Englacial stratigraphy, debris entrainment and ice sheet stability of Horseshoe Valley, West Antarctica

Despite the importance of ice streaming to the evaluation of West Antarctic Ice Sheet (WAIS) stability, we know little about mid-to long term changes in grounding line migration, ice streaming and ice accumulation in the upper Institute Ice Stream (IIS) catchment. In this thesis ground penetrating radar (GPR) and airborne radio-echo sounding (RES) methods have been employed to investigate the subglacial topography, internal stratigraphy and Holocene flow regime of the upper IIS catchment, in and around Horseshoe Valley. High resolution step-andcollect mode GPR was employed to assess the continuity of a Blue Ice Area (BIA) horizontal ice core climate record at Patriot Hills, where analysis has revealed two unconformities in the otherwise conformable 30,000 year climate sequence. By combining these data with airborne RES returns and pre-existing ice sheet models it is suggested that these unconformities represent periods of erosion, occurring as the former ice surface was scoured by katabatic winds in front of Liberty and Mable Hills. Snow_Blow simulations suggest that katabatic winds have scoured the leeward slopes of these mountain ranges for over 10,000 years. This temporal stability can account for the large volume of BI moraine deposits in Horseshoe Valley, where compressive BI flows promote glacial erosion and near-surface debris entrainment through freeze-on processes at the ice/bed interface and compressive thrust faulting. By investigating thicker ice flows in the upper IIS catchment and the Evans Ice Stream, this thesis has also analysed debris entrainment mechanisms at depth, where clasts are incorporated into the ice flow by englacial stratigraphic folding and shearing at the glacial thermal boundary, governed by spatial and temporal changes in ice flow, ice temperature and sediment availability. Mid-to long term changes in ice flow in the wider IIS catchment have been investigated from airborne RES transects, revealing internal layer buckling, and therefore former enhanced ice-sheet flow in three distinct tributaries of the IIS. Buckled ice layers throughout the slow flowing ice in the Independence Trough and the fast-flowing ice in the Ellsworth Trough suggest that enhanced ice flow through these topographically confined regions was the source of ice streaming and iceflow reconfiguration during the mid-to-late Holocene. Although buckled layers also exist within the slow-flowing ice of Horseshoe Valley, a thicker sequence of surface-conformable layers in the upper ice column suggests slowdown more than 4000 years ago, indicating that enhanced flow switch off here cannot be attributed to late-Holocene ice flow reorganisation. The dynamic nature of ice flow in the IIS and its tributaries suggests that ice stream switching and mass change may have been regular during the Holocene, and that these changes may characterise the decline of the WAIS in this area. These results have important implications for our understanding of ice-sheet dynamics and the response of the ice sheet to climate change and provides explanations for fluctuations in debris entrainment and transportation processes in Antarctica.