Dinoflagellate and other palynomorph abundance in sediments from ODP Leg 189 sites

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 pro-deltaic 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 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 close to the Oceanic Anoxic (Oi)-1 18O (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 abundance and events of ODP Hole 189-1168A sediments

Palynomorphs were studied in samples from Ocean Drilling Program (ODP) Leg 189, Hole 1168A (slope of the western margin of Tasmania; 2463 m water depth). Besides organic-walled dinoflagellate cysts (dinocysts), broad categories of other palynomorphs were quantified in terms of relative abundance. In this contribution, we provide an overview of the early late Eocene-Quaternary dinocyst distribution and illustrate main trends in palynomorph distribution. Dinocyst species throughout Hole 1168A are largely cosmopolitan with important contributions of typical low-latitude taxa and virtual absence of endemic Antarctic taxa. Dinocyst stratigraphic distribution broadly matches that known from the Northern Hemisphere and equatorial regions, although significant differences are noted. Selected potentially biochronostratigraphically useful events are summarized. The distribution of dinocysts in the middle-upper Miocene interval is rather patchy, probably due to prolonged exposure to oxygen. An important general aspect in the dinocyst assemblages is the near absence of Antarctic endemic species and the apparent influence of relatively warm waters throughout the succession at Site 1168. General palynomorph distribution indicates continued deepening from an initial shallow, even restricted, marine setting from late Eocene-Quaternary times. A curious massive influx of small skolochorate acritarchs is recorded throughout the late early-early middle Miocene; the significance of this signal is not yet understood. A general long-term oligotrophic nature of the surface waters influencing Site 1168 is suggested from the low abundance of (proto) peridinioid, presumably heterotrophic, species. The overall dinocyst distribution pattern corresponds to the long-term existence of a Leeuwin-like current influencing the region, including Site 1168, confirming results of earlier studies on other microfossil groups. The occasional influence of colder surface water conditions is, however, also apparent, notably during the late Pliocene-Quaternary, indicating the potential of high-resolution dinocyst analysis for future paleoceanographic studies.