Radar Studies on Kamb Ice Stream

During the past two Antarctic field seasons we acquired approximately 1600 km of ground-based ice- penetrating radar data on the lower trunk of Kamb Ice Stream (KIS) as part of radar, GPS and modeling study with scientists at the University of California Santa Cruz examining the possibility of ice stream reactivation. We present here a summary of radar results from this work and preliminary interpretations. Our working hypothesis is that the reactivation of the stagnant KIS may be triggered by excess influx of basal water produced by increased basal strain heating when mass builds up in the upper reaches of the locked ice stream. Using radar data, we have quantified variations in the amplitude of radar reflections from the ice-bed interface to estimate different provenances of occurrence of basal water. The weakest-reflecting ice-bed interface is found at a "sticky spot" in the middle of the ice stream trunk where ice appears to have become grounded over a large bedrock bump. At the sticky spot, bore holes drilled by California Technical Institute in 2000 showed a dry bed. A more highly reflective bed is located to either side of the sticky spot in regions of faster flow of KIS including one location where bore holes showed water at the ice-bed interface. However, the brightest bed is located approximately 80~km upstream of the sticky spot, where ice velocities are still on the order of 120~m a-1. Here radar reflected power is up to 1.5 times higher than elsewhere in the trunk despite the ice being 40% deeper. From this pattern of bed reflectivity we hypothesize that conditions allowing for rapid flow still exist under most areas of KIS and that sticky spots, like the one studied here, have played a key role in the ice stream shut down. We have also produced a map of detailed bed topography and tracked internal reflection layers over the sticky spot. We are able to trace the evolution of folds in the radar internal stratigraphy in this region in both time and space by comparison with ground-based radar data we collected in the late 1980's. These studies depict changes in the horizontal strain rate as ice passes around the sticky spot. Bed topography, rheological properties and internal layer characteristics will all provide constraints to models describing the evolution and possible future of ice flow in KIS. Two profiles crossing from the actively-flowing region of highly reflective bed into the northern margin of KIS (defined by strong velocity gradients) show continuous internal stratigraphy across the margin, indicating that substantial volumes of ice are still flowing across the margin into KIS in this area. A 250 km long longitudinal profile has also been acquired following a flow line down stream KIS and then crossing the margin up to Siple Dome summit. Along the profile we have traced internal layers to the KIS north margin. On the Siple Dome side, we have also traced internal stratigraphy to the site for the ice core drilled at Siple Dome summit, allowing us to date the internal layers. However the high shear that has taken place across this margin prevents us from convincingly tying layers in the ice stream to the dated Siple Dome stratigraphy.