Our studies resulted in an unprecedented integrated biomagnetostratigraphic record across the E-O transition, providing the first ever calibration for Southern Ocean dinocyst records. It now appears that several LOs of cosmopolitan species (e.g., H. semilunifera and S. speciosa) are older here than they are in the Tethyan realm (e.g., Brinkhuis and Biffi, 1993). This aspect may be due to the overall global cooling trend during the latest Eocene, resulting in equatorward migration of cosmopolitan taxa from the higher latitudes. Further integration and comparison of these and coeval circum-Antarctic endemic events (see compilation in Williams et al., this volume), widely recorded around Antarctica in previous studies, will lead to better assessment of the timing of paleoenvironmental changes in the Paleogene of the Southern Ocean, notably in shallow-marine domains.
For the first time, integrated biomagnetostratigraphy allows detailed age assessment of the critical deepening of the Tasmanian Gateway. Our correlations indicate that this event occurred quasi-synchronously throughout the Tasmanian region, starting at ~35.5 Ma, some 2 m.y. before the E/O boundary. Quantitatively, the succession of three distinct dinocyst assemblages reflects the relatively rapid and pronounced stepwise environmental changes associated with the E-O transition in the Tasmanian region.
The oldest association reflects a pro-deltaic setting that prevailed in the region since the Cretaceous (see also Brinkhuis, Sengers, et al., this volume). This association has been recognized all around the Antarctic margin and is known as the Transantarctic Flora (cf., e.g., Wrenn and Beckman, 1982). In marked contrast, at Site 1168 off western Tasmania, these endemic Antarctic taxa are conspicuously absent in upper Eocene deposits (Brinkhuis, Munsterman, et al., this volume). This aspect indicates that surface waters influencing Site 1168 were distinctly different than those at the other Leg 189 sites (see Stickley et al., submitted [N1]; Huber et al., submitted [N2]). At Sites 1170-1172, this Transantarctic Flora assemblage drastically changes into a more cosmopolitan assemblage at ~35.5 Ma, generally with a more offshore character. This reflects the arrival of different oceanographic and environmental conditions associated with the deepening of the Tasmanian Gateway. In turn, this assemblage grades at ~34 Ma into one more typical for even more offshore and/or upwelling conditions at Site 1172. In slightly younger deposits at all sites, organic microfossils are virtually absent, reflecting winnowing and oxidation, indicative of a next step of oceanographic development. This phase may be dated as very near or at the Oi-1 18O (Antarctic glaciation) event of Zachos et al. (1992, 1994) (~33.3 Ma), and it appears that the two phenomena may be intrinsically connected. In a single productive sample from the earliest Oligocene at the northern Site 1172, a relatively warm water cosmopolitan assemblage was recovered. This aspect contrasts findings from coeval deposits from the Ross Sea, where endemic Antarctic species remain dominant (Hannah et al., 2000). Thus, somewhere between the paleo-positions of Site 1172 and the Ross Sea, a strong differentiation of surface waters occurred in the earliest Oligocene, possibly reflecting the onset of the Antarctic Circumpolar Current.