The Cenozoic sea surface temperature evolution offshore Tasmania

4pm AEDT, Thursday 10 November 2022 
Jaeger 1 Seminar Room, Research School of Earth Sciences, Australian National University

During the Cenozoic (66–0 Ma) Tasmania has continuously been at a crucial geographic location. It represented the final tectonic connection between Australia and Antarctica before complete separation of both continents in the late Eocene, and therefore a barrier for circumpolar flow. Since the Eocene-Oligocene transition, the northward drifting Tasmania was bathed by the throughflow of the subtropical front, but remained an obstacle of the ideal flow path of strengthening ocean currents. The sedimentary record around Tasmania thus represents a perfect archive to record the oceanographic consequences of this regional tectonic change.

This research presents a new TEX86 and UK37-based SST compilation from 4 sediment cores: ODP Site 1172 (East Tasman Plateau), Site 1170 and 1171 (South Tasman Rise) and Site 1168 (western Tasman margin). These reconstructions were paired with microplankton (dinoflagellate cyst) assemblage data which reflect qualitatively the surface water conditions: nutrients, temperature, salinity. Together, the >1.300 samples portray the SST evolution around the island, from the time it was still connected to the Antarctic continent in the Paleocene to its near-subtropical location today.

Trends in the SST compilation broadly follow those in benthic foraminiferal stable isotope compilations, but with some interesting deviations. The expected warming due to northward tectonic migration of the island during the Cenozoic is largely compensated for by the general Cenozoic cooling trend. The mid-Paleocene is characterized by cool temperatures and sw Pacific surface waters were remarkably fresh, suggesting abundant riverine input. This diminished when Tasmania migrated northward during the early Eocene, warmed, and the hinterland dried up. Differences in SSTs on either side of the Tasmanian Gateway are small, even when the Tasmanian Gateway is considered closed. Widening of the Tasmanian Gateway around the Eocene-Oligocene transition immediately allows throughflow of what later becomes the Leeuwin Current, which warms the sw Pacific. Oligocene and Neogene SST trends follow those of the benthic d18O, and with continuous influence of the proto-subtropical front. Comparing Tasmanian SSTs to those from the wider sw Pacific region allows for a reconstruction of the evolution of Oligocene-Neogene Southern Ocean latitudinal SST gradients. While the SST evolution of Tasmania is remarkably stable in most of the Oligocene, prominent cooling steps are inferred in the Late Oligocene (26 Ma), at the MMCT (~14 Ma), in the mid-to-late Miocene (9 Ma, 7 Ma and 5.3 Ma) and in the Pliocene (2.8 Ma). Pleistocene SST variability is strong over glacial-interglacial cycles.

Taken together, the sites portray a complete overview of local environmental change of the subtropical front area, and provides crucial context to the history of Southern Ocean heat transport and regional climate.

Dr Peter Bijl is an Assistant Professor at Utrecht University, Netherlands, with unique expertise in climatic and environmental evolution of Antarctica and the Southern Ocean. His research predominantly draws on sedimentary archives from successive IODP scientific ocean drilling programs, and he has participated in IODP Expedition 318: Wilkes Land Glacial History in Antarctica and IODP Expedition 392: Agulhas Plateau Cretaceous Climate

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