Tectonic Gateways and Paleogene Southern Ocean Circulation: The Proto-Antarctic Circumpolar Current

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Strong westerly winds drive the Antarctic Circumpolar Current (ACC) in the Southern Ocean. The ACC thermally isolates Antarctica in an annulus of cold water and brings nutrient-rich thermocline waters to the surface through upwelling at the Antarctic Divergence. The role of the ACC in Cenozoic planetary cooling is unknown, largely due to uncertainties in the timing of critical tectonic gateway openings. Neodymium (Nd) isotope measurements from sediment cores in the Atlantic sector of the Southern Ocean have revealed the early stages of Drake Passage opening during the late middle Eocene. The propagation of the distinct Pacific seawater Nd isotope signal into the Atlantic sector in association with increases in paleoproductivity proxies is the basis for our hypothesis that a proto-ACC (shallow to intermediate depths) had developed by the late Eocene. To test this hypothesis, we will measure Nd isotopes from sediment cores in the Indian and Pacific sectors to track the effects of new interocean connections along the circum-Antarctic pathway. The recognition of similar circulation patterns between the Atlantic and Indian sectors will strongly support our hypothesis. The early opening history of Tasmanian Gateway is also an important paleoceanographic objective. Two conflicting paleocirculation reconstructions are supported by recent drilling in the Tasman Sea. The 'warm scenario' suggests seawater in the Tasman Sea was derived from the low latitude Indian Ocean after the gateway opening while the 'cold scenario' suggests a high latitude source. Nd isotope measurements of sediment cores from the south Tasman Rise will test these opposing hypotheses. This study will provide a more detailed understanding of the evolution of the ACC relative to Cenozoic climate evolution. This project provides postdoctoral support and career development, and results will be incorporated into presentations given to docents at the Seymour Marine Discovery Center, a public education facility at UCSC.