Late Quaternary West Antarctic Ice Sheet Dynamics: Remote sensing and substrate studies of palaeo-ice sheet beds on the Amundsen Sea shelf

Extensive parts of the largely marine-based West Antarctic Ice Sheet are currently subject to the most rapid changes in the global cryosphere. In recent decades, ice streams that drain >35% from the ice sheet’s interior into the Amundsen Sea have been affected most dramatically. As upwelling, relatively warm Circumpolar Deep Water flows onto the continental shelf towards the deep inner shelf cavities, it melts the ice shelves from below, thins them, and causes ice stream acceleration and grounding line retreat in response to decreased buttressing. Since ice streams are marine-based on slopes that significantly deepen inland, they are susceptible to increased future ice mass loss, directly resulting in a sea-level rise of up to 3.4 meters, assuming that all marine-based West Antarctic ice portions would disintegrate and melt (Fretwell et al. 2013). However, ‘state-of-the-art’ ice sheet models that aim to elucidate these future scenarios are only initialised with observational data covering the past 30-40 years (e.g. Favier et al. 2014), thus excluding long-term empirical data of ice sheet change spanning the Last Glacial Maximum and the subsequent deglacial period. To test the reliability of predicted future scenarios it is essential that models are validated against past ice sheet configurations confirmed by empirical data from palaeo-ice sheet beds on modern Antarctic continental shelves. Attempts at reproducing the LGM ice sheet (Golledge et al. 2013), have revealed considerable mismatches between model simulations and empirical data. Such disparities have been attributed principally, to a lack of comprehensive palaeo-glaciological data particularly from outer continental shelves and regions in between the large palaeo-ice stream troughs, regions known as inter-ice stream ridges, that would reveal the spatial and temporal variations of the West Antarctic Ice Sheet in the Amundsen Sea sector of the Southern Ocean more precisely. This thesis presents the mapping and detailed analysis of new marine geophysical and geological data from three formerly unstudied regions on the Amundsen Sea shelf that significantly improve our understanding of Antarctic palaeo-ice sheet dynamics. In Chapter 2 I will present the first sea-floor geomorphological record of former basal ice conditions on an inter-ice stream ridge that entirely differ from those commonly investigated in the nearby palaeo-ice stream troughs. From these data, an improved temporal and spatial reconstruction of flow conditions of the former ice sheet in inter-ice stream areas of the eastern Amundsen Sea Embayment is revealed. Age constraints aiming to reveal the minimum grounding line retreat from the ridge broadly correspond with those from the nearby Pine Island Trough. Palaeo-ice sheet dynamics as inferred from the glacial landform and sediment record on the inter-ice stream ridge are well complemented by a large-scale multibeam bathymetry survey of the middle and outer shelf, north of the inter-ice stream area, presented in Chapter 3. This new dataset compiles bathymetry from 11 separate research cruises to the region, and is supplemented by the analysis of new sedimentological and seismic data. From comprehensive landform mapping, the detailed palaeo-flow pathways of the WAIS in the northern and easternmost Amundsen Sea Embayment is revealed. Furthermore, geomorphological analysis of the bathymetry data allows thermal regimes at the palaeo-ice sheet bed to be defined in unprecedented detail, showing the complex relation of trough geometries and the subglacial geology to palaeo-ice flow behaviour. In combination with findings from Chapter 2, a coherent pattern of episodic post-Last Glacial Maximum retreat between the Pine Island and Abbot glacial troughs across the inter-ice stream ridge is revealed by the landform information, from which uniform retreat across the entire eastern Amundsen Sea Embayment is inferred. The same episodic style of retreat is also evident for a formerly unstudied part of the Amundsen Sea shelf offshore the Hobbs Coast presented in Chapter 4, as here the analysis and interpretation of marine geophysical and geological data reveal a large grounding-zone wedge recording a prolonged grounding line stabilization phase after the West Antarctic Ice Sheet reached the continental shelf edge during the last maximum extent. The initial retreat here is demonstrated to have been initiated at a pre- or early Last Glacial Maximum stage with deglaciation of inner shelf regions completed by ~12.9 cal. ka BP. Set in the context of other studies, a diachronous initial retreat of West Antarctic Ice Sheet grounding lines is indicated, which is discussed as a possible response to different local settings. This thesis will ultimately help to better understand West Antarctic Ice Sheet dynamics during and since the Last Glacial Maximum. The new information will significantly add to a hitherto sparse database of previous work, helping to test, validate, and improve ice sheet models in the vital region of the Amundsen Sea. Only by enhancing their ability to simulate past ice sheet configurations more accurately will more reliable predictions of the future evolution of these dramatically changing parts of the West Antarctic Ice Sheet be possible.