Weak bed control of the eastern shear margin of Thwaites Glacier, West Antarctica

Abstract

ABSTRACT. Recent acceleration and thinning of Thwaites Glacier, West Antarctica, motivates investigation of the controls upon, and stability of, its present ice-flow pattern. Its eastern shear margin separates Thwaites Glacier from slower-flowing ice and the southern tributaries of Pine Island Glacier. Troughs in Thwaites Glacier's bed topography bound nearly all of its tributaries, except along this eastern shear margin, which has no clear relationship with regional bed topography along most of its length. Here we use airborne ice-penetrating radar data from the Airborne Geophysical Survey of the Amundsen Sea Embayment, Antarctica (AGASEA) to investigate the nature of the bed across this margin. Radar data reveal slightly higher and rougher bed topography on the slower-flowing side of the margin, along with lower bed reflectivity. However, the change in bed reflectivity across the margin is partially explained by a change in bed roughness. From these observations, we infer that the position of the eastern shear margin is not strongly controlled by local bed topography or other bed properties. Given the potential for future increases in ice flux farther downstream, the eastern shear margin may be vulnerable to migration. However, there is no evidence that this margin is migrating presently, despite ongoing changes farther downstream.

Self-affine subglacial roughness: consequences for radar scattering and basal water discrimination in northern Greenland

“…Equation (9) is the same power decay formula as coherent H = 0 models (Peters et al, 2005;MacGregor et al, 2013;Grima et al, 2014;Schroeder et al, 2015), where it is sometimes multiplied by a first-order Bessel function (which enables some of the incoherent energy contribution to be captured; MacGregor et al, 2013). Thus the stationary limit of the self-affine model is consistent with previous glacial basal scattering models.…”

“…The central assumption that differentiates the model from coherent stationary (H = 0) models (Berry, 1973;Peters et al, 2005;MacGregor et al, 2013;Grima et al, 2014;Schroeder et al, 2015) is that the rms height increases as a function of radius, r, about any given point, following the self-affine relationship…”

“…Coherent scattering models are applicable where the reflecting region is orientated nearly perpendicular to the incident pulse (the nadir regime) and the reflecting region is fairly smooth at the scale of the illuminating wavelength (Campbell and Shepard, 2003), which is normally assumed to be a good approximation for the RES of glacier beds (e.g. Peters et al, 2005;MacGregor et al, 2013;Schroeder et al, 2015). Volume (Mie) scattering is typically neglected from basal RES scattering analysis and would hypothetically require scatterer dimensions of the order of the radar wavelength (∼ 0.5 to 5 m dependent on the bed dielectric and radar system).…”

“…As such, there is a knowledge gap regarding the topographic control upon radar scattering. Observations indicate that subglacial roughness exhibits self-affine (fractal) scaling behaviour over length scales from ∼ 10 −3 to ∼ 10 2 m (Hubbard et al, 2000;MacGregor et al, 2013). Self-affine scaling corresponds to when the vertical roughness increases at a fixed slower rate than the horizontal length scale, following a power-law relationship that is parameterised by the Hurst exponent (Malinverno, 1990;Shepard et al, 2001).…”

“…It is observed for a wide variety of natural terrain (Smith, 2014), including the surface of Mars (Orosei et al, 2003), volcanic lava (Morris et al, 2008), and alluvial channels (Robert, 1988). If widely present, the self-affinity of subglacial roughness poses a challenge for integrating topographic roughness with existing glacial radar scattering models (Berry, 1973;Peters et al, 2005;MacGregor et al, 2013;Schroeder et al, 2015). This is because these are statistically stationary models which assume that roughness is independent of horizontal length scale, and hence an artificial scale separation between high-frequency roughness and low-frequency topography is present (Berry, 1973).…”