Using Spatial and Temporal Variations of Ice Deformation to Study Basal Processes

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Abstract EchelmeyerOPP-01-15819 The Principal Investigators will study the distribution of basal shear stress and basal motion through detailed monitoring of surface velocities and internal ice deformation. The hypothesis is that the distribution of basal stress is related to the internal deformation field, and that temporal changes in basal conditions manifest themselves not only as changes in surface motion, but as changes in internal deformation. Such changes in internal deformation can be relatively large given the non-linear rheology of ice and the sensitivity of basal processes to variations in effective pressure. The Principal Investigators will investigate the mechanisms of basal motion using these ideas on Black Rapids Glacier, Alaska. They have previously documented the presence of a subglacial till layer and have some knowledge of its properties. Observational and modeling studies have shown that water pressure fluctuations cause changes in the strength of this till, and that these spatial and temporal changes in basal conditions then lead to measurable changes in the ice deformation field. Six boreholes will be drilled along a cross section where the till exists, and a seventh will be drilled further downglacier. They will be instrumented with tilt sensors for continuous monitoring of borehole deformation and with pressure transducers for monitoring basal water pressure. They will also measure surface velocities across the transects, and their temporal fluctuations. The observational study will be initiated before the annual spring speed-up event and carried through the summer, when various motion events occur. Logging will then continue through the more steady conditions of the winter season. A finite element model of ice deformation will then be used to interpret these data in terms of the distribution of basal shear stress and the underlying mechanisms of basal motion throughout the annual cycle. The proposed project will take full advantage of synergetic collaboration with an existing, funded program to install tilt sensors deep in the underlying basal till at the same location as our central borehole. Taken together, the Principal Investigators will be able to obtain a more complete picture of basal processes and of the interdependence of basal motion and ice deformation. Such interdependence is usually neglected in simple models of ice flow and the response of glaciers to climate variations.