Abstract. Improvements in our capability to reconstruct ancient
surface-ocean conditions based on organic-walled dinoflagellate cyst
(dinocyst) assemblages from the Southern Ocean provide an opportunity to
better establish past position, strength and oceanography of the subtropical
front (STF). Here, we aim to reconstruct the late Eocene to early Miocene
(37–20 Ma) depositional and palaeoceanographic history of the STF in the
context of the evolving Tasmanian Gateway as well as the potential influence of
Antarctic circumpolar flow and intense waxing and waning of ice. We approach
this by combining information from seismic lines (revisiting
existing data and generating new marine palynological data from Ocean Drilling Program (ODP) Hole 1168A)
in the western Tasmanian continental slope. We apply improved taxonomic
insights and palaeoecological models to reconstruct the sea surface
palaeoenvironmental evolution. Late Eocene–early Oligocene (37–30.5 Ma)
assemblages show a progressive transition from dominant terrestrial
palynomorphs and inner-neritic dinocyst taxa as well as cysts produced by
heterotrophic dinoflagellates to predominantly outer-neritic/oceanic
autotrophic taxa. This transition reflects the progressive deepening of the
western Tasmanian continental margin, an interpretation supported by our new
seismic investigations. The dominance of autotrophic species like Spiniferites spp. and
Operculodinium spp. reflects relatively oligotrophic conditions, like those of regions
north of the modern-day STF. The increased abundance in the earliest Miocene
of Nematosphaeropsis labyrinthus, typical for modern subantarctic zone (frontal) conditions, indicates a
cooling and/or closer proximity of the STF to the site . The absence of
major shifts in dinocyst assemblages contrasts with other records in the region
and suggests that small changes in surface oceanographic conditions
occurred during the Oligocene. Despite the relatively southerly
(63–55∘ S) location of Site 1168, the rather stable oceanographic
conditions reflect the continued influence of the proto-Leeuwin Current
along the southern Australian coast as Australia continued to drift
northward. The relatively “warm” dinocyst assemblages at ODP Site 1168,
compared with the cold assemblages at Antarctic Integrated Ocean Drilling Program (IODP) Site U1356, testify to the
establishment of a pronounced latitudinal temperature gradient in the
Oligocene Southern Ocean.