Uranium isotopes and past changes in Southern Ocean circulation

University Of Southern Mississippi, Hattiesburg MS

Investigators

Abstract

Ocean circulation has deep connections with processes around the Earth including the melting or growing of continental ice sheets. The exact mechanisms linking ocean circulation to ice sheet variability can be difficult to tease out using modern observations alone. Geologists therefore look to the past for examples of when ocean circulation changed and how those changes relate to other phenomena in past eras. In particular, it has been found that the circulation of Antarctic Bottom Water, dense water that sinks around the Antarctic continent and fills the deepest parts of the world?'s ocean, may have a very intimate link with melting of the West Antarctic Ice Sheet. The concentration of naturally-occurring uranium in Southern Ocean sediments, formed under low oxygen conditions, is a key tool that has been applied to reconstruct past circulation changes during one past warm period, ~125,000 years ago, in one area of the Southern Ocean. This project will apply this recently developed method to other areas of the Southern Ocean and to other time periods in the past (up to 500,000 years ago) in order to investigate a more robust mechanism for how circulation around Antarctica is linked to ice sheet melting and how this might change in the future. The project supports an early-career researcher as well the training of a graduate and undergraduate students in the research. Outreach will be accomplished through the development of a new course and with high school students through events such as the Ocean Sciences Bowl competition. The project will contribute to infrastructure by developing a new proxy for paleoceanography, which has the potential to better constrain models of ice sheet melting and inform on issues related to climate change and sea level rise. To more accurately reconstruct past deep Southern Ocean oxygenation conditions, and therefore Antarctic Bottom Water (AABW) circulation, the project will utilize a novel geochemical approach using the isotope composition of authigenic uranium (U-234/U-238), augmented by other redox sensitive metals (Re and Mn). This combination of measurements allows for periods of changing oxidation to be identified that elemental proxies alone could not detect. This approach will be applied in multiple Southern Ocean cores to test for reproducibility and sensitivity to varying sedimentary environments. Furthermore, the project will investigate multiple interglacial periods over the past 500,000 years to examine the response of AABW across a range of forcings and Antarctic Ice sheet extents. The result will be a robust perspective on AABW variability during interglacial periods and its relationship to external forcings (orbital configuration, CO2, Southern Ocean productivity) and West Antarctic Ice Sheet melting.