Spatial and temporal variations of basal conditions beneath glaciers and ice sheets inferred from radio echo-sounding measurements.

摘要

The basal structure of Black Rapids Glacier, Alaska and Siple Dome, West Antarctica were studied using new radio echo-sounding (RES) methods and analysis techniques. Development of these methods was motivated by the need to make rapid large-scale non-invasive observations of sub-ice systems. RES data from each study site are analyzed to identify and isolate the portion of the data associated with the reflection from the bed by using a calibration determined from principal component analysis of the ice-internal reflectors/scatterers. Reflectivity models are developed to calculate the dielectric structures responsible for the measured reflections and to infer the range of physical conditions responsible for those structures. Repeated RES measurements on Black Rapids Glacier were made during the time of annual spring speedup which is associated with changes in meltwater input. These measurements show that the subglacial structure near the center of the subglacial valley did not change significantly during the period of observation, but allow for some changes toward the glacier margin. Together with other geophysical measurements, these results show that the observed speedup of the glacier was probably not driven locally by changes in the subglacial system at the center of the glacier, but from changes toward the sides and/or down glacier. The absence of glacier-wide changes in basal conditions in the presence of glacier-wide changes in glacier speed suggests that large-scale glacier motion may be affected or even controlled by localized changes in sensitive regions of the glacier bed. RES profiles across Siple Dome and onto adjacent paleo-ice streams are used to reveal information about their history and current basal conditions. The pattern of reflective layering in the ice is used to positively identify the location of paleo-ice stream margins and the depth of buried crevasses is used to estimate the time since shutdown of these ice streams. The pattern of reflected power from the bed is used to show that the bed under Siple Dome must be either one of ice frozen to smooth bedrock or to a thin ({dollar}{dollar}1 m thick. The absence of a thick till beneath Siple Dome is consistent with its apparent stability as an inter-ice stream ridge in the past and suggests that it may remain as a stable limiter of ice stream width in the future. Together these results reveal much about both the potential and the limitations of RES for investigating basal conditions in polar and temperate ice masses.