The Cenozoic sea surface temperature evolution offshore Tasmania

Abstract

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. We here present a new TEX86 and UK’37-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). We paired these reconstructions 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. Differences in SSTs on either side of the Tasmanian Gateway are small in the early Paleogene (66–34 Ma), even when the Tasmanian Gateway is considered closed. Widening of the Tasmanian Gateway around the Eocene-Oligocene transition (34Ma) 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. 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). The remarkably strong Neogene cooling of the subtropical front implies expansion of subpolar temperate conditions and probably gradual strengthening of the Antarctic circumpolar current. Pliocene-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.