Dynamics of the Late Oligocene Antarctic Ice Sheet

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Superimposed on a global long-term cooling trend from the warm conditions of the Paleogene into the colder Neogene, the Oligocene epoch (33.9–23.03 Ma) represents a transitional phase of Cenozoic climate evolution. It marks the onset of the Cenozoic “icehouse” that is thought to have commenced with the initiation of large ice sheets on Antarctica. While the long-term evolution of the Oligocene Antarctic Ice Sheet (AIS) has been well established over the last two decades, its short-term dynamics are not well understood. Furthermore, available proxy records of the Late Oligocene show opposing signals regarding the pacing and magnitude of Antarctic ice-volume variability, which hampers a full and mechanistic understanding of Late Oligocene ice-volume and climate dynamics. This thesis aims to address this paradox and contribute to a more sophisticated understanding of the Late Oligocene climate and cryosphere dynamics. Therefore, high-resolution benthic and planktic foraminiferal proxy records are generated based on deep-sea sediments drilled within the framework of the Integrated Ocean Drilling Program (IODP) Expedition 342 at Site U1406 (northwestern Atlantic). These records provide adequate fidelity to study the fundamental processes and mechanisms underlying ice-sheet waning and waxing on orbital timescales during the Late Oligocene. In the first part of this thesis, novel and unprecedented insights into short-term processes and mechanisms of AIS fluctuations are presented. Investigations of this thesis evidence that the Late Oligocene AIS was highly dynamic on orbital timescales. The AIS generally shifted between two states of glaciation, i.e., volumes of half the modern to slightly bigger than modern global ice-volumes, and smaller than to half the modern global ice volume. Insolation forcing is identified as a key player in modulating changes in the extent and dynamics of the AIS: The influence of eccentricity periodicities is higher when AIS volumes are smaller, whereas bigger ice-volume react more sensitively to changes in obliquity. Very large angels of the Earth’s spin axis are shown to cause severe deglaciation pulses. Potential consequences of those AIS dynamics are explored in the second part of this thesis. The first high-resolution planktic foraminiferal proxy record from the high Northern Atlantic (Site U1406) is presented. Potential changes in surface ocean characteristics in response to glacial dynamics of the Southern Hemisphere during the Late Oligocene are explored. It is shown that the meridional position and strength of the North Atlantic Current (NAC) during the Late Oligocene has varied as a function of glacial dynamics of the Southern Hemisphere. There are indications that the NAC variability caused changes in nutrient concentrations and alterations in the amount of northward heat transport, which has broader implications for the climate in the circum-North Atlantic region.