Abstract
ABSTRACTBed roughness is an important control on ice-stream location and dynamics. The majority of previous bed roughness studies have been based on data derived from radio-echo sounding (RES) transects across Antarctica and Greenland. However, the wide spacing of RES transects means that the links between roughness and flow are poorly constrained. Here, we use Digital Terrain Model/bathymetry data from a well-preserved palaeo-ice stream to investigate basal controls on the behaviour of contemporary ice streams. Artificial transects were set up across the Minch Palaeo-Ice Stream (NW Scotland) to mimic RES flight lines over Institute and Möller Ice Streams (Antarctica). We then explored how different data-resolution, transect orientation and spacing, and different methods, impact roughness measurements. Our results show that fast palaeo-ice flow can occur over a rough, hard bed, not just a smooth, soft bed, as previous work has suggested. Smooth areas of the bed occur over both bedrock and sediment covered regions. Similar trends in bed roughness values were found using Fast Fourier Transform analysis and standard deviation methods. Smoothing of bed roughness results can hide important details. We propose that the typical spacing of RES transects is too wide to capture different landform assemblages and that transect orientation influences bed roughness measurements in both contemporary and palaeo-ice-stream setting.
Highlights
This paper aims to measure the bed roughness of contemporary subglacial and deglaciated terrains at analogous length scales
Subglacial obstacles of ∼0.5–1 m in both amplitude and horizontal wavelength have been shown theoretically to exert critical basal drag (Weertman, 1957; Kamb, 1970; Nye, 1970; Hubbard and Hubbard, 1998; Hubbard and others, 2000; Schoof, 2002); these obstacle dimensions lie below the resolution achievable by radio-echo sounding (RES) across ice sheets
By using Digital Terrain Models (DTMs) -exposed palaeo-ice streams to calculate bed roughness, we propose that it may be possible to explore these complexities in more detail because the bed of a palaeo-ice stream can be directly observed over its entirety at much higher spatial resolutions than contemporary ice-stream beds
Summary
This paper aims to measure the bed roughness of contemporary subglacial and deglaciated terrains at analogous length scales. Accurate quantification of bed roughness beneath ice sheets is important because it is a primary control on basal drag and ice flow velocity (Siegert and others, 2005; Bingham and others, 2017). Several authors have explored the relationship of higher amplitude (several 100 m) and longer wavelength (hundreds of m to several km) bed roughness and ice dynamics across ice sheets using available RES data. These analyses have suggested that beds beneath contemporary ice streams are relatively smooth, with roughness values decreasing downstream, whilst in surrounding areas of slower ice flow, the beds are relatively rougher (Siegert and others, 2004; Rippin and others, 2006, 2011; Callens and others, 2014). Basal roughness is regarded as one of the controls on ice-stream location, in particular for ice streams not topographically controlled by deep valleys (Siegert and others, 2004; Bingham and Siegert, 2009; Winsborrow and others, 2010; Rippin, 2013)
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