Fibre-optic borehole observations and numerical modelling of complex ice-sheet thermodynamics

• Main Author: Law, Robert
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Cambridge 2022
• Subjects: Antarctic Ice Sheet; Fibre-optics; Finite Element Modelling; Glacier borehole; Glacier fieldwork; Glaciology; Greenland Ice Sheet; Antarctic; Greenland; Kujalleq; Isunnguata Sermia; Antarc*; glacier; Ice Sheet; Sermeq Kujalleq
• Online Access: https://www.repository.cam.ac.uk/handle/1810/343281; https://doi.org/10.17863/CAM.90692

Predictions of ice-sheet mass loss, and therefore predictions of global sea level rise, depend sensitively upon how ice-sheet motion is incorporated into numerical models. Using field observations and numerical modelling, this thesis demonstrates that two frequently overlooked processes are central to describing borehole observations of fast ice-sheet motion --- intermediate-scale (<25 m, ⪅2 km) interaction of ice motion with realistic or real bed topography, and modulation of these ice-motion patterns through a basal layer of temperate ice (much softer ice at the pressure-melting point). I first present a fibre-optic data set from a 1,043 m deep borehole drilled to the base of the fast-moving (>500 m a‾¹) marine-terminating Sermeq Kujalleq (Store Glacier) at the western margin of the Greenland Ice Sheet. This reveals hitherto unappreciated complexity in the processes behind fast ice-sheet motion. I observe substantial but isolated strain heating ~220 m beneath the surface within stiffer interglacial-phase ice where previously none was expected. Ice deformation within glacial-phase ice below 889 m is further observed to be strongly heterogeneous, with a possible high-strain interface demarcating the Last Glacial-Interglacial Transition. I also find a 73-m-thick temperate basal layer, notably thicker than the <10-m-thick temperate layer just 8.9 km away, unexplained by existing theory, and interpreted to be important for the glacier's fast motion. To disentangle this observed complexity, I then model three isolated 3D domains from the Greenland Ice Sheet's western margin --- two from Sermeq Kujalleq and one from the land-terminating Isunnguata Sermia, all centred above a central borehole observation. By incorporating high-resolution realistic geostatistically simulated topography, I demonstrate that a layer of basal temperate ice with spatially highly variable thickness forms naturally in both marine- and land-terminating settings, alongside ice-motion patterns which are far more complex than previously ...