What lies beneath

: A radioglaciological study of Greenland

Abstract

The subglacial environment can exert strong controls on glacier dynamics, influencing the orientation and velocity of ice flow, as well as the modulation and distribution of basal waters and sediment. Bed geometry can also provide a long-term record of geomorphic processes, allowing insight into landscape evolution and inheritance with consequences for both palaeo- and contemporary ice dynamics. However, beneath the Greenland Ice Sheet (GrIS), many aspects of the subglacial environment remain poorly constrained, including both its bulk (e.g. geology, geometry, and presence of sediment and water) and inter-facial properties (e.g. basal roughness and geotechnical bed properties). There is, therefore, limited understanding of how spatially heterogenous subglacial properties relate to ice-sheet motion. A marked increase in the availability of radio-echo sounding (RES) (often referred to as ice-penetrating radar (IPR)) data, has not only improved the accuracy of derived basal topography products, but continues to facilitate wider analysis of the subglacial environment. The primary focus of this thesis is to use RES, and complementary data (e.g. ice velocities, basal thermal state, surface topography, etc.), to examine the complex interrelationships between Greenland's ice and the bed. The first results chapter of this thesis presents evidence from RES data for a large, subglacial drainage network, radiating inland from Jakobshavn Isbrae, Greenland's largest outlet glacier. Topographic and basin morphometric analyses of an isostatically uplifted (ice-free) bedrock topography suggest that this catchment pre-dates ice-sheet initiation. This landscape has likely been instrumental in controlling the location and form of Jakobshavn Glacier, as well as influencing ice flow from the deep interior to the margin, both now and over several glacial cycles. The second results chapter presents a new systematic analysis of subglacial roughness beneath the GrIS. Using two decades of RES data, this chapter quantifies two near-independent subglacial roughness metrics enabling assessment of roughness anisotropy, and of finer-scale information, respectively. In fast-flowing regions `topographic roughness' (defined by variation in sampled bed elevation) exhibits an exponential scaling relationship with ice velocity parallel, but not perpendicular, to flow direction. In many slow-flowing regions both roughness metrics indicate spatially-coherent signals, representative of `smooth' bed; combined with analyses for underlying geology, it is concluded that these are likely due to the presence of a hard bed. The third results chapter focuses on the assessment of how the subglacial environment is expressed at the surface of the ice sheet. This is undertaken through the use of a new high-resolution surface topography (ArcticDEM), and its potential to provide enhanced insights into basal and englacial ice-sheet processes. Notable surface features---quantified using a calculated `residual surface elevation' (RSE)---are observed coincident with previously documented subglacial channels. Additionally, `new' smaller-scale tributaries (< ~2,000 m in width) and complex valley-like structures are revealed with particular clarity. RSE also allows the extents of basal ice units to be mapped, providing support to the theorized genesis of these these features, and that they act as `false bottoms,' likely due to a rheological contrast in the ice column. Ultimately, this thesis not only provides significant advances in the quantification of subglacial properties of the GrIS, the results and data presented herein will facilitate further assessment of the bed and its influence on, and interrelationship with ice motion. It is, therefore, hoped that this thesis works to improve the understanding of glacial response to contemporary climatic perturbation, and to improve projections of future contributions to global sea level.

Date of Award	23 Jan 2019
Original language	English
Awarding Institution	University of Bristol
Supervisor	Jonathan L Bamber (Supervisor)