayey siltstone for the entire Tasmanian region. The siliciclastics are associated with abundant neritic diatoms from the middle Eocene in the Pacific region of the ETP. The sediment is enriched in organic matter in the poorly ventilated western Australo-Antarctic Tasmanian region. Such siliciclastics are widespread on the margins of Australia and Antarctica and have been observed from the Eocene Great Australian Bight (Feary, Hine, Malone, et al., 2000) to the Ross Sea margin near Cape Adare (Hayes, Frakes, et al., 1975).
The initiation of the
transitional unit coincides with the preservation of abundant neritic diatoms in
the upper Eocene on the STR, followed by the occurrence of glauconitic
siltstones and sands throughout the Tasmanian region. This is indicative of
greater bottom-water activity near the Eocene/Oligocene boundary. Siliciclastic
sediments decrease rapidly above the glauconitic interval in the lowermost
Oligocene of the ETP and the STR but persisted in the early Oligocene of the
western Tasmanian margin (until 31
Ma.).
The STR and ETP pelagic carbonate sequence (see "Geochemistry") begins abruptly with the first occurrence of nannofossil chalk and limestone in the lowermost Oligocene. The STR sequence also contains common to abundant siliceous microfossils. On the west Tasmanian margin, the pelagic carbonate content increases progressively in the lower Oligocene, and the sediments consist of almost-pure nannofossil chalk by the middle Miocene. The pelagic carbonate ooze contains increased foraminifers and clay content in the whole Tasmanian region from the late Pliocene to the Pleistocene.
The upper Paleocene and lower Eocene sediments on the STR decrease in grain size upsection as the clay assemblage evolves from a complex assemblage of kaolinite, illite, and smectite to predominantly smectite. This pattern reflects decreased erosion and continental relief at the end of transtensional activity that led to the development of basins from Antarctica to much of the STR. The local source region of Site 1171 was tectonically active during the late Paleocene, and that of Site 1170 during the middle Eocene (TFZ), leading to changes in clay mineral evolution at the two sites.
Beginning in the late middle Eocene, increasing grain size of the shallow-marine siliciclastics on the STR and western Tasmania margin, together with a clay assemblage dominated by illite and kaolinite, resulted from erosion of steep-continental relief. This interval correlates with a stage of late middle and late Eocene tectonism resulting from increased spreading rates in the Australo-Antarctic Gulf, and strike-slip activity on the western STR. This stage of tectonism ultimately led to the final separation of Australia from Antarctica at the Eocene/Oligocene boundary. The presence of volcanic glass on the Pacific side of the Tasmanian region and ash layers on the ETP indicate that the late Eocene interval of tectonism was associated with intensified volcanic activity that may have persisted into the early Oligocene. It remains to be determined how much of this volcanism was associated with the presumed hot spot volcano of the Cascade Seamount on the ETP and how much volcanism was related to the general tectonism.
Extensive bioturbation of the entire upper Maastrichtian to upper Eocene sequence on the ETP and of Eocene sediments on the STR indicates significant bottom-waters ventilation. However, the absence of carbonate microfossils and the presence of siliceous microfossils suggest that major dissolution occurred and may have been associated with restricted environmental conditions. Darker sediment colors and decreased bioturbation on the western STR, together with abundant organic matter and intervals of lamination on the western Tasmanian margin, indicate poor ventilation of the Australo-Antarctic Gulf during the middle and late Eocene. Anoxic to suboxic conditions prevailed in the sheltered troughs of the Tasmanian margin. Abundant uppermost Eocene glauconite includes foraminiferal infillings indicative of in situ formation. Widespread glauconite in the entire Tasmanian region indicates strong bottom-current activity and winnowing at shelf water depths.
Bottom-water ventilation increased sharply in the earliest Oligocene, producing the light-colored sediments of the ETP and the STR, whereas ventilation developed slowly from the early Oligocene to the middle Miocene on the western Tasmania margin as attested by the persistence of laminations and weak bioturbation. The predominance of pelagic carbonates from the earliest Oligocene is unusual at southern high latitudes. The carbonates are likely related to the warm marine influence of subtropical currents transporting heat to the Tasmanian region. This is in sharp contrast with the Ross Sea margin (Site 274), then adjacent to the STR, where biosiliceous microfossils have been predominant since the Oligocene (Hayes, Frakes, et al., 1975).
No glacial or ice-rafted deposits were found at any of the Leg 189 sites, even sediments from the Oligocene, when the Tasmanian region was still close to Antarctica, suggesting the absence of significant ice cover in nearby East Antarctica. Smectite or kaolinite dominates in most Eocene sediments of the entire Tasmanian region, indicating that warm climates prevailed on emerged adjacent areas of the Tasmanian land bridge and Antarctic margin. An increasing trend toward illite and random mixed-layered clays in the lower Oligocene of Site 1170 on the western STR suggests the development of physical weathering on the adjacent emerged areas, like that in other sectors of East Antarctica (Ehrmann, 1991; Robert and Kennett, 1997). This trend immediately follows the strike-slip activity on the western STR during the transition to the more ventilated biogenic sequence.
The continued presence of common to abundant kaolinite on the western Tasmania margin and the ETP from the lower Eocene to the Pliocene indicates the persistence of relatively warm climates with significant precipitation in the adjacent Australian coastal areas. Episodes of increased kaolinite content on the western Tasmania margin in the lower Oligocene and in the upper to middle Miocene correlate with intervals of interpreted intensified precipitation at midlatitudes. The early to middle Miocene interval immediately preceded the expansion of Antarctic ice at 14-15 Ma (Kennett, 1977). Kaolinite on the ETP increased continuously from the middle Eocene to the Pliocene as Australia moved north to warmer latitudes. Beginning in the Pliocene, an increase in the sediment clay fraction is associated with a mineralogical change to increased illite and kaolinite, a composition very similar to that of the Lord Howe Rise in the Tasman Sea. This pattern suggests the development of a dust supply from arid central Australia, most probably by tropospheric winds associated with cold fronts (Pye, 1987).
There are pervasive alternations of lighter and darker intervals from the Eocene to the Pleistocene in the entire Tasmanian region. Preliminary spectral analyses have been conducted on lightness and magnetic susceptibility data from the west Tasmania margin during a chronologically well-constrained Pliocene-Pleistocene interval. Three important cycles of ~200, 100, and 40 k.y. may indicate some orbital control on regional sedimentation. High Eocene sedimentation rates on the STR suggest that some variation of the sedimentation at higher frequencies (104-105 yr) might be expected.
Leg 189 has provided a wealth of new and exciting biostratigraphic and paleoenvironmental information in a part of the Southern Ocean that is critical to understanding Earth's Cenozoic history. We drilled five sites in the Tasmanian region, north and south of the Subtropical Convergence, and in the Indian and Pacific Ocean sides of the STR (Fig. F17). The recovered record spans the Late Cretaceous to Quaternary (Fig. F18) but is commonly punctuated by a late Miocene hiatus and a possible Eocene-Oligocene hiatus (Fig. F19). The integrated microfossil record from all five sites will help unravel the southern history of the East Australian Current, which bathed the Pacific Ocean sites, and also the history of the "Proto-Leeuwin" Current, which bathed the Indian Ocean sites. The activity of both of these currents brought warmer waters to the study area, resulting in markedly different faunal associations when compared with other Antarctic circumpolar sites (e.g., Leg 113). The late Miocene hiatus is marked by a strong foraminiferal dissolution event and is restricted to the southernmost sites (Sites 1169, 1170, and 1171), although strong fragmentation of foraminifers was also observed at Sites 1168 and 1172.
The transition from "Greenhouse" to "Doubthouse" is strikingly seen at four of the five sites (Fig. F19). Latest Eocene to earliest Oligocene slow sedimentation (~1 cm/k.y.), which may be continuous or marked by hiatuses, records the history of this dramatic change in Earth's history. Five major microfossil groups are represented over this interval and provide the opportunity for shore-based collaboration to elucidate the details of this event. At Site 1172, a thick sequence of upper Cretaceous to Eocene fine-grained mudstones of the "Greenhouse" period, often glauconitic and organic rich, contains a continuous dinocyst record. Sporadic calcareous nannofossils and planktonic and benthic foraminifers through this interval will provide age control, as well as paleodepth information.
The planktonic foraminiferal distribution reflects the northward drift of Australia during the Cenozoic. Because of the southern location of Australia during the Paleogene, the subantarctic zonal scheme was used in place of the traditional temperate scheme. This temperate scheme was appropriate for the late Paleogene and Neogene. In addition, the Neogene faunal diversity, preservation, and phylogeny reflect changing paleoceanography throughout the "Icehouse" period. The expanded Oligocene and Neogene section of the two northern sites (with sedimentation rates between 1.7 and 4.3 cm/k.y.), and of Site 1168 in particular, means that a standard biostratigraphic section can be established for the shelfal and onshore exposures to the north (this applies also to the other microfossil groups). The Globoconella group is widely distributed through the Neogene section and phylogenetic studies coupled with paleoceanographic results should provide an improved planktonic foraminiferal biostratigraphy for the region. The low-diversity planktonic foraminiferal assemblages of the Late Cretaceous and Paleogene are generally very well preserved, but their abundances are low.
Benthic foraminifers at all sites show a clear change between neritic water depths in the Cretaceous to Eocene interval and deeper waters of the Oligocene and Neogene. At all sites except one (Site 1168), subsidence across the Eocene/Oligocene boundary appears to be a rapid event, whereas the trend at Site 1168 is more gradual. However, this site is the only one with an expanded Oligocene sequence. Eocene benthic foraminiferal assemblages indicate a more pronounced regional influence, whereas low-resolution sampling in the Oligocene and Neogene section suggests that the general trends at all sites are similar for the Neogene, with only Site 1168 showing clear differences in the Oligocene.
The Neogene standard nannofossil zonal scheme and the Paleogene nannofossil zonation for the Southern Ocean were successfully applied to Leg 189 sites to provide some of the most useful subantarctic temperate biostratigraphic records to date. In particular, the Oligocene to Pliocene interval is among the most detailed of the similar latitude Southern Ocean sites. This sequence will serve as an important reference section for the Southern Hemisphere. Eocene through lower Oligocene nannofossil assemblages at Leg 189 sites show warmer water characteristics than comparable paleolatitude sites in other sectors of the Southern Ocean (Sites 512, 513, 699, 703, and 747), which reflects the activity of the warm-water Proto-Leeuwin and the East Australian Currents.
Oligocene to Quaternary diatoms are abundant at all sites drilled during Leg 189, except Site 1168, and will be used to construct the first calibrated Oligocene-Holocene diatom biostratigraphy south of Australia. In particular, high-resolution biostratigraphy looks especially promising for Sites 1170 and 1171. Integration with other Paleogene and Neogene diatom biostratigraphies being developed from recent ODP legs to the Southern Ocean (e.g., Leg 177) will contribute to a scheme applicable to northern parts of the Southern Ocean. Neritic diatoms are prolific in upper Eocene and lowermost Oligocene sediments at Sites 1170, 1171, and 1172. In conjunction with dinocysts, they will be useful for reconstructing paleoenvironmental conditions across the Eocene/Oligocene boundary including productivity (trophic status), water energy levels, relative salinity, and sea level for both the Indian and Pacific Oceans. During the early Oligocene, the marked floral changes and increased diversity within Sites 1170, 1171, and 1172 imply increasing oceanic influence and productivity. The Neogene diatom floras indicate fluctuations in the influence of warm temperate and subantarctic water masses over Site 1170, heralding meridional shifts in the position of the Subtropical Convergence as well as the presence of the Proto-Leeuwin Current. At Site 1172, similar fluctuations may signal variations in the influence of the East Australian Current.
Radiolarians are well represented at all sites except Site 1168, although diversity varies through the sequences. The subantarctic radiolarian biostratigraphic sequence from the middle Eocene through Pleistocene is unique and will provide an important new radiolarian zonation. This subantarctic scheme will provide a correlation between tropical and Antarctic biostratigraphies even though there is a near absence of tropical and cold-water age-diagnostic species in the Tasmanian region. The radiolarian faunas from Leg 189 also provide an insight into radiolarian evolution.
The dinoflagellate-cyst studies have provided a number of important new observations. The first well-calibrated Neogene and Oligocene dinocyst record of the Southern Ocean was discovered at Site 1168. These microfossils are extremely abundant and, with sporomorphs, are common in the Upper Cretaceous to lowermost Oligocene successions. Together with palynofacies analysis, there is a great potential for well-calibrated high-resolution biostratigraphy. Dinoflagellate cysts appear to provide the sole biostratigraphic means for age assessment of the critical uppermost Eocene-lowermost Oligocene interval (the so-called "barren green sands"), although sporomorphs are present as well. Together with the diatoms, the palynomorphs will allow detailed paleoenvironmental interpretation. Postcruise paleoenvironmental and paleoclimatological studies, including "land-sea correlation" may be possible on time scales down to <20-k.y. cycles. Also, the first well-calibrated Paleogene dinocysts and sporomorph records from the Southern Hemisphere should result from postcruise studies. Cores have yielded the best imaginable material to study the variability of dinocyst morphology, notably within the Deflandrea phosphoritica and Areosphaeridium diktyoplokum-Enneadocysta partridgei groups.
Future stable isotopic studies on planktonic foraminifers, together with quantitative analysis of diatoms, nannofossils, and dinocysts, should be useful for detailed reconstruction of the sea-surface paleotemperatures, paleoproductivity, and other paleoenvironmental parameters, such as fluctuations in the position of the Subtropical Convergence. An outstanding biostratigraphic contribution resulting from Leg 189 will be an integrated zonation scheme, including the six microfossil groups for the Oligocene to the Quaternary for this sector of the Southern Ocean.
Paleomagnetism tends to be a difficult profession during carbonate legs. However, during Leg 189 we were fortunate enough to generate some magnetostratigraphy at all sites, and at Sites 1170, 1171, and 1172 the paleomagnetic record was sufficient for construction of useful age-depth plots. Our work is described in terms of its three main aspects: (1) magnetostratigraphy, (2) rock magnetism, and (3) investigation of magnetic overprints and measurement difficulties.
Magnetostratigraphic interpretation of the paleomagnetic record at Sites 1168, 1169, and 1170 was restricted to the Pliocene-Pleistocene and late Miocene. At Sites 1171 and 1172, the paleomagnetic records were adequate to generate a Neogene age-depth plot independent from biostratigraphic datums. At Site 1172, magnetostratigraphy was established down to the middle Miocene. Although the quality of the magnetic record deteriorated in older intervals, a relatively complete magnetostratigraphy was achieved, particularly across the Eocene/Oligocene boundary. Unfortunately, at both sites the interpretation of the Eocene inclination record was problematic, which led to the development of an approach based upon the sign of the z-component. This approach generated distinctive magnetostratigraphic boundaries, where they were almost totally obscured in the inclination record. The magnetostratigraphy for Leg 189 is summarized in Table T3, which displays the observed boundary chron depths at the various sites. Where there was more than one core at a given depth at a given site, the depths are taken from the best magnetostratigraphic record.
Throughout Leg 189, rock magnetism studies have revealed a remarkable homogeneity of magnetic material. Most discrete samples had anhysteretic remanent magnetization (ARM) intensities of order 10-3 A/m and isothermal remanent magnetization (IRM) intensities one order greater. We interpret the ARM/IRM ratio as typical for a detrital source. However, at Site 1168 this ratio was higher, suggesting that the magnetic record was carried in fine, single-domain grains probably of biogenic origin. At Sites 1170 and 1171, attempts to correlate magnetic properties (ARM/IRM, IRM 20/IRM, and IRM 500-IRM 200/IRM 500 ratios) with lithostratigraphic units were unsuccessful. Throughout much of the Cenozoic, magnetic properties were very similar and seem to reflect a similar source, as if the magnetic carriers were not influenced by environmental changes. However, some changes were weakly correlated with lithology, and the origin of the hard carriers observed in some sections remains to be determined from shore-based studies.
The techniques used during this leg varied somewhat from the standard methods. For the most part, we measured unsplit sections instead of archive-half sections. This was performed prior to the multisensor track (MST) measurement. Because the paleomagnetic measurement is faster than the MST track, the paleomagnetic analysis of the unsplit section was conducted in dead time, and thus, core flow was improved. A long-standing problem in the paleomagnetism of ODP-recovered sediments, and in particular in results from magnetically weak sediments such as carbonates encountered during this leg, has been the bias toward the "0°" declination reading. The analysis of the drift of the 2G cryogenic magnetometer indicates that one possible explanation for this phenomenon in very weakly magnetized sediments is improper application of drift corrections, although this needs to be investigated in more detail on shore. The use of nonmagnetic core-barrel assemblies permitted comparisons between magnetic results from standard and nonmagnetic assemblies, which did not appear to make a major difference to the magnetization of recovered cores, although further analysis on shore is also required.
In summary, during Leg 189 we encountered the usual difficulties of carbonate legs, but considerable magnetostratigraphy was achieved and aspects of measurement and analysis techniques have been improved. Of particular note is the measurement of unsplit sections to increase intensities and reduce potential overprinting from splitting and the utilization of the intensity of the z-component to interpret the magnetostratigraphy.
High-resolution (2-3 cm) whole-core gamma-ray attenuation (GRA) bulk density and magnetic susceptibility data collected on the MST show two distinct Cenozoic units: (1) an upper unit marked by very low magnetic susceptibility values and relatively low bulk density associated with the Oligocene to Quaternary pelagic carbonate deposits and (2) a lower unit with high and variable magnetic susceptibility values and relatively higher and more variable bulk density associated with the Cretaceous to late Eocene shallow-water siliciclastic deposits (Fig. F20).
The bulk-density data show the first-order effect of downhole compaction and dewatering, with lithologic variability superimposed on this trend. In addition, the bulk-density values in the lower unit are artificially low as a result of XCB and RCB coring techniques that recover a smaller diameter core. The magnetic susceptibility profiles, on the other hand, provide a clearer picture of the change from shallow-water siliciclastic sedimentation to more open ocean pelagic biogenic sedimentation. Lithologic variability, in particular the variation in carbonate and siliciclastic content, drives the first-order changes in the magnetic susceptibility data.
The high-resolution physical properties data suggest that the transition from shallow-water siliciclastic sedimentation to a more open ocean biogenic pelagic character is locally variable. The southern South Tasman Rise site (Site 1171) shows the first effects of this transition in the Eocene, although a full open-ocean carbonate biogenic sedimentation pattern is not present until the upper Oligocene/lower Miocene. The western South Tasman Rise site (Site 1170) changes to open-ocean carbonates in the upper Eocene/Oligocene. The more northern East Tasman Plateau (Site 1172) and Western Tasmania Margin (Site 1168) sites are marked by abrupt changes to open-ocean carbonates in the lower Miocene.
Both data sets exhibit sedimentary cycles superimposed on the first-order lithologic and depth-related changes. Shipboard biostratigraphic and paleomagnetic results suggest that the period of these cycles range from ~400 to 20 k.y. Detailed postcruise sediment geochemical, biostratigraphic, and paleomagnetic studies are required to differentiate the changing effects of terrigenous supply and biogenic production that influence the period of these sediment cycles and to quantify the evolution of sedimentation patterns (cyclicity) during the Cenozoic.
Downhole logging was conducted at four of the sites drilled during Leg 189 (Sites 1168, 1170, 1171, and 1172). The sequences logged included a range of siliciclastic and biogenic sediments deposited in a variety of Late Cretaceous to the late Neogene sedimentary environments. The results from all sites show that there are two distinct Cenozoic logging units: (1) an upper unit characterized by low natural gamma, magnetic susceptibility, resistivity, and an ~4 photoelectric value associated with the Oligocene to Quaternary pelagic carbonate deposits and (2) a lower unit with higher natural gamma, higher magnetic susceptibility, and lower photoelectric values associated with the Paleocene to late Eocene shallow-water siliciclastic deposits (Fig. F21).
Log parameters in the upper unit are fairly uniform with only small variations apparent in magnetic susceptibility, natural gamma, density, and resistivity indicating that relatively homogeneous, carbonate-rich, pelagic sediments have accumulated since the Oligocene. The small fluctuations in the natural gamma and susceptibility logs imply that, despite the predominance of pelagic carbonates, variations in the sediment's terrigenous content occurred throughout the Neogene. Minor changes in the terrigenous component could result from variations in the supply and delivery of terrigenous material to the site or, alternatively, may represent changes in the overlying production of biogenic carbonates, which dilute a relatively constant terrigenous supply. Discriminating between these various influences, each of which is related to different physical mechanisms with their own relationship to climate changes, will require detailed postcruise sediment geochemical studies.
In contrast to the relative stability of log parameters in the Neogene pelagic sequences, pronounced variability is evident in downhole logs within the underlying shallow-water Paleocene to Eocene siliciclastic deposits. This lower sequence is characterized by a general increase in the downhole natural gamma logs with depth, suggesting that the sediment's terrigenous fraction increases with age at every site, although the trend is most pronounced at the STR Site 1171. Likewise, the general increase in U-spectral gamma logs with depth suggests that the shallow-water siliciclastic Paleogene deposits have higher organic carbon content than the overlying open-ocean Neogene deposits. The Th/U > 2 suggests that the Paleogene depositional environment was dysoxic to oxic and less ventilated than the overlying pelagic carbonate sequence. Superimposed on this general trend of increasing natural gamma values with depth are intervals containing higher frequency cyclic variations in the spectral gamma, magnetic susceptibility, and resistivity logs at Sites 1170, 1171, and 1172. The regional persistence of these Th spectral gamma and magnetic susceptibility cycles suggests a common influence on sedimentation, either through changes in sea level or regional climate, which influenced sediment supply and/or delivery. Shipboard biostratigraphic results suggest that the period of these sediment cycles is near 20 or 40 k.y. using nannofossil and dinocyst datums, respectively.
The transition interval between the relatively homogeneous upper log units and the highly variable lower log units varies in thickness by site, occurring abruptly at Sites 1171 and 1172 but more gradually at Site 1168. In addition, correlative peaks in the density, resistivity, PEF, and K spectral gamma logs, associated with glauconite-rich sediments (typical of sediment-starved environments) are common in upper Eocene sediments just below the transition to the overlying deeper, open-ocean carbonate deposits.
The drilling of five sites in the region off Tasmania, in particular the four Sites 1168, 1170, 1171, and 1172, provided unique insights into both regional and temporal variation in organic and interstitial water geochemistry. The results of the organic geochemical characterization of sediments cored during Leg 189 have led to the following salient observations. First, a two-tiered carbonate distribution is characteristic of Sites 1170, 1171, and 1172. In this distribution, Paleogene sediments are generally carbonate poor, whereas Neogene sediments are carbonate rich. Second, the transition from Paleogene carbonate-poor to Neogene carbonate-rich sediments appears to be mostly abrupt, except at Site 1168, where it is gradual. Third, an antithetic relationship exists between carbonate and TOC content. In this relationship, Paleogene sediments usually contain >0.5 wt% TOC, whereas Neogene sediments are relatively free of organic matter. Fourth, organic matter type determined primarily by Rock-Eval pyrolysis and paleosalinity characterizations using C/S ratios provide a basis for intersite geochemical facies correlation from site to site. The facies show distinct, and perhaps important, changes at the Paleocene/Eocene boundary, in the middle Eocene, and near the Eocene/Oligocene boundary. Fifth, at Sites 1168, 1170, and 1171, methane contents obtained from headspace gas measurements show an abrupt increase in the organic carbon-containing Paleogene sediments.
Paleogene sediments with relatively high TOC and low CaCO3 contents exist at Sites 1168, 1170, 1171, and 1172 (Fig. F22). These sediments are bioturbated (see "Lithostratigraphy") and have higher natural gamma values (Fig. F21), in particular Th/U ratios mostly >2 (see "Physical Properties and Downhole Measurements"). These characteristics indicate dysoxic to oxic conditions on the seafloor during deposition. Most of the relatively high TOC-low CaCO3 sediments may have undergone enhanced burial efficiency of organic matter associated with more rapid siliciclastic sedimentation. The generally poorer ventilation of the waters during this time may have promoted high TOC-low CaCO3 sediments relative to the Neogene.
The high carbonate content of Neogene sediments reflects dominance of calcareous nannofossils; foraminifers are secondary in importance (see "Biostratigraphy"). The upward increase in carbonate content at all sites during Leg 189 is a consequence of a change from shallow-marine to pelagic open-ocean conditions. The low organic carbon content of these carbonate-rich sediments suggests deposition through a well-mixed water column to a well-oxygenated seafloor. The transition from carbonate-poor to carbonate-rich sediments appears to be relatively abrupt at Sites 1170, 1171, and 1172, although higher resolution sampling at Site 1172 demonstrates that the carbonate content changes through ~10 m of section. This observation suggests that higher resolution sampling across this boundary at Sites 1170 and 1171 may better define the nature of this carbonate-rich/carbonate-poor boundary. Specifically, the presence of a condensed section can be inferred if the carbonate content increases gradationally across the boundary, whereas the presence of an unconformity can be considered where an abrupt change in carbonate content is observed. At Site 1168, the change from siliciclastic to carbonate sedimentation is gradational, because terrigenous sedimentation was more continuous.
The similarity of geochemical facies between sites is significant because it suggests regional-scale changes in seafloor and water-column conditions. Upper Paleocene-lower Eocene sediments encountered at Sites 1171 and 1172 contain elevated TOC content and variations in C/S and hydrogen index values indicative of fluctuations between terrestrial and marine inputs (Fig. F22). These characteristics suggest rapidly changing environmental conditions across the Paleocene/Eocene boundary in the Tasmanian region. A middle Eocene episode of elevated organic carbon burial was widespread in shallow-marine environments. Correlating to this time, a clear gradient from terrestrially influenced to dominantly marine organic matter preservation is observed from Site 1170 to Sites 1171 and 1172. These observations may reflect deposition at Site 1170 within the Australo-Antarctic Gulf, whereas Sites 1171 and 1172 were more strongly influenced by the Paleogene Pacific Ocean.
The highest organic carbon contents observed during Leg 189 are from upper Eocene sediments at Site 1168. Here, TOC content exceeds 5 wt% and is nearly wholly terrestrial in origin. Such characteristics were not encountered at any of the other sites and likely reflect the proximity of Site 1168 to hinterland source regions, as well as the site location well within the restricted Australo-Antarctic Gulf. However, upper Eocene sediments at Sites 1168, 1170, and 1171 share a C/S record of brackish water conditions before the transition from carbonate-poor to carbonate-rich sedimentation. This signal is difficult to resolve within a setting of overall water deepening. Perhaps, the seaway conditions were deeper, but climatic fluctuations were sufficient to cause episodes of "freshening" of the water column and/or sediment pore fluids. Alternatively, the brackish water signature in C/S values may actually represent unrecognized periods of decreased pyrite formation. In either case, more analyses will be needed to understand the significance of the geochemical record from this interval.
Headspace gases at all Leg 189 sites are best characterized as biogenic, although thermogenic inputs were observed at the base of most holes. Rock-Eval pyrograms from Paleogene sediments often display a double S2 peak character, suggesting the presence of bitumen. Tmax values obtained by Rock-Eval pyrolysis indicate that organic matter is mostly immature, although values characteristic of the "oil window" were recorded at depth in most holes. Methane gas content is closely tied to pore-water sulfate concentrations at Sites 1168, 1170, 1171, and 1172 (Fig. F23), which in turn is closely tied to the lithostratigraphic separation between carbonate-rich and carbonate-poor sediments. The onset of methanogenesis immediately beneath the sulfate reduction zone exhibits the characteristics of microbially driven diagenetic depth zonation. In this model, carbonate-poor Paleogene sediments contain sufficient organic matter for complete sulfate reduction, whereas the sulfate reducers in carbonate-rich Oligocene and Neogene sediments inhibit methanogens. At Site 1172, however, the sulfate reduction process is likely inhibiting methanogenesis in the organic carbon-bearing Paleogene sediments. This observation is unusual because the TOC content of these low-gas sediments appears to be sufficient to have already driven sulfate concentrations to zero.
One of the surprising discoveries of the Leg 189 interstitial water geochemistry program was the presence of regionally extensive low-chloride (Cl-) pore fluids in the older sediments throughout the region, including on the west Tasmania margin (Site 1168), the STR (Sites 1170 and 1171), and the ETP (Site 1172) (Fig. F23). In general, the fresher pore waters are located in the older part of the cored interval but are in different aged sediments and are not restricted to specific lithologies. These fresher fluids are manifested in the Cl- profiles by multiple distinct maxima and minima rather than smoothly decreasing values. Minimum Cl- values at the four sites range from 486 to 440 mM, a 13%-21% decrease relative to the mean seawater value (559 mM). At three of the sites, pore-water freshening coincides with the onset of methanogenesis; however, low Cl- fluids were encountered at Site 1172, where only trace quantities of methane are present (Fig. F23).
At present, the origin of the low-Cl- fluids is enigmatic. Low Cl- values in marine pore waters have been observed in environments ranging from accretionary prisms to passive continental margins. One possible external source of low-Cl- fluids in passive continental margins is the advection of meteoric waters from the continent (e.g., Austin, Christie-Blick, Malone, et al., 1998). However, the geographic separation of the sites (Fig. F3) and the general geologic setting (see "Geologic Setting" in "Introduction") make it improbable that continental recharge occurred here.
Possible internal sources that may provide low-Cl- fluids include (1) gas hydrate dissociation; (2) dehydration reactions of hydrous minerals, such as clays and biogenic opal; (3) clay membrane ion filtration (e.g., Kastner et al., 1991; Hesse and Harrison, 1981; Paull, Matsumoto, Wallace, et al., 1996); and perhaps (4) connate fluids. Gas hydrate dissociation is a common cause of such profiles in continental margin settings. However, crude estimates of the base of the gas hydrate stability zone (GHSZ) at Sites 1168, 1170, and 1171, assuming a pure methane and seawater system, indicate that the low-Cl- fluids are below the stability zone. The depth of the hydrate stability zone will be sensitive to the chemistry of the pore fluids and incorporation of other gases into the hydrate structure (Dickens and Quinby-Hunt, 1997; Sloan, 1998). Therefore, a more rigorous calculation of gas hydrate stability may extend the depth of the stability zone. However, the presence of low-Cl- fluids at Site 1172, with only traces of methane present, appears to eliminate gas hydrates as a possible mechanism, at least at Site 1172.
We also currently cannot eliminate connate fluids as the possible source of fresher fluids. However, the distinct Cl- maxima and minima should have been smoothed by diffusional processes that are clearly at work with other interstitial water constituents (Fig. F23). In fact, the jagged nature of the Cl- profiles suggests that the emplacement of fresher fluids has occurred relatively recently. Clay mineral reactions are a well-known source of fresh fluids, and clay content increases in the older part of the sedimentary succession (see "Lithostratigraphy"). Although reaction kinetics are also important, previous research, both experimental and natural observations, indicates that dehydration reactions (e.g., alteration of smectite) occur at elevated temperatures of 80°C or higher (e.g., Perry and Hower, 1972; Velde, 1983). Downhole temperature measurements made during Leg 189 indicate geothermal gradients of ~50°-60°C/km, and all sites were drilled <1 km deep. Thus, the origin of these regionally extensive fresher pore fluids remains unresolved, and postcruise isotopic analysis of the interstitial waters will be required to better understand these diagenetically and/or paleoenvironmentally important fluids.
In addition to the low-Cl- fluids, pore-water profiles are characterized by reactions involving the alteration of silicate minerals, carbonate recrystallization, and organic matter degradation (Fig. F23). Alteration of silicate minerals, within and below the cored section, may have lead to covarying decreases in Mg2+ and K+ and increases in Li+ within the pore fluids. The smooth, diffusional Mg2+ and K+ profiles observed at all sites are likely the result of uptake in reactions below the cored interval. Carbonate recrystallization is most pronounced in the Oligocene and Neogene pelagic carbonates, resulting in increases in interstitial Sr2+. The thinnest carbonate sequence (Site 1171) has the lowest Sr2+ concentrations with the thinnest interval of elevated Sr2+ (cf. Sr2+ to carbonate profiles in Fig. F22). As described above, organic matter reactions are most active in the thick, organic-rich Eocene sediments in the lower part of the cored intervals, which may result in differing sulfate gradients (i.e., thicknesses of the sulfate reduction zone) as sulfate is rapidly depleted in the organic-rich Eocene sections.
