ABSTRACT
The Cenozoic Era is unusual in its development of major ice sheets. Progressive cooling at high latitudes during the Cenozoic eventually formed major ice sheets, initially on Antarctica and later in the Northern Hemisphere. In the early 1970s, a hypothesis was proposed that climatic cooling and an Antarctic cryosphere developed as the Antarctic Circumpolar Current progressively thermally isolated the Antarctic continent. This current resulted from the opening of the Tasmanian Gateway south of Tasmania during the Paleogene and the Drake Passage during the earliest Neogene.
The five Leg 189 drill sites, in 2463 to 3568 m water depths, tested the above hypothesis and refined and extended it, greatly improving understanding of Southern Ocean evolution and its relation with Antarctic climatic development. The relatively shallow region off Tasmania is one of the few places where well-preserved and almost-complete marine Cenozoic carbonate-rich sequences can be drilled in present-day latitudes of 40°-50°S, and paleolatitudes of up to 70°S. The broad geological history of all the sites was comparable, although there are important differences among the three sites in the Indian Ocean and the two sites in the Pacific Ocean, as well as from north to south.
In all, 4539 m of core was recovered with an excellent overall recovery of 89%, with the deepest core hole penetrating 960 m beneath the seafloor. The entire sedimentary sequence cored is marine and contains a wealth of microfossil assemblages that record marine conditions from the Late Cretaceous (Maastrichtian) to the late Quaternary and terrestrial conditions until the earliest Oligocene. The drill sites are on submerged continental blocks extending as far as 600 km south of Tasmania. These continental blocks were at polar latitudes in the Late Cretaceous when Australia and Antarctica were still united, although rifts had developed as slow separation and northward movement of Australia commenced. The cores indicate that the Tasmanian land bridge, at polar latitudes, completely blocked the eastern end of the widening Australo-Antarctic Gulf, during both the slow-spreading phase and the fast-spreading phase (starting at 43 Ma), until the late Eocene.
Prior to the late Eocene, marine siliciclastic sediments, largely silty claystone, were deposited in a relatively warm sea on broad, shallow tranquil shelves. Sediment supply was rapid and despite the rifting, drifting, and compaction, largely deltaic deposition kept up with subsidence. Calcareous and siliceous microfossils are sporadic, and dinocysts, spores, and pollen are ever present. The spores and pollen are compatible with, throughout this time, this part of Antarctica being relatively warm with little ice and supporting temperate rain forests with southern beeches and ferns--part of the "Greenhouse" world. Differences in the claystones between east and west indicate that the eastern Australo-Antarctic Gulf was more poorly ventilated than the gradually widening Pacific Ocean with its western boundary current, the East Australian Current. However, currents from low latitudes warmed both sides of the land bridge.
In the late Eocene (37 Ma), the Tasmanian land bridge had separated from Antarctica, the bridge and its broad shelves began to subside, and cool surface currents started to circulate around Antarctica from the west. These swept the still-shallow offshore areas, and glauconitic siltstones were deposited very slowly as condensed sequences. Palynological and other evidence suggests that there were considerable 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 are rare, but diatoms and foraminifers indicate that some sites began to reach bathyal depths in the latest Eocene (34 Ma).
By the early Oligocene, warm currents from the tropics were cut off by the developing Antarctic Circumpolar Current from some parts of Antarctica, leading to cooling and some ice sheet formation. This contributed to global cooling. Conditions were significantly cooler in the Tasmanian offshore region, and there is no positive evidence of land vegetation although this organic matter could have been oxidized during deposition in well-ventilated waters. 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, decreasing precipitation, and erosion. Altogether, far less siliciclastic sediment was transported from the land and generally slow deposition of deep-water pelagic sediments set in. However, the East Australian Current and currents moving down the western Australian margin continued to keep the Tasmanian region relatively warm, resulting in carbonate deposition rather than the siliceous biogenic deposition that marks much of the Antarctic margin. Furthermore, in the Tasmanian region, and even in the Cape Adare region on the conjugate Antarctic margin, there is no sign of glaciation during the early Oligocene.
The Drake Passage opened early in the Neogene, and the Tasmanian Seaway continued to open, strengthening and widening the Antarctic Circumpolar Current and strongly isolating Antarctica from warm-water influences from lower latitudes. At ~15 Ma, the east Antarctic cryosphere evolved into ice sheets comparable to those of present day. This intensified global cooling and thermohaline circulation. The "Icehouse" world had arrived. However, temperatures and current activity fluctuated, and dissolution and erosion varied over time. The Tasmanian region had been moving steadily northward so that its sediments were never south of the Polar Front, and pelagic carbonate continued to accumulate in deep waters at average rates of 1-2 cm/k.y. The upper Neogene sequences contain windblown dust from Australia, which was moving progressively northward into the drier midlatitudes. Along with global climate change associated with high latitude ice sheet expansion, this led to the massive aridity of Australia and an increase in dust abundance in some sequences after 5 Ma.
Comparisons with sequences drilled elsewhere on the Antarctic margin will further improve our understanding of these momentous changes in Earth's history and some of the constraints on modern climates. If Australia had not broken away from Antarctica and moved northward, global climate may well have remained warm. We can now document in some detail the changes related to that tectonic movement. Tectonic information indicates that there were three critical tectonic events during the Cenozoic in the Tasmanian region: Paleocene strike-slip movement, terminating at 55 Ma by seafloor spreading south of the South Tasman Rise (STR); Eocene strike-slip movement along the western boundary between the STR and Antarctica, terminating in the latest Eocene around 34 Ma; and early Oligocene subsidence of the STR and collapse of the continental margin around Tasmania. The early Oligocene subsidence and collapse also occurred in the Victoria Land Basin east of the rising Transantarctic Mountains (Cape Roberts Scientific Team, 2000) and along the Otway coast on mainland Australia, northwest of Tasmania.
Postcruise studies and comparisons will better define and explain regional similarities and differences in tectonism, sedimentation, and climate. Initial studies of physical properties, wireline logs, and microfossils all show that climatic cycles of varying length are present throughout the entire sequence, and postcruise studies will better define Milankovitch and other cycles. In the Neogene pelagic carbonates, the excellent preservation and high depositional rates will allow detailed isotope studies of surface- and bottom-water temperatures through time. There is a unique opportunity to build Southern Ocean correlations between various microfossil groups--calcareous nannofossils, planktonic foraminifers, diatoms and radiolarians, and dinocysts, spores, and pollen. Geochemical studies have identified some unusual trends in the sequences, and the Paleogene sequence contains thin, almost-mature hydrocarbon source rocks at most sites.
Leg 189 results essentially encapsulate the evolution during the Cenozoic of the Antarctic system from "Greenhouse" to "Icehouse." The changes recorded in the cored sequences clearly reflect evolution of a tightly integrated, and at times dynamically evolving, system involving the lithosphere, hydrosphere, atmosphere, cryosphere, and biosphere.