Kennett, Houtz, et al. (1975) proposed that the climate cooled and an Antarctic ice sheet (cryosphere) developed in late Eocene to early Oligocene time as the Antarctic Circumpolar Current (ACC) progressively isolated Antarctica thermally. Australia's movement north from Antarctica formed the Tasmanian Gateway and was considered to have triggered ACC formation and the global cooling that started ice sheet formation, initially on Antarctica and later in the Northern Hemisphere. While Paleogene rifting slowly opened the Australo-Antarctic Gulf (AAG) between the two continents, the Indian and Pacific Oceans remained separated by the almost continuous Tasmanian "land bridge" until the late Eocene, so no ACC existed. Circulation of warm water from the tropics warmed Antarctica through a complex series of climatic feedback mechanisms. Early drilling (Deep Sea Drilling Project [DSDP] Leg 29) in the Tasmanian Gateway between Australia and Antarctica provided a basic framework of paleoenvironmental changes associated with gateway opening (Kennett, Houtz, et al., 1975). However, the cored sequences were of insufficient quality and resolution to more fully test the potential interrelationships of plate tectonics, circumpolar circulation, and global climate.
Ocean Drilling Program (ODP) Leg 189, carried out in early 2000, was designed to assist with better understanding of the nature and timing of changes associated with gateway opening (Fig. F1). We aim in this contribution to provide a summary of the Leg 189 results that includes some postcruise studies, with special emphasis on the papers written for this Scientific Results volume. We present new information about the nature of and processes involved in the Paleocene and Eocene warm episode, late Eocene transitional climate, and Oligocene and later cooling (Fig. F2). Also, a large group of papers based largely on Leg 189 drilling will appear in an American Geophysical Union Geophysical Monograph (Exon et al., in press a); these are only briefly mentioned here. Implications for petroleum prospectivity were presented by Exon et al. (2001) and are not considered here. A detailed comparison with data from other ODP and DSDP sites and other stratigraphic information is provided in Exon et al. (in press b). It shows that, in nonabyssal sequences off southern Australia and New Zealand, the transition from siliciclastic to carbonate sedimentation generally occurred somewhere in the late Eocene or early Oligocene. In the Australian sector of the Antarctic margin, the Eocene–Oligocene transition is from nonmarine clastics to shallow glaciomarine siliciclastics and diatomaceous sediments.
During Leg 189, 4539 m of nearly continuous sediment core was recovered from five deepwater sites off Tasmania (Table T1), ranging in age from Late Cretaceous (75 Ma) to present day. The general character of the drilled sequences is summarized in Figures F3 and F4. The Paleogene sediments vary considerably among sites (Fig. F3), reflecting their varied locations with respect to the pre-Oligocene Tasmanian land bridge between Australia and Antarctica, their meridional position, and their tectonic differences. At that time, all the sites were located in high southern latitudes of 60°–70°S, compared to their present latitudes of 42°–48°S. There are also differences in the carbonates of the post-separation Oligocene and younger sequences.
In the Paleogene, the western Site 1168 was located in the narrow and tranquil AAG, west of the land bridge and connected to the Indian Ocean. This sequence on the west Tasmanian margin (cored to 883 meters below seafloor [mbsf]) recovered upper Eocene shelf mudstone and an almost continuous sequence of Oligocene and younger chalk and ooze.
The other sites were located in the Pacific Ocean and east of the culmination of the Tasmanian land bridge. This sector of the southwest Pacific ocean was relatively narrow and constricted during the Paleogene. Site 1170 on the western South Tasman Rise (STR) (cored to 780 mbsf) contains middle to upper Eocene shelf mudstone and lower Oligocene and younger chalk and ooze. This site lay closer to the developing ACC than Site 1168, and the ACC caused erosion or nondeposition of much of the middle Oligocene. At nearby Site 1169 (cored to 264 mbsf), middle Miocene and younger ooze was recovered.
Site 1171 on the southernmost STR (cored to 959 mbsf) consists of an upper Paleocene to upper Eocene shelf mudstone sequence and an almost complete upper Oligocene and younger chalk and ooze sequence. Hiatuses of latest Eocene and Oligocene age reflect increased bottom water flow. The Neogene section is relatively continuous, apart from a late Miocene hiatus. Site 1172 on the East Tasman Plateau (ETP) was farther from the ACC. An Upper Cretaceous to upper Eocene shelf mudstone sequence was recovered, along with Oligocene and younger chalk and ooze. Hiatuses were identified in the lowermost Paleocene, middle Paleocene, lower middle Eocene, and lowermost Oligocene.
Figure F3 compares the four deepest penetrating sites. As usual in ODP drilling, recovery was poorest in the deepest section. Upper Maastrichtian mudstones were cored only at Site 1172. Paleocene mudstones were cored at Sites 1171 (upper Paleocene only) and 1172 (a 75-m sequence with a major hiatus between the Danian and late Paleocene). Complete sequences of Eocene mudstones were cored at Sites 1171 (~600 m) and 1172 (~200 m), the thin sequence on the ETP probably representing its remoteness from source areas. Middle and upper Eocene mudstones were cored at Site 1170, and upper Eocene mudstones were cored at Site 1168 west of Tasmania. The thick (~300 m), primarily Oligocene sequence at Site 1168 grades eastward into rapidly thinning deepwater Oligocene chalks. 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. As outlined in the Leg 189 Initial Reports volume (Exon, Kennett, Malone, et al., 2001) and a brief synthesis in Eos (Exon et al., 2002) drilling showed that in the Tasmanian-Antarctic region there were three phases of sedimentation, clearly related to plate tectonic configuration and its influence on changes in ocean circulation and global climate. Depositional rates at Leg 189 sites varied considerably during the three phases and from location to location (Fig. F5). Warm "greenhouse" conditions persisted during the Late Cretaceous, Paleocene, and Eocene and were associated with deposition of shelf mudstone, the depositional rate of which (~2–5 cm/k.y.) matched the rapid subsidence on the rifting continental margins. In the late Eocene (37–33.5 Ma), early separation occurred between Australia and Antarctica in shallow water and the currents began to flow in the Tasmanian Seaway. This led to a second phase of deposition, consisting of slow sedimentation (<1 cm/k.y.) of glauconitic siltstone, which failed to keep up with subsidence of the margin. Rapid subsidence at the eastern sites commenced in the latest Eocene. By the earliest Oligocene (33.5 Ma) the seaway had opened substantially, and the third phase of deposition commenced, with slow deposition (1–2 cm/k.y.) of deepwater pelagic carbonates as Australia moved northward. Water depths no longer significantly increased following the Oligocene because subsidence rates had fallen off. The third phase covered both intermediate "doubthouse" conditions (33.5–15 Ma) and "Icebox" conditions (15–0 Ma).