Many characteristics of an ice stream onset are highlighted in Figure 1. The flow rate increases, the surface slope decreases and crevasses appear. Upstream of the onset, ice flows primarily by internal deformation and has a parabolic vertical profile, downstream the vertical profile becomes constant, i.e. plug flow. Transverse flow is convergent, evidenced by buckled inter layers. The rapid ice stream motion must be accommodated by either sliding over its bed or by strain in an underlying till layer. The appearance of basal water, or the evolution of a hydrologic system would be critical to either of these processes. Finally, the bed interface forms the framework for topographic or geologic control on these processes.

During the 2001/02 austral-summer we began a two-season field program to study the onsets of ice streams C and D, West Antarctica. We established a primary survey grid centered on the previously identified onset of ice steam D and secondary grids at corresponding locations for two tributaries of ice stream C. These locations, OND, C2 and C1b, are shown in Figure 2. Here we focus on the ice stream D surveys. Figures 3 through 10 give local-scale context for the onset-D region. In each figure, our survey grid is outlined in white and hatching (at the down-stream end) indicate exposed (black) and subsurface crevasses interpreted from airborne sounding data. Characteristic of trunk and “tributary” of ice steam D are evident in each of these figures.

Figure 2

Results from our last season are only preliminary. Ultimately, with our ice motion measurements will be able to detect any substantial flow variations since a survey of the region conducted during the mid-1990’s by NASA/GSFC. The shallow-sounding radar allows high-resolution stratigraphic imaging down to approximately 100 meters. Layer undulations revealed by these surveys are indicative of shear margin development as well as generation of flow stripes. The deep-sounding radar surveys image the bed and resolve layering to approximately half-depth. With these data we are pursuing characterization of the basal interface including its roughness and the presence of water. In 2002/03, we will conduct high-resolution seismic sounding surveys to detect the mechanical properties of any subglacial till layers.