Abstract
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.
Highlights
Late stages of continental break-up between Australia and Antarctica in the late Eocene–early Miocene led to gradual deepening and widening of the Tasmanian Gateway (Lawver et al, 1992; Cande and Stock, 2004; Whittaker et al, 2013)
Our study shows that ocean waters at the Australian margin in the late Eocene–early Miocene resembled those of modern subtropical water near-shore sites north of New Zealand (Prebble et al, 2013), despite the more southerly position of Australia
We present new seismic interpretations and marine palynological association data to reconstruct the late Eocene to early Miocene palaeoceanographic conditions on the western Tasmanian continental margin
Summary
Late stages of continental break-up between Australia and Antarctica in the late Eocene–early Miocene led to gradual deepening and widening of the Tasmanian Gateway (Lawver et al, 1992; Cande and Stock, 2004; Whittaker et al, 2013) This process redirected Southern Ocean surface water circulation (Stickley et al, 2004a; Bijl et al, 2013; Sijp et al, 2014, 2016) and regionally redistributed ocean heat (Sijp et al, 2014, 2016) with notable consequences for Antarctic surface water temperatures (Houben et al, 2019; Sauermilch et al, 2021). Dinocysts are increasingly and successfully used as a proxy for palaeoceanographic reconstructions in the Southern Ocean (e.g. Houben et al, 2013; Prebble et al, 2013; Bijl et al, 2018; Sangiorgi et al, 2018)
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