The Impact of Tasman Gateway Opening on Early Paleogene Oceans and Climate

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The Antarctic Circumpolar Current (ACC) in the Southern Ocean is the largest ocean current in the world; it circles Antarctica uninterrupted flowing east to west isolating the continent from the southward flow of warm subtropical waters. The ACC is a major driver of ocean overturning (i.e., thermohaline circulation) redistributing heat, salinity and nutrients around the world, thus playing a critical role in regulating global climate. The ACC developed as Antarctic, Australia and South America continents separated creating an open marine passage in the Southern Hemisphere sometime between 34 and 50 million years ago. These gateway openings have been linked to driving the Earth's climate from a greenhouse state with high atmospheric carbon dioxide (CO2) levels and little or no ice sheets on Antarctica to a cooler 'icehouse' state with large ice sheets and lower atmospheric CO2 levels. The initiation of the ACC has been proposed as the mechanism for thermally isolating Antarctica, making it easier to grow large ice sheets on the continent and cooling the global ocean. However, more recent work suggests that declining atmospheric CO2 levels were the primary driver of global cooling. We currently lack highly resolved climatic and ocean circulation records from close to the opening gateways which spanning the critical transition, in particularly the poorly recovered early Eocene (~45-50 million years ago), making distinguishing between these scenarios challenging. In September-November 2017, International Ocean Drilling Project Expedition 369 will recover brand new deep-sea sediments from the west of the Tasman Gateway (the final pinch-point in the development of the ACC), that span the interval during which the gateway opened and the ACC developed. I will measure the geochemical composition of calcareous shells from the marine microfossils foraminifera (single-celled organisms) found in deep-sea sediments. Foraminifera precipitate their shell from the seawater in which they live, and thus provide a record of the changing temperature, salinity and productivity at the seafloor. Foraminiferal shell chemistry can be used to trace bottom waters as each has a distinctive chemical signature dependent on the area where they were formed and their relative 'age', i.e., how long they have been isolated from surface waters. This signature can be detected and the extent of each water mass within ocean basins mapped, and the source regions identified. This established tool will constrain the source and intensity of deep-water formation throughout the focal interval and the relationship to ACC onset. Ultimately, this work will provide a greater understanding of how and when the ACC evolved, and its role in shaping ancient climate change. This project will also provide key data to fill spatial gaps for testing and configuration of numerical palaeoclimate models.