SUMMARY OF RESULTS (continued)
Leg 189 has provided a wealth of new and exciting biostratigraphic and paleoenvironmental information in a part of the Southern Ocean that is critical to understanding Earth's Cenozoic history. We drilled five sites in the Tasmanian region, north and south of the Subtropical Convergence, and in the Indian and Pacific Ocean sides of the STR (Fig. 17). The recovered record spans the Late Cretaceous to Quaternary (Fig. 18) but is commonly punctuated by a late Miocene hiatus and a possible Eocene-Oligocene hiatus (Fig. 19). The integrated microfossil record from all five sites will help unravel the southern history of the East Australian Current, which bathed the Pacific Ocean sites, and also the history of the "Proto-Leeuwin" Current, which bathed the Indian Ocean sites. The activity of both of these currents brought warmer waters to the study area, resulting in markedly different faunal associations when compared with other circum-Antarctic sites (e.g., Leg 113). The late Miocene hiatus is marked by a strong dissolution event in the foraminifers and is restricted to the southernmost sites (Sites 1169, 1170, and 1171), though strong fragmentation of foraminifers was also observed at Sites 1168 and 1172. The transition from "Greenhouse" to "Doubthouse" is strikingly seen at four of the five sites (Fig. 19). The latest Eocene to earliest Oligocene period of slow sedimentation (~1 cm/k.y.), which may be continuous or marked by hiatuses, records the history of this dramatic change in Earth's history. Five major microfossil groups are represented over this interval and provide the opportunity for shore-based collaboration to elucidate the details of this event. At Site 1172, a thick sequence of upper Cretaceous to Eocene fine-grained mudstones of the "Greenhouse" period, often glauconitic and organic rich, contains a continuous dinocyst record. Sporadic calcareous nannofossils and planktonic and benthic foraminifers through this interval will provide age control, as well as paleodepth information.
The planktonic foraminiferal distribution reflects the northward drift of Australia during the Cenozoic. For the Paleogene, because of the southern location of Australia at that time, the subantarctic zonal scheme was used in place of the traditional temperate scheme. This temperate scheme was appropriate for the late Paleogene and Neogene. In addition, the Neogene faunas reflect changing paleoceanography throughout the "Icehouse" period in their diversity, preservation, and phylogeny. The expanded Oligocene and Neogene section of the two northern sites (with sedimentation rates between 1.7 and 4.3 cm/k.y.), and of Site 1168 in particular, means that a standard biostratigraphic section can be established for the shelfal and land exposures to the north (this applies also to the other microfossil groups). The Globoconella group is widely distributed through the Neogene section and phylogenetic studies coupled with paleoceanographic results should provide an improved planktonic foraminiferal biostratigraphy for the region. The low-diversity planktonic foraminiferal assemblages of the Late Cretaceous and Paleogene are generally very well preserved, but their abundances are low.
Benthic foraminifers at all sites show a clear change between neritic water depths in the Cretaceous to Eocene interval and deeper waters of the Oligocene and Neogene. At all sites except one (Site 1168), subsidence across the Eocene/Oligocene boundary appears to be a rapid event, whereas the trend at Site 1168 is more gradual. However, this site is the only one with an expanded Oligocene sequence. Benthic foraminiferal assemblages indicate a more pronounced regional influence in the Eocene, whereas low-resolution sampling across the Oligocene and Neogene suggests that the general trends at all sites are similar for the Neogene, with only Site 1168 showing clear differences throughout the Oligocene.
The Neogene standard nannofossil zonal scheme and the Paleogene nannofossil zonation for the Southern Ocean were successfully applied to Leg 189 sites to provide some of the most useful subantarctic temperate biostratigraphic records to date. In particular, the Oligocene to Pliocene interval is among the most detailed of the Southern Ocean sites of similar latitudes. This sequence will serve as an important reference section for the Southern Hemisphere. Eocene through lower Oligocene nannofossil assemblages at Leg 189 sites show warmer-water characteristics than comparable paleolatitude sites in other sectors of the Southern Ocean (Sites 512, 513, 699, 703, and 747), which reflects the activity of the warm-water Proto-Leeuwin and the East Australian Currents.
Diatoms are abundant in Oligocene to Quaternary sediments at all sites drilled during Leg 189, except Site 1168, and will be used to construct the first calibrated Oligocene-Holocene diatom biostratigraphy south of Australia. In particular, high-resolution biostratigraphy looks especially promising for Sites 1170 and 1171. Integration with other Paleogene and Neogene diatom biostratigraphies being developed from recent ODP legs to the Southern Ocean (e.g., Leg 177) will contribute to a scheme applicable to northern parts of the Southern Ocean. Neritic diatoms are prolific in upper Eocene and lowermost Oligocene sediments at Sites 1170, 1171, and 1172. In conjunction with dinocysts, they will be useful for reconstructing paleoenvironmental conditions across the Eocene/Oligocene boundary including productivity (trophic status), water energy levels, relative salinity, and sea level for both the Indian and Pacific Oceans. During the early Oligocene, the marked floral changes and increased diversity within Sites 1170, 1171, and 1172 imply increasing oceanic influence and productivity. The Neogene diatom floras indicate fluctuations in the influence of warm-temperate and subantarctic water masses over Site 1170, heralding meridional shifts in the position of the Subtropical Convergence as well as the presence of the Proto-Leeuwin Current. At Site 1172, similar fluctuations may signal variations in the influence of the East Australian Current.
Radiolarians are well represented at all sites except Site 1168, although diversity varies through the sequences. The subantarctic radiolarian biostratigraphic sequence from the middle Eocene through Pleistocene is unique and will provide an important contribution to this group's biostratigraphy in the form of a new zonation. This will provide a linkage between the tropical and Antarctic biostratigraphies. There is a near absence of age-diagnostic species from the Antarctic and tropical realms in the Tasmanian region. The radiolarian faunas from Leg 189 also provide an insight into radiolarian evolution.
The dinoflagellate-cyst studies have provided a number of surprises. The first Neogene and Oligocene dinocyst record of the Southern Ocean was discovered at Site 1168, with excellent calibration. These microfossils are massively abundant and, with sporomorphs, are common in the Upper Cretaceous to lowermost Oligocene successions. Together with palynofacies analysis, there is a great potential for well-calibrated high-resolution biostratigraphy. Dinoflagellate cysts appear to provide the sole biostratigraphic means for age assessment of the critical uppermost Eocene-lowermost Oligocene interval (the so-called "barren green sands"), whereas sporomorphs are present as well. Together with the diatoms, the palynomorphs will allow detailed paleoenvironmental interpretation. Postcruise paleoenvironmental and paleoclimatological studies, including "land-sea correlation" are possible on all conceivable time scales, down to <20-k.y. cyclicities. Also, the first well-calibrated Paleogene dinoflagellate and sporomorph records from the Southern Hemisphere should result from postcruise studies. Cores have yielded the best imaginable material to study the variability of dinocyst morphology, notably within the Deflandrea phosphoritica and Areosphaeridium diktyoplokum-Enneadocysta partridgei groups.
Future stable isotopic studies on planktonic foraminifers, together with quantitative analysis of diatoms, nannofossils, and dinocysts, should be useful for detailed reconstruction of the sea surface paleotemperatures and paleoproductivity and other paleoenvironmental parameters, such as fluctuations in the position of the Subtropical Convergence. An outstanding biostratigraphic contribution resulting from Leg 189 will be an integrated zonation scheme, including the six microfossil groups for the Oligocene to the Quaternary for this sector of the Southern Ocean.
Summary of Results-Paleomagnetism | Table of Contents