BED-DEFORMATION EXPERIMENTS BENEATH A THICK TEMPERATE GLACIER

Genesis of various subglacial landforms and high sediment fluxes from Pleistocene ice masses have been commonly attributed to shear deformation of wet, subglacial sediment. Bed-deformation data gathered beneath modern glaciers, however, are difficult to interpret. Difficulties stem from measuring deformation near glacier margins, where conditions may be abnormal, or at the bottoms of boreholes, where the scope of instrumentation is limited, drilling disturbs sediment, and local boundary conditions are poorly known.

A different approach is possible at the Svartisen Subglacial Laboratory, where tunnels melted in basal ice provide human access to the bed of Engabreen, a temperate outlet glacier of the Svartisen Ice Cap in Norway. A trough (~2 m x 1.5 m x 0.5 m deep) was blasted in the rock bed, where the glacier is 213 m thick and sliding at 0.1-0.2 m/d. This trough was filled with ~2.5 tons of simulated till. Instruments in the till recorded shear deformation, dilation and contraction, total normal stress, and pore-water pressure as ice moved across the till surface. Pore pressure was manipulated by feeding water to the base of the till with a high-pressure pump, operated in a tunnel below the bed surface.

Results illustrate some fundamental aspects of bed deformation. Deformation of till required low effective normal stresses associated with high pore-water pressures, highlighting the frictional nature of till. Shear strain generally increased upward in the bed toward the glacier sole, consistent with previous measurements beneath thinner ice near glacier margins. At very low effective normal stresses, however, like those measured beneath modern wet-based ice sheets, ice usually decoupled from underlying till and slipped across the till surface. This result agrees with borehole studies at Trapridge Glacier and Storglaciären. We conclude that soft-bedded glaciers may not move principally by bed deformation.