As noted in Exon, Kennett, Malone, et al. (2001), the Leg 189 drill sites, in 2463–3568 m water depths, have tested, refined, and extended the hypothesis that climatic cooling and an Antarctic ice sheet (cryosphere) developed in late Eocene to early Oligocene times, as the ACC progressively isolated Antarctica thermally (Kennett et al., 1975). This has led to improved understanding of Southern Ocean evolution and its relationship to Antarctic climatic development. The relatively shallow region off Tasmania is one of the few locations where well-preserved and almost complete marine Cenozoic sequences can be drilled at present-day latitudes of 40°–50°S and paleolatitudes of up to 70°S. The Oligocene and younger sequences are carbonate rich and not deeply buried and are hence suitable for stable isotopic investigations. The broad geological history of all the sites is comparable, although there are important pre-Miocene differences between Site 1168 in the AAG and the sites in the Pacific Ocean, as well as from north to south.
The drill sites are on submerged continental blocks extending to 600 km south of Tasmania. These blocks were at polar latitudes in the Late Cretaceous when Australia and Antarctica were still united, although rifts had developed associated with commencement of slow separation and northward movement of Australia. In all, 4539 m of core was recovered with average recovery of 89%. The deepest core hole penetrated 960 m beneath the seafloor. The entire sedimentary sequence cored is marine and contains varied microfossil assemblages that record conditions from the Late Cretaceous (Maastrichtian) to the late Quaternary. Until the earliest Oligocene, terrestrially derived siliciclastic sediments predominate at all sites. During the Oligocene there was an abrupt change to pelagic carbonate deposition at the eastern sites, but siliciclastic debris remained important into the early Miocene at Site 1168.
The record in the cores indicates that the Tasmanian land bridge almost completely blocked the eastern end of the widening AAG during both the slow-spreading phase and the fast-spreading phase (starting at ~43 Ma) until the late Eocene. Drilling evidence, especially that from clay minerals, and other geological and geophysical evidence points to a number of tectonic events during the Cenozoic in the Tasmanian region:
The early Oligocene subsidence and collapse also occurred in the Victoria Land Basin east of the rising Transantarctic Mountains (Cape Roberts Science Team, 2000) and along the Otway coast on mainland Australia, northwest of Tasmania.
Prior to the late Eocene, marine siliciclastic sediments (largely mudstones at Leg 189 sites) were deposited in a temperate sea on broad, shallow, tranquil shelves. There was little or no circulation of marine waters between the AAG and the Pacific Ocean. Sediment supply kept up with subsidence despite the rifting, drifting, and compaction during largely deltaic deposition. Dinocysts, spores, and pollen are ever present. Reducing conditions in the often organic-rich sediments helped ensure that especially calcareous and, to some extent siliceous, microfossils were preserved only sporadically. The spores and pollen indicate that this part of Antarctica was temperate (with little ice) during this time and supported rain forests with southern beeches and ferns—part of the Late Cretaceous to Eocene "Greenhouse" world. During the late Eocene the sequences still document marked differences between east and west, when the eastern AAG was warmer and more poorly ventilated than the gradually widening Pacific Ocean. Hence, marine circulation across the former land bridge must still have been very limited. Microfossil biogeography suggests that the east Tasmanian region (Sites 1170–1172) was influenced by a northwestward-flowing cool countercurrent during the Eocene (Fig. F7A). This circulation pattern may well have been in operation from the Maastrichtian, when the proto-Pacific Ocean first existed east of Australia and north of Antarctica (Cande and Stock, in press), until the beginning of the Oligocene.
By the late Eocene (37 Ma), the Tasmanian land bridge had largely separated from Antarctica and the bridge and its broad shelves began to subside. Surface currents affected the deepening shelves. These swept the still-shallow offshore areas, and glauconitic siltstones were deposited slowly as condensed sequences. Palynological and diatom evidence suggest that there were fluctuations in temperature superimposed on a general cooling and that the amount of upwelling also fluctuated in response to the changing oceanic circulation. Calcareous microfossils remained rare. Benthic foraminifers and other evidence indicate that the sites began to deepen slightly at ~37 Ma, although shelf depths continued. Final separation of the southwestern tip of the STR from Antarctica occurred at ~33.5 Ma, leading to profound changes in sediment deposition, climate, and ocean history (Fig. F7B).
By the earliest Oligocene, bathyal pelagic carbonates were being deposited at the eastern sites and marls at Site 1168. The developing Antarctic Circumpolar Current cut off warm currents from the north, leading to cooling and some ice sheet formation. These events contributed to global cooling. Conditions were significantly cooler in the Tasmanian offshore region, and there is no positive evidence of terrestrial vegetation in the sediments, although vegetation then existed on Tasmania. However, almost all organic matter deposited during the early Oligocene would have been oxidized in well-ventilated waters.
There were several reasons for the change to pelagic carbonate deposition. Much of the land bridge had subsided beneath the ocean, so there was a smaller hinterland to supply sediment. Furthermore, the colder ocean provided less moisture and, hence, decreasing precipitation and erosion. Therefore, far less siliciclastic sediment was transported from the land. The reduced flow of detrital organic matter ended the earlier reducing conditions in the sediments and ensured that calcareous organisms were now preserved. Generally slow deposition of deepwater pelagic sediments was initiated. Currents from the north kept the Tasmanian region relatively warm, supporting carbonate deposition rather than the siliceous biogenic deposition that marks much of the Antarctic margin. In the Tasmanian region, and even in the Cape Adare region on the conjugate Antarctic margin, there is no sign of widespread glaciation during the early Oligocene.
The Drake Passage probably opened in the Oligocene, and the Tasmanian Seaway continued to open. In the late Oligocene (Fig. F7C) and Neogene, the ACC strengthened and widened, strongly isolating Antarctica from warm-water influences. At ~15 Ma, the east Antarctic cryosphere evolved into ice sheets comparable to those of the present day. This intensified global cooling and thermohaline circulation. The "Icehouse" world had arrived, but temperatures and current activity fluctuated and dissolution and erosion varied over time. The steady northward movement of the Tasmanian region kept sedimentation north of the Polar Front, and pelagic carbonate continued to accumulate in deep waters at average rates of 1–2 cm/k.y. Australia's movement northward into the drier mid-latitudes, along with the global climate change associated with high-latitude ice sheet expansion, led to massive aridity in Australia and an increase in windblown dust abundance at Site 1172 after 5 Ma.
Comparisons with sequences drilled elsewhere on the Antarctic margin are improving our understanding of these momentous changes in Earth history and some of the constraints on modern climates. We suggest that if Australia had not broken away from Antarctica and moved northward, the Earth might not have experienced its Cenozoic ice ages.