The sedimentary succession in the Tasmanian region generally records three major phases of sedimentary deposition:
In general, sedimentation rates throughout the region changed dramatically in relation to these distinct phases of sedimentation. These were rapid (~10 cm/k.y.) during the siliciclastic sedimentation of the early rifting phase of the late Paleocene to early late Eocene, slow to condensed during the Eocene-Oligocene transition, when more glauconitic sediments were deposited, and generally slow during the biogenic sedimentation from the earliest Oligocene to the present. There were periods of minimal late Oligocene and late Miocene sedimentation or erosion.
Considerable thickness variation is evident for different time periods among the sites. The two major sediment types, pelagic carbonates of the Oligocene and Neogene and the shallow-water siliciclastic mudstones of the Paleogene and Late Cretaceous, are dealt with separately below. In regard to the pelagic carbonates, the Quaternary is thickest at the two southern sites (Sites 1170 and 1171). The Pliocene is thickest at the western sites on the STR and especially thick at Site 1169, which was protected from east-flowing current scour by its great depth and the ridge of the TFZ. The Miocene is affected by the onset of the strong Antarctic Circumpolar Current, with the thickest and most complete section found in the protected northern Site 1168. The other northern site (1172) has a fairly complete but thinner section. The lower Miocene is especially thin at the southern Sites 1169, 1170, and 1171, probably because of current scouring. The Oligocene, too, was strongly affected by the Antarctic Circumpolar Current and/or the East Australian Current, with the protected northern Site 1168 as the only one with a thick and reasonably complete sequence (Figs. F16, F17).
The upper Eocene siliciclastic mudstones are thin everywhere, apparently because fine-grained sediments were swept away by the newly forming shallow-water Antarctic Circumpolar Current at all sites. In contrast, the lower and middle Eocene are thick everywhere because siliciclastic supply was rapid and there was little current erosion. The same interpretation probably applies to the Paleocene at the southern Site 1171, whereas there is little Paleocene at the northeastern Site 1172. The uppermost Cretaceous mudstones penetrated at Site 1172 may be quite thick judging from seismic reflection profiles.
During the Mesozoic, the Tasmanian region was well within east Gondwana, and although the locations of all the sites eventually remained part of Australia, the four deep sites are all on different blocks of continental crust that behaved somewhat differently as Gondwana fragmented. According to the plate tectonic reconstruction of Royer and Rollet (1997), during the mid-Cretaceous (Fig. F5) the locations of Sites 1169 and 1170 were close together on the Antarctic side of the northwest-southeast strike-slip zone along which Australia was moving northwest relative to Antarctica, here named the Tasmania-Antarctic Shear Zone (TASZ). Across the TASZ were the locations of Sites 1168 and 1171, some 600 km apart. Site 1172 was ~300 km east of Site 1170 and well away from the rift zone.
An east-west rift along the southern margin of Australia and the northern margin of Antarctica, and slow northwest-southeast separation, may have begun as early as the Late Jurassic (Willcox and Stagg, 1990). This rift continued eastward in the Otway, Bass, and Gippsland Basins between Australia and Tasmania. Crustal thinning and subsidence occurred along the developing east-west rift, which was filled with thousands of meters of nonmarine Lower Cretaceous sediments. From the mid-Cretaceous (95 Ma) to the middle Eocene (43 Ma), Australia moved northwest relative to Antarctica at a slow rate of ~10 m/m.y. As the east-west rift widened into the Australo-Antarctic Gulf, the sea transgressed from the west depositing shallow-marine and deltaic Late Cretaceous to Eocene sediments. In the Late Cretaceous (75 Ma), the Tasman Sea began to form and rifting in the Bass Strait failed.
In the Tasmanian region, the TASZ terminated the east-west rift. The block east of the TASZ, consisting of southeast Australia, Lord Howe Rise, and Campbell Plateau, moved northwestward along the shear zone. Some crustal thinning occurred along the TASZ, with possible deposition of Early Cretaceous sediments. By the Late Cretaceous, the TASZ was subsiding steadily, marine transgression took place through the Gulf from the west, and shallow-marine deltaic sediments began to be deposited in the extreme eastern part of the Australo-Antarctic Gulf. This phase of deposition continued into the Eocene.
In the Late Cretaceous (75 Ma), the Lord Howe Rise, Campbell Plateau, and the ETP all began to move away from Australia and Antarctica to the east-northeast (relative to Australia). At 65 Ma, in the latest Cretaceous, the ETP (with Site 1172) stopped moving with the other blocks, but a legacy of rifting before that movement is its thinned crust and the partially oceanic depression between the ETP and what became the STR. Results from Site 1171 showed that strike-slip movement within the central and eastern STR (e.g., on the Balleny Fracture Zone) occurred during a short period in the late Paleocene, when spreading began south of the STR. This agrees with the plate tectonic reconstruction of Cande et al. (2000), which shows that from the late Paleocene east-west spreading extended from south of the STR and north of the Transantarctic Mountains (TAM) across to New Zealand. This interval marks the beginning of the uplift of the TAM (Fitzgerald, 1992). However, evidence from the 1999 Cape Roberts drilling, in the western Ross Sea, indicates that Victoria Land Basin first formed by ~3000 m of rapid subsidence east of the TAM at ~34 Ma, close to the Eocene/Oligocene boundary (Cape Roberts Science Team, 2000), suggesting that much of the TAM uplift may have occurred at that time.
This tectonic scenario for Antarctica seems to have been reflected in the structurally interconnected Tasmanian region. If late Paleocene uplift of the TAM did occur, this uplift would have corresponded to the period of strike-slip movement, and associated vertical displacement, on the central and eastern STR. The end of this movement corresponds to the onset of spreading between the TAM and the STR, which separated the strike-slip faults within most of the STR from tectonic movement in Antarctica. However, early Oligocene collapse of the continental margins everywhere in the Tasmanian region, documented in part by Leg 189 drilling, coincides precisely with the age of formation of the Victoria Land Basin on the conjugate Antarctic margin (~34 Ma). A causal relationship between these events appears probable, but determining that relationship awaits postcruise research.
The Site 1168 region was part of the Tasmanian block throughout its Cretaceous and Cenozoic history and within the Sorell Basin depocenter formed by crustal thinning along the TASZ. The Sorell Basin was fully separated from the conjugate eastern end of the Wilkes Land Basin in Antarctica once large quantities of oceanic crust began to form in the easternmost Australo-Antarctic Gulf during the middle Eocene (43 Ma). In regard to the oldest seafloor spreading in this region, Pyle et al. (1995) obtained Ar-Ar dates of 60.7 Ma from basalt at the base of nearby DSDP Site 282 on thinned continental crust, but doubts remain about this date because of the extent of alteration of the basalt. Royer and Rollet (1997) suggest the presence of some oceanic crust as old as Chron 24 (55 Ma). Like the basins to the north, the Sorell Basin subsided throughout the Cretaceous and Cenozoic and was filled with thick shallow-marine Upper Cretaceous to Eocene deltaic sediments derived largely from Tasmania. Strike-slip faulting, active into the Paleocene, cut the basin into a number of subbasins and ridges in which Site 1168 is located. In the earliest Oligocene, the Sorell Basin subsided rapidly into deep water and the deposition of pelagic carbonate commenced. This observation has also been made to the northwest in the Otway Basin. In both cases, the subsidence appears to have been about hinge lines near the present coastlines. The cause of the initial rapid subsidence remains unclear, but it was a regional event. Subsidence may have resulted from the final clearance of southeast Australia (the southwestern tip of the STR) from Antarctica (Wilkes Land) near the Eocene/Oligocene boundary (Fig. F6). Site 1168 contains a remarkably continuous Eocene to Holocene sequence, but with a clear transition from siliciclastic deposition in the Eocene to pelagic carbonate deposition in the early Oligocene as the margin sank rapidly.
According to Royer and Rollet (1997), Site 1170 was part of the Antarctic block (Wilkes Land) in the early Late Cretaceous, separated from Southeast Australia by the TASZ. After collision with the central STR block in the latest Cretaceous (66 Ma), the western STR block formed, on which Site 1170 is located. This block began to separate from Antarctica by strike-slip motion, with the TASZ transferring to the block's western side. By the latest Paleocene, the STR block was firmly welded to southeast Australia. During the middle Eocene (43 Ma), the onset of north-south fast spreading in the Australo-Antarctic Gulf increased the rate of movement along the TASZ (which was now the TFZ) to ~30 m/m.y. At the same time, fast spreading started south of the STR. Much of this block is covered by the Ninene Basin, which is cut by Cenozoic strike-slip faults into subbasins and ridges. The basin is filled at least in part by fairly thick Upper Cretaceous to Eocene prograded shallow-marine siliciclastic sediments. This western block was substantially thinned by tectonism and now lies much lower than the adjacent central block of the STR. Site 1170 is located only 10 km from the eastern side of the Ninene Basin (Exon et al., 1997b), where the basin abuts against the north-south fault (and modern fault scarp) that marks the suture with the central block.
During the earliest Oligocene, Site 1170 and the Ninene Basin subsided rapidly into deep water, as at Site 1168. The initial fast subsidence is a regional event involving at least Southeast Australia and the Victoria Land Basin in Antarctica. The final clearance of the western block from Antarctica near the Eocene/Oligocene boundary probably had an effect at Site 1170. The unusually thin upper Oligocene sequence may have been caused by increased current erosion, caused by shallowing as the spreading axis, with its heat source, passed 100 km to the west (~26 Ma). Site 1170 contains a less complete Upper Eocene to Holocene sequence than Site 1168 because of the generally greater current activity caused by the site's greater proximity to the Tasmanian Gateway between the STR and Antarctica and the associated Antarctic Circumpolar Current. The transition from Eocene siliciclastic sediments to lowermost Oligocene pelagic carbonate, as the basin sank rapidly, was much sharper than at Site 1168. This more transitional change at Site 1168 was probably caused by the ongoing supply of siliciclastic sediments from the high hinterland of Tasmania.
During the Early Cretaceous, the southernmost Site 1171 was located on the east Gondwana block consisting of Australia, the Lord Howe Rise, and the Campbell Plateau across the strike-slip TASZ from Antarctica. Some Early Cretaceous sediments must have been deposited along the TASZ, but there is no evidence of them near Site 1171. According to Royer and Rollet (1997), the central and eastern blocks of the STR separated from the Tasmanian block in the early Late Cretaceous (95-83 Ma). Site 1171 is located near the boundary between the two blocks, which "jostled" as they moved. As Australia moved northwest, the two blocks were "braked" by Antarctica until the latest Cretaceous (65.6 Ma), with 150 km of stretching between this and the Tasmanian block forming the South Tasman Saddle. Strike-slip basins and ridges developed within the two STR blocks, but there was only limited thinning of these cratonic blocks. Seismic profiles suggest that the basins were filled by prograded Upper Cretaceous and Paleocene sediments (probably shallow marine) derived from the subaerially exposed ridges and along the basins from higher areas to the north. During the Late Cretaceous, at 75 Ma, seafloor spreading started to the east with the Campbell Plateau and the ETP separating from the composite block and moving east-northeast relative to Site 1171. The continental crust of the blocks was thick, so subsidence was limited.
The timing of the initiation of seafloor spreading to the south of Site 1171 is open to interpretation, as profiles with magnetic anomalies are limited and the older anomalies are close together and questionable. Royer and Rollet (1997) reliably identify Anomaly 20 (45 Ma), questionably identify Anomaly 24 (55 Ma), and very doubtfully identify Anomaly 31 (67 Ma). Fast spreading clearly began in the middle Eocene (43 Ma). Pyle et al. (1995) obtained Ar-Ar dates of 64.2 Ma from the basalt at the base of DSDP Site 280 on nearby oceanic crust, but because of its alteration, doubts remain about this date. Further information on the breakup comes from Site 1171 itself, which lies in a small north-south basin bounded to the east by an arm of the Balleny Fracture Zone, the boundary between the central and eastern STR blocks. From this site, seismic and palynological data show an unconformity between very broadly and gently folded sediments of Cretaceous to late Paleocene (58 Ma) age and onlapping and flat-lying early Eocene sediments. This shows that much of the movement within continental crust along the Balleny Fracture Zone (which has been active in oceanic crust ever since) was confined to the period between 58 and 54 Ma (i.e., latest Paleocene; ~55 Ma). The end of this episode of strike-slip faulting and associated folding agrees with the Anomaly 24 identification above, confirming that final separation south of the STR (from Wilkes Land) was in the late Paleocene.
During the earliest Oligocene, Site 1171 and the central STR block subsided into deep water, similarly to Sites 1168 and 1170. The amount of subsidence was less than at those sites because of the greater crustal thickness at Site 1171. The likely cause of at least part of the early Oligocene subsidence (contemporaneously with that of the margins elsewhere in southeast Australia and the Victoria Land Basin) is again the final clearance of the attached western STR block from Antarctica. Site 1171 contains a much thinner and less complete upper Eocene and Oligocene sequence than Sites 1168 and 1170 because of its immediate proximity to the Tasmanian Gateway between the STR and Antarctica and its exposure to the Antarctic Circumpolar Current thereafter. The transition from Eocene siliciclastic sediments to pelagic carbonate, as the basin sank rapidly in the earliest Oligocene, was more rapid than in the less current-swept and expanded sequences at Site 1168.
During the Early Cretaceous, the eastern Site 1172 was in the east Gondwana block consisting of Australia, the Lord Howe Rise, and the Campbell Plateau well away from the TASZ. Seismic data suggest that dipping and faulted Early Cretaceous sediments exist at depth. It is probable that a highly reflective sequence above these sediments consists of Late Cretaceous breakup volcanics, perhaps including the undated rhyolite dredged from the eastern scarp of the ETP (Exon et al., 1997a). According to Royer and Rollet (1997), breakup began to form the ETP during the Late Cretaceous (75-65 Ma), both to the plateau's west and east. The plateau moved east-northeast relative to Tasmania and the STR, initially with the continental block of the Lord Howe Rise and Campbell Plateau. However, this block soon separated, leaving the Tasman Sea oceanic crust to the east of the ETP. In the west, some oceanic crust formed between the ETP and the STR, forming L'Atalante Depression, and crustal thinning occurred between the ETP and Tasmania, eventually forming the East Tasman Saddle.
Although the ETP formed in the Late Cretaceous and might well have had a very different subsidence history from the other Tasmanian blocks drilled during Leg 189, this proved not to be the case by the close similarity between the two Pacific Ocean Sites 1172 and 1171. At both sites, Paleocene and Eocene sediments are similar shallow-marine (neritic) siliciclastics, and at both sites there was rapid subsidence and a switch to deposition of pelagic carbonate beginning in the earliest Oligocene. The plateau had been planated by erosion and subject to latest Cretaceous and Paleogene deposition, with siliciclastic sediment coming from the marginal granitic highs and perhaps from Tasmania, if the East Tasman Saddle subsided late. An additional feature of the ETP was the formation of a large hot spot volcano in its center during the late Eocene. According to Quilty (1997), calcareous microfossils show that the upper part of this 1500-m-high guyot, the Cascade Seamount, was in shallow-marine depths during the late Eocene. This clearly conflicts with the evidence of upper bathyal deposition of latest Eocene sediments at Site 1172 below the base of the seamount. An explanation awaits postcruise investigation.
A tabulation of vital information from ODP Leg 189 and DSDP Leg 29 sites (Kennett and Houtz, 1975) summarizes much of the above (Table T4). It shows that three breakup ages (75, 55, and 34 Ma) affected the various sites in various ways. The first breakup was at 75 Ma for the central and eastern Sites 1171, 281, 1172, and 283 and at 55 Ma for the western and southern Sites 282, 1168, and 280. The last breakup was at 34 Ma on the STR for Sites 1170, 281, and 1171. The subsidence since the Eocene is large on oceanic crust or thinned continental crust (Sites 282, 1168, 1170, 280, and 283) at 3100-3800 m. It is less on the thick continental crust of the central STR and the ETP (Sites 281, 1171, and 1172) at 1600-2500 m. The transition from siliciclastic to biogenic sedimentation is Eocene/Oligocene at most Leg 189 sites in basinal settings in shallow Eocene water depths, but it is Oligocene/Miocene in several Leg 29 sites on local highs in deeper Eocene water depths. This variation is probably caused in part by the position of the CCD relative to the sites during the Eocene to Miocene.
In the Early Cretaceous, east Gondwana was intact and ocean currents flowed west and north of Australia and east of the continental block of the Lord Howe Rise, the Campbell Plateau, and New Zealand (LCNZ). The situation began to change early in the Late Cretaceous, when an east-west rift caused marine transgression into the Australo-Antarctic Gulf. Despite a continuation of the rift eastward through present-day Bass Strait into the Lord Howe Rise, the Tasmanian-Antarctic land bridge east of the Gulf survived, and the eastern part of the rift filled with nonmarine sediments. By the Late Cretaceous, the northwest-southeast TASZ was well developed and Gulf waters began to transgress southward along it. During the Late Cretaceous at 75 Ma, continental breakup occurred and seafloor spreading began between Australia and the LCNZ. This rift propagated northward forming the Tasman Sea. Final breakup off northeastern Australia was during the Paleocene at ~60 Ma, and thereafter major ocean currents could flow along the eastern coast of Australia, Tasmania, the ETP, the STR, and the Antarctic margin to the south. However, the Tasmanian Land Bridge between Southeast Australia and Antarctica remained essentially intact, separating the Australo-Antarctic Gulf from the Pacific Ocean.
By the latest Paleocene, the areas of continental crust in the land bridge that had been thinned during the Cretaceous—the future Bass Strait, the South Tasman Saddle between Tasmania and the STR, and parts of the TASZ between Antarctica and STR—had subsided and were near sea level. From then on, limited interchange of shallow-marine waters may have occurred between the Australo-Antarctic Gulf and the Pacific Ocean. The plate tectonic reconstructions of Royer and Rollet (1997), Cande et al. (2000), and S.C. Cande (unpubl. data) suggest, and a variety of other tectonic and sedimentary evidence (including that from Leg 189) supports, the idea of a shallow-marine connection during the middle Eocene (Fig. F7). However, the very different character of the shallow-marine Eocene sediments recovered at Sites 1168 and 1170 in the restricted waters of the Gulf, and at Sites 1171 and 1172 in the open waters of the Pacific Ocean, indicates that this interchange was not significant.
However, the same lines of evidence suggest that Antarctica and the STR separated during the Eocene-Oligocene transition (Fig. F6). The ODP sites clearly show a rapid change in the earliest Oligocene to similar open-ocean conditions on both sides of the former Tasmanian Land Bridge, and a shallow-water Antarctic Circumpolar Current was established at that time. By the late Oligocene, the Tasmanian Seaway was hundreds of kilometers wide and at abyssal water depths south of the STR, and a shallow to deep Antarctic Circumpolar Current from the west was eroding older sediments. Nearly all of the former land bridge south of Tasmania had by then submerged, and the expanding current was scouring the ocean floor in places. The Tasmanian Gateway was fully open at all depths by the time the Drake Passage, south of South America, is inferred to have opened to deep water during the early Miocene (Barker and Burrell, 1977) and could accommodate an ever-increasing circumpolar flow. This increasing flow continued through the remainder of the Cenozoic as the Australian mainland and Tasmania continued to move northward and the Drake Passage opened further. The expanding influence of circumpolar circulation led to bottom-water erosion and winnowing over certain areas and was particularly strong during the late Neogene. The expansion of the Antarctic cryosphere during the middle and late Cenozoic, and its effect of strengthening thermohaline circulation at deep and intermediate water depths, contributed to the deep-ocean erosion and formation of hiatuses in the sequences. Examples of this include the middle Miocene scouring east of Site 1170 that removed the Oligocene and the scouring during the Miocene-Pliocene transition in the southern part of the STR.
Deposition during the Paleocene through early late Eocene (43-36 Ma) was dominated by continental influences. The sequences drilled are probably quite similar to even older underlying Paleogene sequences, which, based on seismic and other evidence, consist of deltaic siliciclastic sediments that kept up with subsidence and compaction as rifting progressed. The southern sequences (Sites 1170 and 1171) provide unusually good records of Antarctic paleoenvironmental conditions at the high-latitude Antarctic continental margin (~67°S), whereas Sites 1168 and 1172 provide comparative records at lower latitudes on the western and eastern margins of Tasmania. At Sites 1170 and 1171, the Paleocene-Eocene dark, fine-grained, organic-rich siliciclastic sediments were deposited on a highly restricted, moderately tranquil, broad shelf near the opening rift between Antarctica and Australia in the extreme southeastern corner of the Australo-Antarctic Gulf (Site 1170) and in the extreme southwestern margin of the Pacific (Site 1171). Benthic foraminiferal assemblages indicate deposition in neritic water depths, probably shallower than 100 m. An absence of sedimentary characteristics indicating turbulence suggests that deposition was below wave base (which may have been shallow in the prevailing equable climatic conditions) and without important current or tidal influences. Persistent reworked early Eocene to Cretaceous radiolarians and organic dinocysts at Site 1170 suggest continuous weathering and erosion of marine sediments of this age in the hinterland.
Highly restricted nearshore conditions in the middle to early late Eocene at all sites are indicated by a wide range of criteria including pervasive pollen and spore assemblages, abundant continuous low-diversity assemblages of organic dinocysts indicative of eutrophic and brackish surface waters, and limited representation of open-ocean planktonic microfossils. Diverse pollen and spore assemblages are particularly abundant in the Paleocene sequence at Site 1171, indicating particularly strong continental influence with abundant, diverse plant communities. In situ assemblages of planktonic foraminifers and radiolarians are largely absent, diatoms are rare, and calcareous nannofossils are uncommon. Good preservation of calcareous nannofossils suggests that they are uncommon, not because of dissolution, but because of the restricted coastal depositional setting in conjunction with high sedimentation rates. However, the presence of foraminiferal linings suggests dissolution of foraminifers at some levels.
Ventilation on the shelves over much of the region was poor to limited throughout the middle to early late Eocene judging from high TOC, limited bioturbation, and benthic foraminiferal assemblages dominated by agglutinated forms and nodosariids. The sediments at Sites 1168 and 1170 suggest that the eastern, remote end of the narrow, restricted Australo-Antarctic Gulf was poorly ventilated. Low oxygen levels in waters of the eastern Gulf would have developed because of the restricted circulation (2000 km from the open Indian Ocean) and the proximity of inferred abundant terrestrial organic matter sources from the surrounding land masses (including Antarctica), warm climatic conditions, and possible high marine biotic productivity in eutrophic conditions. Ventilation was especially poor at Site 1168, off western Tasmania, as indicated by laminated sediments. Deeper water sediments at nearby Site 282 were also poorly oxygenated. Anoxic to dysoxic depositional environments would be expected within an expanded oxygen minimum zone that extended up onto the continental shelf at Site 1168. Eocene sediments at Site 1170, in the extreme southeastern corner of the Gulf, were also deposited in poorly oxygenated conditions on a tranquil shelf. Here, the absence of laminations and a slight increase in bioturbation in some intervals suggest greater shelf ventilation than at Site 1168, but circulation was almost certainly sluggish.
The poor ventilation in such shallow-water depths at Site 1170 suggests that the Tasmanian Gateway generally was closed to even shallow waters (~100 m) during the middle and early late Eocene, at least in the developing rift basin between Antarctica and the STR. This is supported by the contrasting Eocene record at Site 1171, near the developing seaway on the Pacific side of the Tasmanian land bridge, where the shelf sediments are well bioturbated, suggesting with other evidence that more strongly ventilated conditions existed here than in the Gulf and no oxygen minimum zone was present. If the Tasmanian Gateway had been open widely at shallow depths, there would have been little basin to basin fractionation and, hence, little contrast in sedimentary environments. Middle Eocene sedimentation at Site 1172 bears considerable resemblance to that at Site 1171, but it was in a more oceanic, better ventilated environment within the open Pacific Ocean. The proximity of Site 1172 to the large and newly formed subaerial Cascade Seamount may have increased the speed of bottom currents nearby, thus enhancing ventilation.
The middle through upper Eocene sediment sequence at Sites 1170 and 1171 reveals distinct cycles in physical properties, sediments, and microfossil assemblages. Alternations between dark, poorly bioturbated, nannofossil-poor sediments lacking glauconite and lighter, more nannofossil-abundant bioturbated sediments containing glauconite probably result from changes in water-mass ventilation. Quantitative changes in dinocyst assemblages are cyclic. The darker sedimentary intervals are associated with higher abundances of dinocysts (including massive monotaxic blooms) characteristic of eutrophic conditions and suggesting high nutrient supply to surface waters. In contrast, dinocysts characteristic of more oligotrophic surface waters dominate the lighter intervals in association with more abundant calcareous nannofossils. Clear cyclicity with probable Milankovitch periodicity is also evident in Th abundance in the logging data.
These middle to late Eocene shelfal cycles almost certainly will be shown to correlate with changes in sediment and biotic characteristics. The cause of the cycles is yet to be determined, but they probably resulted from minor climatic oscillations at high southern latitudes, perhaps associated with small sea-level changes, which caused changes in siliciclastic sediment supply, upwelling, and nutrient supply, and associated changes in bottom-water ventilation. The relative importance of marine vs. terrestrial sources of organic carbon varied at the different margin locations during the Eocene. At Site 1168, terrestrial organic carbon sources dominated, but marine organic carbon dominated at Sites 1170 and 1171, where marine productivity was high in a restricted nearshore setting. In contrast, Paleocene sediments, sampled only at Site 1171 on the southern STR, clearly exhibit a much stronger terrestrial influence with abundant terrestrial plant materials (e.g., pollen and spores). Throughout the Paleocene to middle Eocene, sedimentation kept up with subsidence and compaction.
Evidence for glaciation is completely lacking in the Paleocene and Eocene. Both marine and terrestrial microfossils indicate cool temperate conditions throughout the middle and late Eocene. Calcareous nannofossil assemblages are of relatively high diversity and appear to be slightly warmer in this sector of the Antarctic than at comparable latitudes elsewhere (although almost completely lacking warmth-loving discoasters). Their diversity suggests an absence of seasonal sea ice over the shelf and marine conditions during much of the time. Middle Eocene clay assemblages at Site 1170 are completely dominated by smectite, suggesting warm temperatures, seasonal rainfall, and a predominance of chemical over physical weathering.
Relatively warm, cosmopolitan middle Eocene dinocyst assemblages were in part replaced during the late Eocene by cooler, endemic Antarctic forms. This inferred cooling is consistent with stable isotopic records that indicate progressive cooling of the Southern Ocean during the middle and late Eocene (Shackleton and Kennett, 1975; Stott et al., 1990; Kennett and Stott, 1990). Cooler continental conditions during the late Eocene are indicated by conspicuous increases in illite relative to smectite clays at Sites 1170 and 1171, suggesting a reduction in continental chemical weathering, perhaps in combination with increased tectonism near the expanding rift. Nevertheless, the character and abundance of the organic dinocyst and diatom assemblages throughout the entire middle to upper Eocene suggest that at no time was cooling sufficient to form important seasonal sea ice in this Antarctic region. Pollen and spore records suggest that the middle and late Eocene hinterlands were inhabited by diverse and cool temperate plant communities. Floras were dominated by Nothofagus, podocarps, and other forms with an understory of ferns, similar to an approximately coeval floral assemblage from the Weddell Sea sector of Antarctica (Mohr, 1990). Although upper Eocene pollen assemblages appear to represent cooler conditions, they still indicate relative warmth along the Antarctic margin compared with the distinctly cooler Oligocene that followed. Oligocene floras left no record of palynomorphs in the pervasive carbonate sediments, probably because of oxidation. On Tasmania, early Oligocene mountain glaciers existed in a vegetated region (Macphail et al., 1993). Overall, a nonglaciated, cool temperate climate is inferred on this sector of the Antarctic margin during the middle to late Eocene. This contrasts with the Prydz Bay margin, where clear evidence exists for late Eocene glaciation (Barron et al., 1991a; O'Brien, Cooper, Richter, et al., 2001) and early Oligocene development of a major ice sheet (Zachos et al., 1992, 1993, 1996).
In summary, the contrast between the poorly oxygenated, shallow-marine waters of the shelf of the Australo-Antarctic Gulf and the better oxygenated waters in the Pacific Ocean suggests a general lack of interchange between southern Indian and Pacific Ocean waters. The evidence discussed above suggests that the Tasmanian Gateway remained more or less closed, even to shallow surface waters, until the late Eocene at ~36 Ma. Sites 1168 and 1170 in the Gulf are much more poorly ventilated than Sites 1171 and 1172 in the Pacific Ocean.
Major paleoenvironmental changes occurred during the Eocene-Oligocene transition, the most profound change in the entire Cenozoic (Kennett, 1977; Zachos et al., 1993), at all four deep sites (Sites 1168, 1170, 1171, and 1172). The sites contain a variably continuous record of this interval, which shows that almost all aspects of the depositional environment changed. The late Eocene is represented by a condensed sequence of continentally influenced glauconite-rich silty claystones to glauconitic muddy siltstones and the early Oligocene by biogenic carbonates. A transitional change upward in the upper Eocene from more muddy to more sandy sediments, associated with increasing glauconite and quartz and the deposition of glauconitic quartz-rich sandy silts ("green sands"), reflects an upward increase in bottom-current activity and winnowing. The near absence of calcareous microfossils in the "green sands" at Sites 1170, 1171, and 1172 made dating difficult. However, dinocysts and diatoms suggest a relatively complete upper Eocene sequence up to and including the "green sands." This observation is supported by the clear upward gradation in lithology into the "green sands," the general succession of microfossil datums, and the unexceptional changes in the assemblages of organic dinocysts, pollen, and spores.
Within the "green sands," despite the winnowing and hiatuses, the angularity of quartz at some levels indicates periods with little reworking. In the upper Eocene, diatoms and benthic foraminifers indicate a slight deepening of the "green sand" environment. With steadily increasing water depths in the early Oligocene, deposition changed to open-water pelagic carbonates. This reflects a change during the early late Eocene to early Oligocene, from siliciclastic to biogenic sedimentation, from a poorly to a well-oxygenated benthic environment, from tranquil to moderately dynamic environments, and from relatively warm to cool climatic conditions. The sequence of change over the Eocene-Oligocene transition is remarkable in its consistency over the width and breadth of the Tasmanian-Antarctic margin, as determined from our four deeply cored sites. Differences in detail are clearly related to individual settings at the time of deposition, such as latitude, tectonic setting, and proximity to the ocean and landmasses.
The glauconitic sediments are strongly bioturbated and were deposited in well-oxygenated bottom waters. The lack of carbonate microfossils in the "green sands" suggests a dissolution episode related to the expansion of the Antarctic cryosphere. This episode is recorded widely in deep-sea carbonate sequences and is associated with the well-known positive oxygen isotopic shift in the early Oligocene (33 Ma) (e.g., Shackleton and Kennett, 1975; Miller, 1991; Zachos et al., 1994). Based on our biochronology, the changes in the sediments suggest the opening of the Tasmanian Gateway to some cool shallow-water flow in the late Eocene and intensifying current flow toward the Eocene/Oligocene boundary. This was followed in the earliest Oligocene by expansion of the Antarctic cryosphere and deep-water interchange between the southern Indian and Pacific Oceans. This interchange heralds the initiation of the circumpolar current in this segment of the Southern Ocean. Although planktonic microfossils indicate climatic cooling during this interval, there is no evidence of glacial activity, such as ice-rafted debris. Indeed, the calcareous nannofossil assemblages suggest somewhat warmer conditions than in sequences at equivalent latitudes elsewhere in the Southern Ocean (Wei and Wise, 1990; Wei and Thierstein, 1991; Wei et al., 1992). The Oligocene clay assemblages at Site 1170 also suggest a transitional climate based on the co-occurrence of both smectite, indicating chemical weathering, and illite, indicating physical weathering.
Water depths throughout the region began to increase rapidly from the lowermost Oligocene upper bathyal depths. They reached lower bathyal depths during the early Neogene with a total water-depth increase averaging 2000 m. Clearly, such a depth increase must reflect rapid subsidence of the STR and the Tasmanian margin during the Oligocene along with expansion of the Tasmanian Gateway. The major and rapid reduction in siliciclastic sediment supply to the margin would also have contributed to the initial depth increase.
The upper Eocene "green sands" in the sequences closest to Antarctica (Sites 1170 and 1171) are overlain, with little gradation, by ooze and chalk of early Oligocene age. Near western Tasmania (Site 1168), a similar sequence shows more gradation upward in the Oligocene. From the earliest Oligocene onward, sedimentation in these sequences was completely dominated by relatively slow deposition of nannofossil ooze and chalks, faster than in the "green sands," but much slower than during the early and middle Eocene. Although the age of the base of the carbonates requires better constraint, existing stratigraphic data suggest deposition commenced soon after the earliest Oligocene oxygen isotopic shift at 33 Ma. This isotopic shift represents major cooling and the initial major cryospheric development of East Antarctica (Shackleton and Kennett, 1975; Miller, 1991; Wei, 1991; Zachos et al., 1994) and major expansion of the psychrosphere with its deep-ocean circulation (Kennett and Shackleton, 1976). Therefore, the synchronous commencement of biogenic carbonate deposition at all sites appears to reflect major climatic and oceanographic changes that affected broad regions of the STR and Tasmanian margins. This created a more dynamic, well-ventilated ocean with increased upwelling and higher surface-water biogenic productivity that increased rates of sedimentation of calcareous nannofossils and diatoms and decreased preservation of organic carbon. Open-ocean planktonic diatoms replaced neritic diatoms, which suggests coastal upwelling had begun on the STR and Tasmanian margin during the earliest Oligocene. Furthermore, associated cooling of the Antarctic continent apparently decreased weathering rates and transport of siliciclastic sediments to the margin. The environment of deposition was, thus, transformed from the late Eocene to the earliest Oligocene, from dominance of siliciclastics to dominance of calcareous nannofossils.
Sequences from other areas of the Antarctic margin show a similar drastic reduction in siliciclastics and increase in biogenic sedimentation during the Eocene-Oligocene transition. The lowermost Oligocene is often marked by an increase in biogenic sediments or of biogenic components in otherwise relatively slowly deposited siliciclastic sediments including diamictites (Diester-Haass and Zahn, 1996; Kennett and Barker, 1990; Salamy and Zachos, 1999). However, outside the Tasmanian region, the biogenic component is usually biogenic silica (diatoms) rather than calcium carbonate (calcareous nannofossils and foraminifers). On the shallow (probably neritic) northwest margin of the Weddell Sea, carbonate-free diatom ooze was deposited during the earliest Oligocene, suggesting significant cool-water upwelling (Barker, Kennett, et al., 1988). On the margins at Prydz Bay and southern Ross Sea, diatoms became an important component in diamictites (Barron et al., 1991a, 1991b; O'Brien, Cooper, Richter, et al., 2001).
Why was there such a sharp change from siliciclastic to carbonate sedimentation at the Eocene/Oligocene boundary? A very broad, shallow Australian-Antarctic shelf had been supplied with siliciclastics for tens of millions of years, and even though rifting, subsidence, and compaction had started early in the Cretaceous, sedimentation had kept pace and shallow-marine sediments were rapidly deposited through until the end of the middle Eocene. In the Tasmanian region, there was also subsidence related to the Late Cretaceous opening of the Tasman Sea and uplift on the western and eastern onshore margins in the late Paleocene to early Eocene (O'Sullivan and Kohn, 1997). Our studies show that the peak of Cenozoic tectonism occurred during the latest Paleocene (55 Ma) at Site 1171 and in the middle Eocene (~40 Ma) at Site 1170. Rifting between Australia and Antarctica led to almost complete separation of the continents and fast spreading in the middle Eocene (43 Ma), and this could be expected to increase the rate of subsidence. At Sites 1170, 1171, and 1172, glauconitic and siliciclastic late Eocene sedimentation almost kept up with subsidence until the Eocene/Oligocene boundary (33 Ma), some 10 m.y. after the onset of fast spreading, even though the sedimentation rate was then low.
Subsequently, the rate of subsidence increased at the Eocene/Oligocene boundary, an effect mirroring coeval rapid subsidence that formed the Victoria Land Basin in nearby Antarctica (Cape Roberts Scientific Team, 2000). This subsidence clearly had a regional cause (see "Tectonic Evolution") and reduced the area available for erosion in the Tasmanian region. The contemporaneous climatic cooling would have led to greatly reduced rainfall and, thus, weathering and erosion, further reducing siliciclastic supply. Thereafter, the sea deepened rapidly, and slow pelagic carbonate deposition dominated completely. This appears to have been the most significant interval of sediment starvation along the Antarctic margin.
The sequences at Sites 1170 and 1171 have revealed a marked difference in the Eocene-Oligocene sediment transition as compared with other parts of the Antarctic margin. As far as we can tell, no other margin sectors experienced pelagic carbonate deposition immediately following the Eocene/Oligocene boundary. Other areas experienced deposition of biosiliceous sediments or more slowly accumulating siliciclastic sediments with an increased siliceous biogenic component. The environment at the Tasmanian-Antarctic margin apparently favored 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 different climatic regimes in different sectors of the Antarctic margin during the Eocene and Oligocene. Antarctic circumpolar climatic and oceanographic similarity, a hallmark of the modern Antarctic, did not exist during the Paleogene. Biogeographic evidence from calcareous nannofossils and lack of any offshore evidence for glaciation in the Tasmanian sector indicate that conditions were slightly warmer than in other margin sectors, even during the Oligocene. The earliest Oligocene was a time of major cryosphere expansion in the southern Indian Ocean sector and in the southern Ross Sea. Thus, Antarctic circumpolar circulation was still insufficiently developed in the Oligocene to unify the character of Antarctic paleoenvironmental change.
Why are carbonates generally better preserved on the STR in the Paleogene than elsewhere on the Antarctic margin? This region may have been under the strong influence of warm low-latitude surface waters carried southward along the eastern margin of Australia by the East Australian Current and southward around western Australia into the Australo-Antarctic Gulf. Southward flow of warm waters along the east Australian margin would have been enhanced by constriction in the Indonesian Seaway in the Oligocene (Hall, 1996). These warm tropical 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 antiestuarine mode (Berger et al., 1996) marked by a downward flux of deep waters and an 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. Other sectors of the Antarctic margin, marked by strong carbonate dissolution at shallow water depths and high biosiliceous accumulation, may have operated in estuarine mode, marked by an upwelling of nutrient-rich deep waters and an outflow of surface waters. There, carbonate dissolution is favored by the upwelling of old, deep, low-alkalinity, high pCO2 waters such as 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 aridity of the continent and a major reduction of freshwater flow to the surrounding continental margin, which is reflected by marked reduction in the transport of siliciclastic sediments to the Antarctic margin. Surface waters near the margin would therefore 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 STR sector of the Antarctic margin, caused by its unusual plate tectonic setting, may well have especially enhanced bottom-water production and, in turn, increased carbonate-biogenic accumulation on the margin.
Drilling during Leg 189 was conducted to test the hypothesis formulated during DSDP Leg 29 (Kennett et al., 1974; Kennett, Houtz, et al., 1975; Kennett, 1977) that initial development and evolution of the Antarctica cryosphere during the middle and late Cenozoic resulted from thermal isolation of Antarctica by the development of the Antarctic Circumpolar Current and the Southern Ocean. The Cenozoic expansion of the Southern Ocean resulted from plate tectonic movements that caused northward movement of Australia and its southern continental extension—Tasmania and the STR—and the opening of Drake Passage (Weissel and Hayes, 1972; Barker and Burrell, 1977; Lawver et al., 1992; Cande et al., 2000). The opening of the Tasmanian Gateway created initial thermal isolation and resulted in the major cooling of Antarctica that also produced, through feedback mechanisms, the first ice sheets on at least some sectors of the continent. The four major sites drilled during Leg 189 penetrated to middle Eocene or even Upper Cretaceous sediments, thus providing a climatic and paleoceanographic record for the last 40 to 70 m.y., depending on the site. The sediment sequences reveal a remarkably coherent regional picture reflecting major paleoceanographic changes, especially during the mid-to late Paleogene and continuing into the Neogene, resulting from critical plate tectonic changes related to final separation of the STR from East Antarctica.
Changes in the sediment records drilled during Leg 189 are consistent with plate tectonic reconstructions (Royer and Rollet, 1997; Cande et al., 2000). These reconstructions suggest that before the late Eocene, the Tasmanian land bridge effectively blocked even shallow water connections between the Australo-Antarctic Gulf and the southwest Pacific Ocean (Fig. F24). Initial opening of the Tasmanian Gateway to shallow waters occurred some time in the late Eocene after ~37 Ma, and deeper waters by 33 Ma (earliest Oligocene) (Fig. F25). The two crucial effects of plate tectonics were therefore the initial development of a gateway to shallow waters, followed by subsidence of the STR and establishment of deep interocean communication. The sediment sequences recovered during Leg 189 suggest that major subsidence of the margins cored was very rapid, beginning near the Eocene/Oligocene boundary and becoming well advanced by the earliest Oligocene. The gateway was then fully open (Fig. F25), and the Tasmanian Seaway, involving both shallow to abyssal water flow, continued to expand during the remainder of the Cenozoic (Fig. F26) as it does today.
Major reorganization of Southern Ocean circulation occurred during the Eocene-Oligocene transition, as proposed by Kennett, Houtz, et al. (1975), Kennett (1977), and Murphy and Kennett (1986), as Antarctic circumpolar circulation commenced through the developing Tasmanian Seaway. Until the early late Eocene (Fig. F24), no Antarctic Circumpolar Current existed to interfere with the influence of the southward-flowing warm-subtropical arm of the South Pacific gyre (East Australian Current). These warm waters were transported close to Antarctica, warming this region and contributing toward the well-known relative warmth of the continent before the late Eocene, a trend supported by Leg 189 data. The opening of the Tasmanian Gateway and the development of Antarctic circumpolar circulation closely coincides with major cooling and cryospheric development of Antarctica during the Eocene-Oligocene transition (Shackleton and Kennett, 1975; Miller, 1991; Zachos et al., 1994). This allowed relatively cooler waters from the Australo-Antarctic Gulf into the South Pacific as the Antarctic Circumpolar Current, which led to initial decoupling of the warm-subtropical gyre from the Southern Ocean (Fig. F25) and cooling of South Pacific waters at high latitudes (Kennett, 1977, 1978; Murphy and Kennett, 1986). The decoupling strengthened as the Tasmanian Seaway continued to open during the Oligocene (Fig. F26) and throughout the remainder of the Cenozoic, with the expansion of the Southern Ocean. The ever-increasing strength and width of the Antarctic Circumpolar Current led to increased Cenozoic thermal isolation of Antarctica that, in turn, led to further positive feedbacks reinforcing Antarctic cooling and cryosphere expansion. Although the late Oligocene through Neogene record of this climatic evolution was not immediately apparent during shipboard investigations, we expect that this will be revealed as a result of postcruise isotopic and quantitative microfossil studies.
Major changes in the sediments from Leg 189 are consistent with the hypothesis that major Antarctic cooling occurred during the Eocene-Oligocene transition at the time of the Antarctic Circumpolar Current development through the Tasmanian Gateway. The most conspicuous sedimentological change for the entire last 65 m.y. was the Eocene-Oligocene transition from rapidly deposited siliciclastic sediments to pelagic carbonates. In Leg 189 sequences, the transition is associated with an increase in bottom currents, while the margins were still relatively shallow, in neritic to upper bathyal depths, leading to a conspicuous reduction in sedimentation rates and deposition of glauconitic silts and sands. Widespread hiatuses in the lower to mid-Oligocene indicate increased and pervasive current activity down to abyssal depths. Increased current strength in the narrow Gateway resulted in increased thermohaline circulation synchronous with psychrospheric development. This, in turn, was linked to enhanced bottom-water production around the now seasonally freezing Antarctic margin (Kennett and Shackleton, 1976).
As discussed above (see "Eocene-Oligocene Transition"), the relatively sudden decrease in deposition of siliciclastic sediments along the margins of the Tasmanian land bridge is inferred to have resulted largely from sediment starvation caused by a dramatic decrease in precipitation, humidity, chemical weathering, and, hence, run-off and sediment supply from Antarctica associated with the major cooling. This change corresponds to the large positive shift in oxygen isotopic values detected globally in the deep ocean, which reflects both cooling of the deep ocean upon development of the psychrosphere (deep-ocean cool waters) and significant ice accumulation on Antarctica, initiating the cryosphere. The change closely coincides with rapid subsidence of the STR and the Tasmanian margin, which also reduced the supply of siliciclastic sediments, led to open-ocean conditions and encouraged upwelling, and increased biogenic sedimentation. Increased upwelling elsewhere, over broad areas of the Antarctic margin, led to the onset of biosiliceous sedimentation, unlike the pelagic carbonate sedimentation of the Tasmanian margin.
Major glacial development of the Antarctic is inferred to have begun in the earliest Oligocene in a number of areas from the evidence of diamictites (Barron et al., 1991b; Hambrey et al., 1991; Cape Roberts Science Team, 2000) and ice-rafted sediments (Hayes, Frakes, et al., 1975; Barker, Kennett, et al., 1988; Zachos et al., 1992). Although early Oligocene mountain glaciers existed on Tasmania, it remained vegetated (Macphail et al., 1993). Shipboard investigations during Leg 189 revealed no evidence for Oligocene glacial activity on the Antarctic margin in the offshore Tasmanian region. This is consistent with the observations made during DSDP Leg 29 (Kennett, Houtz, et al., 1975). Furthermore, evidence is lacking for early Oligocene glacial activity at DSDP Site 274 in an equivalent sequence on the conjugate Antarctic margin near Cape Adare, where, even in the late Oligocene, glacial activity was extremely rare (Piper and Brisco, 1975). Our combined results strongly suggest that this sector of the Antarctic margin was relatively warm compared with others, especially in the Kerguelen sector in the southern Indian Ocean (see "Paleogene Margin Carbonates"). We present the hypothesis that relative warmth of the Tasmanian region resulted from long-term transport of heat toward the Southern Ocean by the warm East Australian Current (Figs. F24, F25). However, the influence of this warm current apparently did not extend into the southern Ross Sea in the early Oligocene, when marine diamictites were deposited at 77°S at Cape Roberts (Cape Roberts Scientific Team, 2000). Thus, a strong meridional climatic gradient existed in the Ross Sea sector of Antarctica. Indeed, a corollary of Leg 189 interpretations is that Antarctica was clearly marked by strong regional differences in climate during the Oligocene.
Earlier ideas of continent-wide ice sheets of present-day proportions in the Oligocene are unlikely based on descriptions of several margin sediment sequences drilled during the DSDP and ODP programs (e.g., Kennett and Barker, 1990; Hayes, Frakes, et al., 1975), including results from Leg 189. This probably includes the earliest Oligocene interval of inferred major ice growth (Zachos et al., 1992), evidence for which is also apparently lacking in the Leg 189 sequences. This conclusion appears reasonable considering that the Antarctic Circumpolar Current was still developing in the Oligocene, so its unifying circumpolar influence probably had not developed until the Neogene. Until then, regional climatic differences probably existed around the margin. Indeed, if it is correct that deep Antarctic circumpolar circulation formed in the earliest Miocene (Barker and Burrell, 1977), fundamental paleoceanographic changes associated with this development may have been the basis for differentiation of the Paleogene and Neogene by earth scientists in the dawn of the scientific era.
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. The Miocene-Pliocene transition is missing in a hiatus 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 in the sequences. The uppermost Miocene hiatus appears to have resulted from increased thermohaline circulation associated with Antarctic cryosphere expansion at that time (Hodell et al., 1986). Altogether, the Neogene sequences cored during Leg 189 provide a fine suite of sequences in present-day temperate (cool subtropical) 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 (~2-4 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 consistently present 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 southeastern 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 importance of calcareous nannofossils compared to planktonic foraminifers during the late Neogene.
Postcruise investigations of Leg 189 Neogene sequences will lead 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. Shipboard investigations of clay assemblages suggest relatively warmer conditions during the early Neogene until ~15 Ma. After that, clay assemblages suggest general regional cooling. During the late Neogene, clays became increasingly important in the pelagic carbonates, in part because of increasing dust transport from Australia. A distinct influx of kaolinite in 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.