Seismic and radar observations of subglacial bed forms beneath the onset zone of Rutford Ice Stream, Antarctica

We present seismic and radar data from the onset region of Rutford Ice Stream, West Antarctica, which show the form and internal structure of a variety of bed forms beneath an active ice stream. The ice flow in the area of our survey accelerates from 72 to >200 m a(-1), the ice is 2200-3200 m thick, and the bed of the ice stream lies up to 2000m below present sea level. We have imaged the internal structure of the bed forms with seismic reflection techniques and also observed radar reflections from below the bed in some circumstances. We observed a transverse moraine 2 km wide and 1.5 km long beneath the slower-flowing part of the ice stream, which we interpret to be composed of unconsolidated sediment undergoing active deformation near the ice-sediment interface. We observed drumlins of classical form with elongation ratios of between 1 :1.5 and 1 : 4.0 where the surface flow speed exceeded 95 m a(-1). The conformity of the internal structure of the bed forms with the ice base suggests that the bed forms are active depositional features in congruence with the observation of a contemporary drumlin-forming episode in the distal part of the same ice stream. These observations provide the first direct evidence of the association between ice-stream flow speed and bed-form shape