Ice-sheet sensitivity to Earth's surface: an assessment of landscape records

Understanding drivers of glacier stability is important to consider for future contributions to global sea-level rise. The underlying terrain on which ice sheets, continental-scale amalgamations of glaciers, rest have the ability to modulate ice flow and rates of ice sheet growth and decay, but the influence of subglacial bed conditions is far from straightforward. Difficulties in accessing and studying contemporary subglacial environments leaves gaps in knowledge on the sensitivity of ice sheet behavior across time and space. Deglaciated landscapes that were once covered by ice sheets preserve records in the form of landforms and sediments and provide broad spatiotemporal perspectives on ice-sheet response to subglacial bed conditions. Through the development of a semi-automated mapping tool, 11,628 sedimentary and bedrock streamlined subglacial bedforms, serving as proxies for subglacial processes and ice flow, were mapped across nine sites of varying lithologic and topographic setting in the deglaciated Northern Hemisphere (Chapter 2; McKenzie et al., 2022). A minimum bedform length-width ratio and similarities in bedform metric distribution across bed lithology and topography indicate streamlined bedform synthesis is supported across all bed conditions. High ice-flow velocities based on highly elongate streamlined features occur within confined valley settings. Unconfined topography across sedimentary beds encourage consistency in ice-flow velocity and spatially uniform and organized interactions at the ice-bed interface. Elevated bed topography, or “bumps”, have the potential to slow ice flow or, conversely, increase ice-flow speed through strain heating and subglacial meltwater production. Streamlined subglacial bedforms on and proximal to isolated bumps of variable size across the deglaciated landscape of the Cordilleran Ice Sheet (CIS) of Washington state were assessed to characterize differences in ice flow and sedimentary processes due to the variable topography (Chapter 3; McKenzie et al., preprint). Ice flow organization and sedimentary processes (e.g., deformation, erosion, and deposition) are significantly disrupted by bumps greater than 4.5 km3. Landform-building sedimentary processes are most mature downstream of bumps, likely due to increased availability and production of subglacial sediment and meltwater. Additionally, due to direct CIS coverage and high rates of glacial isostatic adjustment (GIA), sediment archives in the Puget Lowland of Washington state record spatial and temporal shifts landscape evolution and ice behavior. As identified through combined sedimentological and geochronological analyses, the late-stage CIS experienced step-wise retreat within a marine environment about 12,000 years B.P., placing glacial ice in the region for about 3,000 years longer than previously thought (Chapter 4; McKenzie et al., in prep). Additional rapid rates of vertical landscape evolution support a millennial-long stand still of marine-terminating ice, followed by continued retreat of ice in a subaerial environment. Overall, these works provide novel insight to the role of subglacial bed conditions in influencing ice-sheet behavior from landform and sediment records of deglaciated landscapes in the Northern Hemisphere. With this dissertation, I contribute to process-based knowledge of subglacial processes, landform generation, and controls on ice flow and retreat that are useful for understanding both marine- and terrestrial-based components of the contemporary Antarctic and Greenland ice sheets.