Interpretation of Radar Basal Reflectivity in Ice-Sheet Grounding Zones

Abstract:

Properly mapping bed topography and basal conditions in ice-sheet grounding zones is crucial to understanding ice-sheet evolution. Detailed maps of bed topography are needed to properly assess the impact of bed pinning points in grounding-line retreat scenarios. Other processes, including sediment deposition, till compaction, and infiltration of ocean water upstream of grounding via tidal flexure, may also have important effects on grounding-line stability. Ice-penetrating radar is the most commonly used technique to examine ice-sheet grounding zones because it supplies large amounts of useful data comparatively easily. Although mapping the bed topography is relatively straightforward, more-complete interpretations of radar data incorporating information from basal reflectivity and basal-echo phase remain difficult, even with recent advances in radar technology. Here we present a detailed interpretation of radar basal reflectivity at the grounding zone of Whillans Ice Stream, informed using active-source seismic data and dielectric modeling. Our results indicate that basal reflectivity in complex environments must be interpreted with caution, because bed returned power is substantially affected by many possible basal conditions including thin films of various materials (e.g., freshwater, seawater, debris-bearing ice, sediment of varying compaction state), widespread crevasses, and off-nadir reflections. After careful examination of these issues, our data indicate substantial mixing in the shallow water column in a subglacial embayment where several subglacial lakes drain. Basal reflectivity in nearby areas with no subglacial drainage indicates a more abrupt transition along ice flow from ice overlying till to ice overlying seawater. Thus, properly considered, radar basal reflectivity still yields valuable information about grounding-zone conditions, including water properties as the ice begins to float. We conclude by discussing ramifications of this study for interpretation of airborne radar data collected over other grounding zones, including those of Pine Island and Thwaites Glaciers.