Subglacial roughness of the Greenland Ice Sheet: relationship with contemporary ice velocity and geology

Abstract

The subglacial environment of the Greenland Ice Sheet (GrIS) is poorly constrained, both in its bulk properties, for example geology, presence of sediment, and of water, and interfacial conditions, such as roughness and bed rheology.There is, therefore, limited understanding of how spatially heterogeneous subglacial properties relate to ice-sheet motion.Here, via analysis of two decades worth of radio-echo sounding data, we present a new systematic analysis of subglacial roughness beneath the GrIS. We use two independent methods to quantify subglacial roughness: first, the variability of along-5 track topography-enabling an assessment of roughness anisotropy from pairs of orthogonal transects aligned perpendicular and parallel to ice flow; and second, from bed-echo scattering-enabling assessment of fine-scale bed characteristics. We establish the spatial distribution of subglacial roughness and quantify its relationship with ice flow speed and direction. Overall, the beds of fast-flowing regions are observed to be rougher than the slow-flowing interior. Topographic roughness exhibits an exponential scaling relationship with ice surface velocity parallel, but not perpendicular, to flow direction in fast-flowing 10 regions, and the degree of anisotropy is correlated with ice surface speed. In many slow-flowing regions both roughness methods indicate spatially coherent regions of smooth bed, which, through combination with analyses of underlying geology, we conclude is likely due to the presence of a hard flat bed. Consequently, the study provides scope for a spatially variable hard bed/soft bed boundary constraint for ice-sheet models.

Exceptionally High Geothermal Heat Flux Needed to Sustain the Northeast Greenland Ice Stream

“…A similar approach with basal shear stress defined as a function of bed elevation was previously used by Åkesson et al (2018) and by Aschwanden et al (2016). Our simple friction relationship is supported by observations, as bed topography roughness for the NEGIS region shows a pattern inversely correlated with bed elevation (Cooper et al, 2019).…”

Five decades of radioglaciology

“…Additionally, basal roughness at the wavelength-scale can affect the character of the reflected echo including its specularity (or spread in Doppler), waveform abruptness, statistical distribution of echo amplitudes, as well as the radar-derived topography itself (Goff and others, 2014; Grima and others, 2014a; Rippin and others, 2014; Schroeder and others, 2014a; Jordan and others, 2017; Heister and Scheiber, 2018; Eisen and others, 2020; Franke and others, 2020; King, 2020). Principles from these studies have also been translated to paleoglacial landscapes and have also been compared to contemporary bed morphology and lithology (Gudlaugsson and others, 2013; Schroeder and others, 2014c; Falcini and others, 2018; Cooper and others, 2019; Muto and others, 2019; Holschuh and others, 2020).…”

Complex Basal Conditions and Their Influence on Ice Flow at the Onset of the Northeast Greenland Ice Stream

“…The intensity of the integrated bed return echo as well as the length of the integration window is sensitive to small‐scale variations in basal roughness (Cooper et al., 2019), which are not resolved in the radar measurements and depend on the size of the Fresnel zone. The diameter of the Fresnel Zone for a bandwidth of 180–210 MHz and an ice thickness range of 2–3 km is ∼ 60 m. Ice‐bed interface roughness and the wavelength of the radar signal determines the nature of the electromagnetic scattering.…”

“…We follow Cooper et al. (2019) and define the abruptness parameter A by …”

“…A large P max in a small integration window will result in a large value of A . If the reflected energy is distributed over a larger integration window, it will decrease the maximum amplitude and result in a small value of A (for details see Figure 4 in Cooper et al., 2019). High values indicate an abrupt return (i.e., along‐track focusing was sufficient to recover energy to the nadir reflection point), whereas low values indicate a broad return (likely due to energy recovered from a range of cross‐track angles, which could not be focused due to the narrow antenna array in the cross‐track direction).…”