INTRODUCTION

The Cenozoic Era is remarkable in its transition from a "greenhouse" to "icehouse" Earth. Progressive high-latitude cooling eventually resulted in the formation of major ice sheets, initially on Antarctica and later in the Northern Hemisphere. This process, associated with global oceanic circulation changes, severely affected biota, resulting in closely spaced extinctions and originations of species (e.g., reviews in Berggren and Prothero, 1992). In the early 1970s, a hypothesis was proposed that climatic cooling and an Antarctic cryosphere developed as the Antarctic Circumpolar Current (ACC) progressively thermally isolated the Antarctic continent. This current is thought to have resulted from the opening of critical Antarctic conduits (i.e., the Tasmanian Gateway south of Tasmania and the Drake Passage) (Kennett, 1977; Murphy and Kennett, 1986; Shipboard Scientific Party, 2001a).

The five Ocean Drilling Program (ODP) Leg 189 sites (1168-1172; March-May 2000) (Fig. F1) were designed to test the above hypothesis and improve the understanding of Southern Ocean evolution and its relation with Antarctic and global climatic development. The relatively shallow region off Tasmania is one of the few places where well-preserved and almost-complete marine Cenozoic sequences can be drilled in paleolatitudes up to 70S (Shipboard Scientific Party, 2001a). The basic architecture of the Paleogene sedimentary succession at Sites 1168 and 1170-1172 is similar in recording three major phases of paleoenvironmental development: (1) (Maastrichtian to) middle Eocene deposition of shallow-water siliciclastic organic-rich marine sediments during rifting between Antarctica and the South Tasman Rise, (2) a transitional condensed interval with relatively shallow water late Eocene glauconitic siliciclastic sediments giving way to earliest Oligocene deep marine pelagic carbonates representing the activation of bottom currents as the Tasmanian Gateway opened and deepened during early drifting, and (3) Oligocene-Quaternary deposition of pelagic carbonate sediments in increasingly deeper waters and more open ocean conditions as the Southern Ocean developed and expanded with the northward migration of the Australian continent (Shipboard Scientific Party, 2001a).

Shipboard palynological analysis indicated that well-preserved palynomorphs, notably organic walled dinoflagellate cysts (dinocysts) and sporomorphs are present in Maastrichtian to lowermost Oligocene strata (see Shipboard Scientific Party, 2001a; Brinkhuis, Munsterman, et al., and Brinkhuis, Sengers, et al., both this volume). The dominance of different groups of other microfossils drastically changes with depositional environment. Calcareous groups are most prominent from the Oligocene to Quaternary, whereas siliceous groups are common from the middle Eocene to Quaternary (Shipboard Scientific Party, 2001a).

Initial onboard micropaleontological investigations indicated that the critical Eocene-Oligocene (E-O) transitional interval, related to the rapid deepening of the Tasmanian Gateway and possibly to ACC formation, is virtually devoid of calcareous microfossils at all Leg 189 sites. In contrast, the interval is marked by the occurrence of rich assemblages of palynomorphs, notably dinocysts, and rich diatom associations. Initial studies on this critical time interval indicate that the dinocyst assemblages are suitable for biostratigraphic and paleoenvironmental analyses (Shipboard Scientific Party, 2001a; Brinkhuis, Munsterman, et al., and Brinkhuis, Sengers, et al., both this volume).

A significant number of studies concentrating on Late Cretaceous to late Eocene dinocysts from the broad Antarctic realm or Southern Ocean are available, notably from Argentina, Southeast Australia, New Zealand, the Ross ice shelf and Seymour Island, besides several Deep Sea Drilling Program (DSDP)/ODP sites (see overviews in, e.g., Askin, 1988a, 1988b; Wilson, 1988; Wrenn and Hart, 1988; Coccozza and Clarke, 1992; Mao and Mohr, 1995; Truswell, 1997; Hannah and Raine, 1997; Hannah et al., 2000; Levy and Harwood, 2000, and Guerstein et al., 2002). These studies have documented Southern Ocean Paleogene dinocyst distribution and taxonomy in great detail. In contrast, previous studies concentrating on the E-O transition in the region are few (e.g., Clowes, 1985; Edbrooke et al., 1998). Moreover, meaningful chronostratigraphic calibration of dinocyst events in this region is largely absent. Considering that the Upper Cretaceous to Paleogene succession of, for example, Site 1172 has a robust magnetostratigraphy, intercalibrated by biotic events (Stickley et al., this volume; Stickley et al., submitted [N1]), great potential to tie dinocyst events to the Geomagnetic Polarity Time Scale (GPTS) is available here.

Broad overviews of the dinocyst distribution from the Maastrichtian to the lowermost Oligocene and Quaternary intervals of Site 1172 and the upper Eocene-Quaternary of Site 1168 are presented elsewhere (Brinkhuis, Munsterman, et al., and Brinkhuis, Sengers, et al., both this volume). Here, we aim to present a more detailed stratigraphic analysis of the lithologically similar E-O transition at Sites 1170-1172, principally using dinocysts, and refine this correlation using relevant data that have recently become available, such as diatom events and paleomagnetic information (Stickley et al., this volume, submitted [N1]). In addition, we propose correlations to Site 1168 and other Southern Ocean localities. Several new dinocyst taxa have been recorded; some of them are informally characterized herein, others are placed in broad generic groups. Future work on this material will describe these taxa formally and more details, also on other constituents of applied generic groupings, will be presented elsewhere (e.g., see Sluijs and Brinkhuis, submitted [N3]).

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