During the Oligocene, Antarctica and the South Tasman Rise separated further (Fig. F7C). By the late Oligocene, the ACC was well established at all water depths south of the STR and currents also moved through the South Tasman Saddle between Tasmania and the STR. Much of the early Oligocene (~33–30 Ma) at all sites except Site 1168 is represented by a hiatus considered to be equivalent to the regional Marshall Paraconformity (Pfuhl and McCave, this volume) and to have been caused by initiation of the ACC. At Site 1168, farther to the north and to the west of Tasmania, the interval usually represented by the Marshall Paraconformity is represented only by an interval of reduced sedimentation rates (Pfuhl and McCave, this volume). In spite of Oligocene cooling, conditions remained temperate in the vicinity of Tasmania and the South Tasman Rise. By this time the development of the proto-ACC prevented a countercurrent like that of the late Eocene from flowing northward across the South Tasman Rise. As a result, the warm East Australia Current began to influence the Tasmanian region.
Relatively thin deepwater Oligocene chalks at Sites 1170–1172, where current action greatly compressed the section, grade westward into the thick (~300 m) marly Oligocene sequence at Site 1168. A brief hiatus seems to be present at the abrupt Eocene/Oligocene lithologic boundary at Sites 1170–1172 (Shipboard Scientific Party, 2001c, 2001d, 2001e; Fuller and Touchard, in press), although it can be clearly dated only at Site 1172 (Stickley et al., submitted [N1]). At the current-swept southern sites (1170 and 1171) the late Oligocene unconformity, common in much of the Southern Ocean, is well developed. Overall, the assemblages suggest well-ventilated cool temperate conditions and bathyal water depths at Sites 1170–1172. At Site 1172, the Oligocene sequence is ~20 m of pale foraminifer-bearing nannofossil chalk with some thin, greenish glass-bearing mudstone horizons (Shipboard Scientific Party, 2001e). Nannofossils and planktonic foraminifers dominate, but palynomorphs are largely absent. Diatoms and nannofossils (and dinocysts present in one sample) indicate relatively warm well-ventilated conditions and bathyal water depths. CaCO3 increases upcore from 55 to 85 wt%, with a rapid decline in siliciclastic debris, siliceous organisms, and organic walled palynomorphs as open-ocean and oxidizing conditions were established. At Site 1171, the Oligocene sequence is only ~10 m thick, and at Site 1170 it is ~60 m thick. At both sites it consists of pale foraminifer-bearing nannofossil chalk.
At Site 1168, the Oligocene sequence is represented by ~310 m of multicolored calcareous mudstone (Shipboard Scientific Party, 2001b). The ~40-m-thick lower Oligocene sequence consists of varicolored silty claystone, clayey siltstone, and sandy claystone with <20 wt% CaCO3. The ~270-m-thick upper Oligocene sequence is more calcareous (<40 wt% CaCO3). Robert (in press) shows that quartz, clay, and biogenic calcite are roughly subequal; illite again predominates over kaolinite, indicating reduction of relief but ongoing intense weathering. Calcareous nannofossils and planktonic foraminifers dominate, and molluskan fragments are present. Benthic foraminifers increased in diversity because oxygenation increased while the water depth deepened from neritic to upper bathyal. Dinocysts increasingly dominated over spores and pollen as the sea level rose and distance from land increased. Conditions were cool to warm temperate. This was a quiet, restricted, relatively oxygen poor environment. In contrast to the other sites, there is no abrupt lithologic change at the Eocene/Oligocene boundary, but rather a steady increase in water depth, a steady decrease in sand fraction (mainly quartz), and an increase in carbonate dominated by calcareous nannofossils through the Oligocene.
Oligocene carbonates are common in the Tasmanian region because of the interplay of tectonics, climate, and oceanography. The sequences at the southerly Sites 1170 and 1171 have a markedly different Eocene–Oligocene sediment transition compared with nearby parts of Antarctica. We do not know of Antarctic margin sectors that experienced pelagic carbonate deposition in the earliest Oligocene. The Antarctic margin was marked by deposition of biosiliceous sediments or more slowly accumulating siliciclastic sediments with an increased siliceous biogenic component. Why did the environment near the Tasmanian margin apparently favor biogenic carbonate preservation and relatively low biosiliceous productivity? Here, even Eocene siliciclastic sediments generally contain a better record of better preserved calcareous nannofossils and foraminifers than elsewhere.
These observations suggest that different climatic regimes existed near the Tasmanian and Antarctic margins during the Eocene and Oligocene. The earliest Oligocene was a time of major cryospheric expansion in the southern Indian Ocean sector and in the southern Ross Sea. But in the Tasmanian region, biogeographic evidence from calcareous nannofossils, as well as lack of any evidence for glaciation, indicate that conditions were slightly warmer than elsewhere, even during the Oligocene.
Why are carbonates preserved off Tasmania during the Oligocene but not on the nearby Antarctic margin? We hypothesize that warmer surface waters were carried southward from the subtropics, along the eastern margin of Australia by the East Australian Current, and southward around western Australia into the Australo-Antarctic Gulf. The beginning of constriction of the Indonesian Seaway in the Oligocene (Hall, 1996) would have increased southward flow of warm waters along the east Australian margin. These subtropical waters would have been relatively saline and thus would have helped promote production of deep waters. Hence, this sector of the margin may have operated in an anti-estuarine mode (Berger et al., 1996), marked by downward flux of deep waters and inward flow of surface waters, as in the modern North Atlantic. In this case, upwelling of nutrient-rich waters is diminished and carbonate accumulation is favored over biosiliceous accumulation.
The Antarctic margin was already separated from warm waters by the onset of the ACC. There was strong carbonate dissolution at shallow water depths and high biosiliceous accumulation, and the margin may have operated in estuarine mode, marked by upwelling of nutrient-rich deep waters and outflow of surface waters. There, carbonate dissolution is favored by the upwelling of old, deep, low-alkalinity, high-pCO2 waters like those in the modern North Pacific Ocean.
A major strengthening of oceanic thermohaline circulation occurred at the climatic threshold of the Eocene–Oligocene transition. This resulted largely from the major cooling and cryospheric development of Antarctica (Kennett and Shackleton, 1976). This cooling, in turn, led to increased onshore aridity and a major reduction of freshwater flow to the surrounding continental margin, which is reflected by the marked reduction in transport of siliciclastic sediments to the Tasmanian margin. Surface waters near the margin would have increased in salinity. A major positive feedback almost certainly would have resulted, with further strengthening of bottom water production and expansion of the oceanic psychrosphere (deep-ocean circulation). Thus, the delivery of high-salinity surface waters to the Tasmanian margin, caused by its plate tectonic setting, may well have enhanced bottom water production and, in turn, increased carbonate biogenic accumulation.
Neogene sedimentation at Leg 189 sites on the STR and the Tasmanian margin was completely dominated by nannofossil oozes with a significant foraminiferal component. Pelagic carbonate sedimentation was largely continuous, except during the late Miocene and earliest Pliocene, at a number of sites. Miocene deepwater calcareous ooze is thickest at Site 1168 (~300 m) and thinnest at Site 1171 (~170 m). Pliocene oozes are remarkably consistent in thickness (~70 m) at all sites. Pleistocene oozes are thickest at the deepwater STR Site 1170 and thinnest at the shallower, current-swept STR Site 1171. The Miocene–Pliocene transition is missing at Sites 1169 and 1171, and the lower upper Miocene is missing at Site 1168. Otherwise, the lower and upper Miocene and the Pliocene to Quaternary appear to be largely complete sequences. The uppermost Miocene hiatus may have resulted from increased thermohaline circulation associated with Antarctic cryosphere expansion at that time (Hodell et al., 1986). Altogether, the Neogene sediments cored during Leg 189 provide a fine suite of sequences deposited in cool temperate and subantarctic water masses of the Southern Ocean. These represent a treasure chest for high-resolution Neogene paleoclimatic and biostratigraphic investigations of the Southern Ocean. The Neogene carbonates exhibit changes that record changing environmental conditions in response to the northward movement of the STR, Tasmania, and the ETP from Antarctica and shifting positions of the Subtropical Convergence and the Subantarctic Front. The pelagic carbonates accumulated at relatively low rates (~1–2 cm/k.y.) typical of the open ocean. Relatively low diversity benthic foraminiferal assemblages indicate deposition in abyssal depths under generally well ventilated conditions characteristic of the Antarctic Circumpolar Current region. Other than a small, pervasive clay fraction, siliciclastic sediments are absent throughout the Neogene, except in the lower Neogene of Site 1168, which is the site closest to a present land mass. Nannofossil oozes are conspicuously pure white on the STR in the lower Neogene, which corresponds to a period when the STR was well clear of the siliciclastic influences of Antarctica and yet had not come under the late Neogene influence of increasing aridification and associated dustiness of Australia. Diatoms are present consistently throughout the Neogene carbonates but exhibit a distinct increase in abundance and diversity after the middle Miocene. This almost certainly reflects an increase in upwelling within the Southern Ocean at that time in response to the well-known expansion of the Antarctic cryosphere. A marked increase in carbonate ooze deposition during the early Pliocene at Sites 1169 and 1170, on the southwestern STR, is not observed at other sites, suggesting local concentrations of calcareous nannofossils rather than any regional trend like that in the southwest Pacific (Kennett and von der Borch, 1986). During the latest Neogene, planktonic foraminifers become much more important relative to calcareous nannofossils. This may reflect increased winnowing by deep currents and/or a decrease in relative production of calcareous nannofossils compared to planktonic foraminifers.
Postcruise investigations of Leg 189 Neogene sequences are leading to a significant increase in understanding of paleoclimatic and paleoceanographic history of the Southern Ocean. Upper Neogene sections have been satisfactorily spliced from multiple cores from four of the sites (Sites 1168 and 1170–1172) to provide essentially continuous paleoclimatic records. Pervasive sedimentary cycles are apparent throughout the entire Neogene, based on observations of the sediment record and changes in the physical properties of the sediments. Investigations of clay assemblages suggest relatively warmer conditions during the early Neogene until ~15 Ma. After that, clay assemblages show increases in chlorite, illite, and/or kaolinite, suggesting general regional cooling and Antarctic glacial expansion (Robert, this volume).
Sequences cored during Leg 189 have provided stable isotopic records with the highest chronologic resolution so far of the early Miocene (Ennyu and Arthur, this volume, in press) and the middle Miocene (Shevenell and Kennett, in press) from the Southern Ocean. These well-dated stable isotopic records clearly exhibit the well-known major oxygen isotopic shift of the middle Miocene at ~14 Ma as well as regional ocean circulation changes (at depths >1500 m) commensurate with the middle Miocene global climate transition (16.8–12 Ma). Regional oxygen and carbon isotopic trends have been considered to support hypotheses relating middle Miocene cooling and Antarctic cryosphere expansion to reorganization of ocean circulation and related changes in meridional heat flux (Shevenell and Kennett, in press).
Kelly and Elkins-Tanton (in press) describe an occurrence in a single sample of microtektites from the upper middle Miocene–lowermost Pliocene of Site 1169. Although precise biostratigraphic dating of deposition is not possible, by using major element composition they attribute the origin of the microtektites to the HNa Australite field considered to be of late Miocene age (~10.2 Ma) (Bottomley and Koeberl, 1999).
During the late Neogene, clays become increasingly important in the pelagic carbonates, in part because of increasing dust transport from Australia. A distinct influx of kaolinite at several sites, including the southern STR, during the late Pliocene and Quaternary probably reflects increasing southeastward wind transport of relict clays from an increasingly arid Australia. The uppermost Neogene sediments at Site 1172, which is downwind from Tasmania, exhibit distinct cycles in clay abundance in the biogenic oozes, almost certainly in response to glacial–interglacial oscillations in Australian continental aridity. This increase in aridity was almost certainly linked with a cooling trend during the late Neogene. The disappearance in the Tasmanian region of large gephyrocapsids above the middle Pleistocene (Stant et al., this volume) is consistent with this long-term trend toward cooler conditions. Stant et al. (this volume) also showed from the distribution of warm-loving discoasters in Pliocene sediments the presence of a subtropical watermass front (~45°S), south of which the discoasters did not extend during the late Pliocene. Furthermore, their distribution indicates the stronger influence of the East Australia Current compared with the Leeuwin Current in transporting subtropical waters to the region during the late Pliocene.
The Leg 189 sites also provided opportunities for studies of Quaternary paleoclimatology. Nürnberg et al. (in press) used geochemical proxy data from four Leg 189 sites to reconstruct the regional history of glacial and interglacial changes near the subtropical convergence in the last 500 k.y. There is a complex story of variations in paleoexport production, terrigenous flux, sea-surface temperature, and movements of water masses and oceanographic fronts through time. Each glacial period and each interglacial period was different from the others; however, the authors did find that interglacial periods were commonly times of lower productivity and that their deposits contained less terrigenous matter than those of glacial periods, indicating that the subtropical convergence was south of most sites during most interglacials.
Malone et al. (2004) used a diffusion-advection model to calculate the glacial–interglacial change in bottom water 
18O from pore water oxygen isotopic profiles at Sites 1168 and 1170. The results indicate that Circumpolar Deep Water temperatures were –0.2°C (Site 1170) and –0.5°C (Site 1168) at the Last Glacial Maximum. Since the last glacial maximum, 18O changed by 1.0
–1.1 (±0.15) and bottom water temperatures increased by ~1.9° and ~2.6°C, respectively, at the sites.
