The area between Australia's southernmost prolongation (Tasmania and the South Tasman Rise [STR]) and Antarctica is a key to understanding global Cenozoic changes in climate and current patterns, involving
The opening of the Tasmanian Gateway between Australia and Antarctica and the only other important constriction in the establishment of the Antarctic Circumpolar Current, the Drake Passage, had enormous consequences for global climate. These consequences came in part by isolating Antarctica from warm gyral surface circulation of the Southern Hemisphere oceans and also by providing the necessary conduits that eventually led to ocean conveyor circulation between the Atlantic and Pacific Oceans. Both factors, in conjunction with positive feedbacks and other changes in the global system, have been crucial in the development of the polar cryosphere, initially in Antarctica during the Paleogene and early Neogene and later in the Northern Hemisphere during the late Neogene. Furthermore, the continued expansion of the Southern Ocean during the Cenozoic, because of the northward flight of Australia from Antarctica, has clearly led to further evolution of Earth's environmental system and of oceanic biogeographic patterns.
The geographic position of the Tasmanian offshore region makes this a crucial location to study the effects of Eocene-Oligocene Australia-Antarctic separation on global paleoceanography. Australia and Antarctica were still locked together in the Tasmanian area until late in the Eocene, preventing the establishment of Antarctic circumpolar circulation (Fig. F1). At that time, and earlier, the water masses were separated on either side of the barrier in the southern Indian and Pacific Oceans and most likely exhibited distinct physical, chemical, and biological properties. The Tasmanian region is also well suited for the study of post-Eocene development of Southern Ocean climate, feedbacks that contribute to ice-sheet development and increased stability, and formation and variation of high-latitude climate zones. This region is one of the few ideally located in the Pacific sector of the Southern Ocean for comparison with the models of Cenozoic climate development and variation in the Indian and the South Atlantic Oceans. Therefore, an outstanding question is whether paleoceanographic variability, known from the Atlantic and Indian sectors, is characteristic of the entire Antarctic circumpolar ocean or whether there are zonal asymmetries in the Southern Ocean and, if so, when these developed.
The meridional spread of the sites on the STR (Fig. F3) is well suited for monitoring the migration of oceanic frontal zones through time, analogous to transects on the Southeast Indian Ridge (SEIR) (Howard and Prell, 1992). The total meridional displacement of oceanic fronts on the STR is expected to be somewhat less than on the SEIR because the STR is a shallower topographical barrier to the Antarctic Circumpolar Current. The East Tasman Plateau (ETP) site is ideally located to monitor paleoceanographic changes at the interface between the East Australian Current and the Antarctic Circumpolar Current because the East Australian Current transports heat into the Southern Ocean, an important "gateway" objective.
The sites cored during Leg 189 also provide high-quality Neogene paleoclimatic and paleoceanographic records, including the Quaternary, from the southern temperate and subantarctic regions. These sequences are being employed to examine the development of surface-water productivity, oscillations in subtropical and polar fronts, changing strength of the East Australian Current, and changes related to further expansions of the Antarctic cryosphere during the middle and late Miocene.
Previous investigations have demonstrated that the Southern Ocean late Quaternary paleoceanographic record, manifested in its temperature response (Howard and Prell, 1992) and carbon cycling (Howard and Prell, 1994; Oppo et al., 1990), mirrors that of the Northern Hemisphere. This suggests similar cryospheric, atmospheric, and oceanographic variability in Southern Ocean climate during the past 500 k.y. as that of the Northern Hemisphere (Imbrie et al., 1992, 1993). However, on the Milankovitch band there appears to be a lead in the Southern Ocean, perhaps reflecting the importance of this region. For example, the potential role of Southern Ocean paleoproductivity on global climate remains a topic of significant interest. Despite the excellent documentation of latest Pleistocene Southern Ocean paleoclimate history, where variability is dominated by 100-k.y. cycles, there is a lack of fully cored sections to address the mid-Pleistocene (900 ka) transition from obliquity-dominated cycles (40-k.y. periodicity) to eccentricity-dominated (100-k.y. periodicity) cycles (Ruddiman et al., 1989) in this region. A documented Southern Ocean section over this "transitional" period was based on poor recovery (Hodell and Venz, 1992), so this important transition in global climate remains to be properly documented. However, two giant piston cores taken on the STR (Marion Dufresne, 1997) provide excellent records back to 900 ka, including this transition in orbital forcing mechanisms. The STR Ocean Drilling Program (ODP) sites will add to this record and complement subantarctic South Atlantic transect sites (Leg 177) in documenting this transition.
Major questions addressed during Leg 189 include the following:
An understanding of Cenozoic climate evolution has required better knowledge of the timing, nature, and responses of the Paleogene opening of the Tasmanian Seaway (Figs. F1, F2). Early ocean drilling in the Tasmanian Seaway (Deep Sea Drilling Project [DSDP] Leg 29) provided a basic framework of paleoenvironmental changes associated with its opening but was of insufficient quality and resolution to fully test the hypothesis of potential relationships among the development of plate tectonics, circumpolar circulation, and global climate. Until now, the timing of events has remained insufficiently constrained.
The relatively shallow region off Tasmania (mostly above the present carbonate compensation depth [CCD]) is strategically well located for studies of the opening and later expansion of the Tasmanian Seaway. It is also one of the few places where almost-complete marine Eocene to Holocene carbonate-rich sequences can be drilled in present-day latitudes of 40°-50°S and paleolatitudes of up to 70°S (Fig. F4).
The Tasmanian region lay within the continent of Gondwana until breakup started during the Late Cretaceous (Fig. F5). Rifting related to the separation of Antarctica and Australia may have started as early as the Late Jurassic, and by the Early Cretaceous there was a well-developed east-west rift system along the southern margin of Australia that passed north of Tasmania through the Bass Strait (Willcox and Stagg, 1990). The rift sequences in outcrop and petroleum exploration wells are volcaniclastic fluviatile and lacustrine sediments thousands of meters thick in places. The volcanism was basic to andesitic, and the Lower Cretaceous sediments are dominantly immature lithic conglomerates, sandstones, and mudstones, with some better sorted quartz-rich sandstone bodies. They were probably derived from volcanism along what is now the east coast of Australia.
During the beginning of the Late Cretaceous, the early rifting in the Bass Strait failed and a northwest-southeast zone of strike-slip faulting, west of Tasmania, absorbed motion related to the continuing east-west rifting (Fig. F5). This motion eventually separated Australia and Antarctica (Figs. F6, F7). During the Late Cretaceous, the sea intruded into the rift from the west, along the gulf between Australia and Antarctica, here named the Australo-Antarctic Gulf (Fig. F1). Data from petroleum exploration wells show that coastal plain to shallow-marine detrital sediments were deposited along the east-west rift (Smith, 1986; Lavin, 1997; McKerron et al., 1998) and also along the northernmost part of the zone of strike-slip faulting (Moore et al., 1992). In depocenters near Tasmania, these sediments are relatively mature, quartz-rich and 1000-2000 m thick (Moore et al., 1992). However, southwest of Tasmania, dredging has recovered immature, shallow-marine lithic sandstones and mudstones (Hinz et al., 1985; Exon et al., 1992) of Late Cretaceous age that are reminiscent of the Lower Cretaceous rocks farther north. Seismic reflection profiles show that these sequences are frequently progradational and deltaic (Hill et al., 1997b).
Australia's Eastern Highlands were uplifted at the end of the Early Cretaceous at ~95 Ma (O'Sullivan et al., 1995), and rifting commenced between Australia to the west and the Lord Howe Rise and the Campbell Plateau to the east. During the Campanian (75 Ma, Chron 33), drifting of the latter elements to the east-northeast was well established (Royer and Rollet, 1997), and the eastern margin of Australia/Tasmania/STR began to collapse. In the upper Paleocene to lower Eocene, apatite fission dating indicates uplift and erosion along the western and eastern margins of Tasmania (O'Sullivan and Kohn, 1997).
In the latest Cretaceous to Eocene, the east-west rift continued to fill with prograding shallow-marine detrital sediments and coal-bearing strata. The depression along the strike-slip zone also filled with prograding sediments, and seismic interpretation suggests the depocenter moved southward with time relative to Tasmania, with Paleocene sedimentation dominating in the north and Eocene in the south (Hill et al., 1997b). Paleogene sediments are as thick as 1500 m in places. In the Oligocene, Australia cleared Antarctica, its margins subsided, and deposition of relatively thin hemipelagic, pelagic, and shallow-water carbonate predominated thereafter.
Today, the Tasmanian offshore region consists of continental crust of the Tasmanian margin (Moore et al., 1992; Hill et al., 1997b), the STR (Hinz et al., 1985; Exon, et al., 1996, 1997b), and the ETP (Exon et al., 1997a) and is bounded on all sides by oceanic abyssal plains (Fig. F3). Oceanic crust to the east was created by the seafloor spreading that formed the Tasman Sea in the Late Cretaceous and Paleogene. The crust to the south and west was formed during the Cenozoic, and perhaps the latest Cretaceous, by the seafloor spreading that led to the separation of Australia and Antarctica.
The continental shelf around Tasmania (Fig. F3) is mostly nondepositional at present. The continental slope west of Tasmania slopes fairly regularly, at ~4°, from 200 to 4000 m. The continental rise lies at 4000-4500 m, and the abyssal plain is generally 4500-5000 m deep. Sampling cruises have shown that the slope is underlain by continental basement and that Upper Cretaceous and Paleogene shallow-marine sandstone, siltstone, and mudstone are widespread in deep water west of Tasmania, overlain by Oligocene to Holocene pelagic carbonates. Seismic interpretation shows that basement is overlain by several kilometers of sediments in depocenters (Fig. F8).
The current-swept STR is a large north-northwest-trending bathymetric high that rises to <1000 m below sea level (mbsl) and is separated from Tasmania by the northwest-trending, >3000-m-deep South Tasman Saddle (Fig. F3). The STR is a continental block, and seismic profiles show it is cut into basement highs and deep basins with several kilometers of sedimentary section (Fig. F8). The overlying sequences in faulted basins include known Oligocene to Holocene pelagic carbonates and Paleogene marine mudstones, and seismic evidence suggests they also contain Cretaceous sediments. The top of the rise is a gentle dome with low slopes, but slopes are generally steeper between 2000 and 4000 m. The western slope is more gentle to 3000 m, but below that there is a very steep scarp trending 350°, which drops away to 4500 m as part of the Tasman Fracture Zone.
The ETP is a nearly circular feature, 2500-3000 m deep, separated from southeast Tasmania by the East Tasman Saddle (Fig. F3). Slopes are generally low, but considerably greater on the plateau's flanks. Atop the plateau is the Cascade Seamount guyot, which formed as the result of hot spot volcanism and has yielded Eocene and younger shallow-water sandstone and volcanics. Seismic profiles show that the plateau has as much as 3 s two-way traveltime (TWT) of sediment cover (Fig. F8), which are believed to comprise mainly Oligocene to Holocene pelagic carbonates and Cretaceous to Eocene siliciclastic sediments. These are underlain by continental basement rocks.
The structural setting of Sites 1168-1172 is shown in Figure F9. The west Tasmanian margin is cut by strike-slip faults, trending north-northwest or northwest, that were most active in the latest Cretaceous to mid-Paleocene (Hill et al., 1997b). They were generated by the northwest movement of Australia away from Antarctica. The STR is cut by these early faults, and also by younger, middle Eocene- to late Oligocene-age faults. These younger faults are largely north-south-trending strike-slip faults, and on the northwestern STR are related east-west-trending normal faults (Exon et al., 1997b). The three sets of faults all have throws reaching as much as 3 km. Sites 1168-1172 were all located in depocenters to ensure that thick Cenozoic sections with high sedimentation rates would be cored.
Early extension between Australia and Antarctica began in a northwest-southeast direction during the Late Jurassic (Willcox and Stagg, 1990), and this motion created much of the western margin off Tasmania. Subsidence studies along the southern Australia margin, as well as the conjugate pattern of magnetic anomalies off Australia and Antarctica, suggest that the breakup between Australia and Antarctica propagated toward Tasmania from the Great Australian Bight (Mutter et al., 1985). Seafloor spreading may have started west of Tasmania during the Late Cretaceous and continued at a slow rate until the early Eocene, when fast spreading began. The northerly trajectory of the central STR since Australia-Antarctic separation, and its paleolatitudes since the Campanian, are shown in Figure F4. Royer and Rollet (1997) reexamined the seafloor magnetic anomaly data and satellite-derived gravity data in the region along with plate tectonic reconstructions and concluded the following about the region south of Tasmania (Fig. F10):
During DSDP Leg 29, four partially cored sites were drilled in the Tasmanian region (Kennett, Houtz, et al., 1975) (Fig. F3; Table T1). The three sites most relevant to the goals of Leg 189 are Site 282 on the west Tasmanian margin, Site 281 on the STR, and Site 280 on the abyssal plain immediately south of the STR (Fig. F3). The Leg 29 sites were generally located on regional highs to minimize the depth of penetration necessary to reach older strata, and hence much of the succession was cut out by hiatuses. Furthermore, during the first scientific drilling in the area, total sediment recovery for the three critical sites was fairly low (Table T1).
Site 282 was drilled to 310 mbsf on a basement high in deep water west of Tasmania. This sequence includes much of the Cenozoic but contains four major unconformities. The sequence consists of a veneer of Pleistocene ooze, underlain by upper Miocene ooze, lower Miocene marl, lower to mid-Oligocene mudstone, and upper Eocene mudstone. The sediments rest on presumed Tertiary pillow basalts. There is little in the sediments to suggest that the site was located in deep water until the margin began to subside during the Oligocene. Calcareous microfossils are present throughout, and total core recovery was 20%.
Site 281 was drilled to 169 mbsf on a basement high of quartz-mica schist of latest Carboniferous age southwest of the crest of the STR. The sequence consists of Pliocene-Pleistocene foraminifer-nannofossil ooze, Miocene foraminifer-nannofossil ooze, upper Oligocene glauconite-rich detrital sand, and upper Eocene basement conglomerate and glauconitic sandy mudstone. Evidence from the recovered intervals suggests that the site subsided into deep water after the Miocene. Calcareous microfossils are present throughout, and total core recovery was relatively high (62%).
Site 280 was drilled to 524 mbsf, on a basement high in deep water southwest of the STR (Fig. F3), and bottomed in an "intrusive basalt," almost certainly associated with oceanic crust. The site penetrated a veneer of upper Miocene to upper Pleistocene clay and ooze, underlain (beneath a sampling gap) by 55 m of siliceous lower Oligocene sandy silt, and 428 m of middle Eocene to lower Oligocene sandy silt, containing chert in the upper 100 m and glauconite and manganese micronodules in the lower succession. The lower 200 m is rich in organic carbon (0.6-2.2 wt%). The younger part of the lower Oligocene to upper Eocene sequence contains abundant diatoms, but the lower part is almost completely devoid of pelagic microfossils. All sediments were assumed to have been deposited in abyssal depths. A brown organic staining suggests that reducing conditions were present in parts of the upper Oligocene and lower Miocene. Total core recovery was only 19%.
Site 281, in particular, assisted with the development of a broad, globally significant history of Cenozoic paleoceanographic events. Shackleton and Kennett (1975) produced composite foraminiferal oxygen and carbon isotopic curves for the late Paleocene to the Pleistocene from Sites 277, 279, and 281. This record, although of relatively low resolution, exhibits the now classically known general increase in oxygen isotopic values, reflecting a decrease in bottom- and surface-water temperatures and/or ice buildup during the Cenozoic. A general increase occurred in isotopic values following the early Eocene, with a rapid increase during the early Oligocene reflecting major cryosphere expansion and cooling. Average oxygen isotopic values remained steady but oscillatory until the middle Miocene, when another rapid oxygen isotopic increase records further expansion of the Antarctic cryosphere. This was followed by additional increases in oxygen isotopic values reflecting the development of the West Antarctic Ice Sheet during the late Miocene and the Northern Hemisphere cryosphere during the late Pliocene (Fig. F2). Isotopic analyses were made at Site 281 (STR) on the lower Miocene to the Pliocene and at Site 277 (Campbell Plateau) on upper Paleocene to lowermost Miocene.
The two northernmost sites drilled during Leg 189 are located in temperate (cool subtropical) waters north of the present-day position of the Subtropical Front or Subtropical Convergence (Fig. F11). The sedimentary sequences drilled during Leg 189 should record migrations of these fronts as a result of climatic change. Furthermore, as a result of plate tectonic motion, the Tasmanian continental block migrated northward in relation to these fronts during the Cenozoic, leaving records in the marine sediments. The southern sites drilled during Leg 189 are located in subantarctic waters between the Subtropical Front and the Subantarctic Front. The area drilled during Leg 189 therefore lies north of and straddling the Subtropical Front and south to the region near the Subantarctic Front. The Polar Front lies farther to the south of our southernmost site. Rintoul and Bullister (1999) showed that the Subtropical Front is centered on 46°S and lies just south of the saddle between Tasmania and the STR. The Subantarctic Front is centered on 51°S and lies ~200 km south of the STR. The Polar Front is centered on 53°S, ~200 km south of the Subantarctic Front. The Subtropical Front is marked by a zone of rapid north-south decrease in temperature and salinity and an increase in dissolved nutrients (Barrows et al., 2000) and is approximated by the 34.8-35.1 isohalines and the ~10°C winter and the ~15°C summer isotherms in the southern Tasman Sea (Garner, 1959). During summer, sea surface temperatures are >15°C north of the Subtropical Front, ~10°C between the Subtropical Front and the Subantarctic Front, ~8°C between the Subantarctic Front and the Polar Front, and <6°C south of the Polar Front. During midwinter, sea surface temperatures are several degrees lower.
Subantarctic surface water south of the Subtropical Front is driven eastward across the STR by the prevailing westerly winds as the northern part of the Antarctic Circumpolar Current. These surface currents extend to great depths, sweeping the seabed as deep as 2000 m in places. The East Australian Current is a western boundary current that flows southward along the east coast of Tasmania to the vicinity of the ETP. Here, subtropical surface water converges with cooler, less saline subantarctic surface water at the Subtropical Front (Orsi et al., 1995).