Beschreibung
Earth’s temperature has varied in the past in response to orbital variations and to changes in concentrations of atmospheric greenhouse gases such as carbon dioxide (CO2). However, theory and modelling studies suggest the CO2 threshold for Antarctic ice sheet melting and ice sheet growth are not the same, and there is instead a dependence on the direction of change due to the cooling effect an ice sheet has on its surrounding environment. This leads to a phenomenon known as hysteresis. The magnitude of this hysteresis for the entire Antarctic Ice Sheet (AIS) is currently unknown, despite the fact that the long-term future stability of AIS in the face of a warming climate, and hence the ultimate magnitude of future sea level rise, critically rests on the magnitude of this phenomenon. It has been shown that ~34 million years ago, a decline of atmospheric CO2 concentrations through a threshold of ~700 ppm drove the first largescale continental glaciation of Antarctica. However, existing records show that the ice sheet margin waxed and waned at that time while atmospheric CO2 varied within the range projected for year 2100 under business as usual emission scenarios. This interval therefore offers an opportunity to document the magnitude and nature of AIS hysteresis. This proposal will achieve that, using a groundbreaking integration of modelling with cutting edge geochemical analysis of boron isotopes (for pH-CO2 reconstructions), coupled to oxygen isotopes (for ice volume and sea surface temperature reconstructions) and Mg/Ca ratios (for sea surface temperature reconstructions) in exceptionally wellpreserved planktonic foraminifera. A novel high fidelity, orbitally resolved CO2 and ice volume record will be reconstructed for the interval 33 to 34.5 Ma, which will be merged with state-of-the-art computational techniques of the observed ice volume and CO2 relationships across the interval to provide a new understanding of the mechanisms driving the interaction between ice sheets and CO2 concentrations in high CO2 worlds.