Properties and High-Resolution Topography of Subglacial Bedforms Beneath a West Antarctic Ice Stream

Subglacial bedforms such as mega-scale glacial lineations and drumlins are commonly thought to form during active ice flow. They are often present in deglaciated areas with various elongation ratios, consisting of different materials , information which led to the development of different formation theories. However, these exposed examples were formed not only by subglacial processes during glaciation but also altered by processes during and after deglaciation. Here, we analyse in-situ properties and topography beneath Rutford Ice Steam, a fast flowing ice stream in West Antarctica to evaluate current theoretical models of bedform formation. We present a combination of seismic and radar data, including high-resolution 3D radar topography covering the upstream end of a bedform. Data acquisition and processing of the high-resolution 3D radar dataset result in a horizontal resolution of 24 m along- and across-track and a vertical resolution of 12 m. Using seismic acoustic impedance and calibrated radar reflectivity subglacial properties of the bedforms as well as the surrounding area are identified.

A depression around the upstream end of a 360 m wide, 50 m high and more than 13 km long bedform was observed for the first time analysing the high-resolution 3D radar topography.  The depression consists of a deepening up to 45 m deep and 360 m wide and is seen to extend around 10.5 km downstream. Radar reflectivity reveals that the material the depression is excavated into at least partly consists of low porosity material. Radar reflectivity and seismic acoustic impedance  along the bedform imply a stiffer upstream end which softens along flow. The subglacial topography and properties give evidence that the bedform and the depression are formed by a combination of  erosional and depositional processes. Both processes are likely interlinked, as implied by the comparable volume of the moat and the bedform at the upstream end of the bedform. Based on these observations we support or reject common bedform formation theories beneath Rutford Ice Stream.