FILLING THE SOUTHERN OCEAN CIRCULATION GAP: INSIGHTS FROM THE KERGUELEN PLATEAU

Deciphering the evolution of mass water circulation around the Eocene-Oligocene transition has important implications in understanding the timing and inception of the Antarctic Circumpolar Current (ACC), a controlling factor in the development of Earth’s ‘icehouse’ climate. Previous work has largely focused on inferring ACC development based on the timing of oceanic gateway opening within tectonic reconstructions of the Southern Ocean, and correlating the opening of gateways with the timing of significant Antarctic glaciation (ca. 34 Ma). However, there is controversy surrounding the age of deep water flow through the Drake Passage (ranging from 34 to 11 Ma), and the response of sediment transport systems to ACC development is ambiguous. This has prompted an analysis of the oceanographic expression of the ACC based on the homogenisation of water mass tracers from the Pacific and Indian/Atlantic Ocean basins.

We present a new neodymium (Nd) isotope record (¹⁴³Nd/¹⁴⁴Nd ratios) from the upper Eocene to lower Oligocene section of ODP Site 748 (Kerguelen plateau, Indian Ocean). Nd isotope ratios provide a useful means to monitor Indian/Atlantic and Pacific Ocean water mass communication through Southern Ocean gateways. We assess our results from the Kerguelen plateau (Indian Ocean) with existing Nd isotope ratio datasets, including those from Maud Rise, Agulhas Ridge, western Tasmanian Margin, East Tasman Rise, and Hikurangi Plateau, in order to determine the timing of mass water mixing in the Southern Ocean. These results are interpreted within the context of a refined tectonic reconstruction of the Southern Ocean, which incorporates the well-constrained history of seafloor spreading in the South Pacific, the Southern Ocean between Australia and Antarctica and an updated model of the Scotia Sea. Our results provide new insights into the Cenozoic evolution of Southern Ocean circulation, and can be used in the validation of high-resolution climate simulations.