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At Ocean Drilling Program (ODP) Leg 189 Sites 1170–1172, the climatologically critical Eocene–Oligocene (E-O) transition is barren of any calcareous microfossils but contains rich marine organic walled dinoflagellate cyst (dinocyst) and diatom assemblages, suitable for detailed biostratigraphic and paleoenvironmental analysis. The resulting first-ever integrated dinocyst/diatom magnetostratigraphy allows confident correlation of the E-O interval between all Leg 189 sites, including Site 1168. Our correlations indicate that the (deep) opening of the Tasmanian Gateway occurred quasi-synchronously throughout the Tasmanian region, starting at ~35.5 Ma. At Sites 1170–1172, quantitatively, three distinct dinocyst assemblages may be distinguished that reflect the relatively rapid and pronounced stepwise environmental changes associated with the E-O transition in the Tasmanian region, from a prodeltaic setting to a deep marine pelagic setting. Moreover, synchronous with the deepening of the gateway, at the southern and eastern Sites 1170–1172, typical endemic Antarctic assemblages were replaced by more cosmopolitan dinocyst communities. In marked contrast, at Site 1168 off western Tasmania, endemic Antarctic taxa are virtually absent during the E-O transition. At Sites 1170–1172, the endemic Antarctic dinocyst assemblage (Transantarctic Flora) drastically changes into a more cosmopolitan assemblage at ~35.5 Ma, with a more offshore character, reflecting the ar1Sluijs, A., Brinkhuis, H., Stickley, C.E., Warnaar, J., Williams, G.L., and Fuller, M., 2003. Dinoflagellate cysts from the Eocene–Oligocene transition in the Southern Ocean: results from ODP Leg 189. In Exon, N.F., Kennett, J.P., and Malone, M.J. (Eds.), Proc. ODP, Sci. Results, 189, 1–42 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3584CD Utrecht, The Netherlands. Correspondence author: A.Sluijs@bio.uu.nl 3Lamont-Doherty Earth Observatory, 61 Route 9W, PO Box 1000, Palisades NY 10964, USA. Present address: School of Earth, Ocean, and Planetary Sciences, Cardiff University, PO Box 914, Cardiff CF10 3YE, UK. 4Atlantic Geoscience Centre, PO Box 1006, Dartmouth NS B2Y 4A2 Canada. 5Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu HI 96822, USA. Initial receipt: 13 November 2002 Acceptance: 15 July 2003 Web publication: 22 October 2003 Ms 189SR-104 A. SLUIJS ET AL. EOCENE–OLIGOCENE DINOFLAGELLATE CYSTS 2 rival 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 close to the Oceanic Anoxic (Oi)-1 δ18O (Antarctic glaciation) event (~33.3 Ma). In a single productive sample from the earliest Oligocene the northern Site 1172, a relatively warm-water cosmopolitan assemblage has been recovered. This aspect contrasts findings from coeval deposits from the Ross Sea, where endemic Antarctic species remain dominant. Somewhere between the paleogeographic 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.

Dinoflagellate Cysts from the Eocene–Oligocene Transition in the Southern Ocean: Results from ODP Leg 189