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. Improvements in our capability to reconstruct ancientsurface-ocean conditions based on organic-walled dinoflagellate cyst(dinocyst) assemblages from the Southern Ocean provide an opportunity tobetter establish past position, strength and oceanography of the subtropicalfront (STF). Here, we aim to reconstruct the late Eocene to early Miocene(37–20 Ma) depositional and palaeoceanographic history of the STF in thecontext of the evolving Tasmanian Gateway as well as the potential influence ofAntarctic circumpolar flow and intense waxing and waning of ice. We approachthis by combining information from seismic lines (revisitingexisting data and generating new marine palynological data from Ocean Drilling Program (ODP) Hole 1168A)in the western Tasmanian continental slope. We apply improved taxonomicinsights and palaeoecological models to reconstruct the sea surfacepalaeoenvironmental evolution. Late Eocene–early Oligocene (37–30.5 Ma)assemblages show a progressive transition from dominant terrestrialpalynomorphs and inner-neritic dinocyst taxa as well as cysts produced byheterotrophic dinoflagellates to predominantly outer-neritic/oceanicautotrophic taxa. This transition reflects the progressive deepening of thewestern Tasmanian continental margin, an interpretation supported by our newseismic investigations. The dominance of autotrophic species like Spiniferites spp. andOperculodinium spp. reflects relatively oligotrophic conditions, like those of regionsnorth of the modern-day STF. The increased abundance in the earliest Mioceneof Nematosphaeropsis labyrinthus, typical for modern subantarctic zone (frontal) conditions, indicates acooling and/or closer proximity of the STF to the site . The absence ofmajor shifts in dinocyst assemblages contrasts with other records in the regionand suggests that small changes in surface oceanographic conditionsoccurred during the Oligocene. Despite the relatively southerly(63–55∘ S) location of Site 1168, the rather stable oceanographicconditions reflect the continued influence of the proto-Leeuwin Currentalong the southern Australian coast as Australia continued to driftnorthward. 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 theestablishment of a pronounced latitudinal temperature gradient in theOligocene Southern Ocean.

Late Eocene–early Miocene evolution of the southern Australian subtropical front: a marine palynological approach