Soft-bed experiments beneath Engabreen, Norway: regelation infiltration, basal slip and bed deformation N.R. IVERSON, 1 T.S. HOOYER, 2 U.H. FISCHER, 3 D. COHEN, 1 P.L. MOORE, 1 M. JACKSON, 4 G. LAPPEGARD, 5 J. KOHLER 6 1 Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, 50011, USA E-mail: niverson@iastate.edu 2 Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin, 53705, USA 3 Antarctic Climate and Ecosystems CRC and Australian Antarctic Division, Box 252-80, Hobart, Tasmania 7001, Australia 4 Norwegian Water Resources and Energy Directorate (NVE), PO Box 5091, NO-0301 Oslo, Norway 5 Department of Geography, University of Oslo, PO Box 1042, Blindern, NO-0316 Oslo, Norway 6 Norwegian Polar Institute, Polar Environmental Center, NO-9005 Tromsø, Norway ABSTRACT. To avoid some of the limitations of studying soft-bed processes through boreholes, a prism of simulated till (1.8 m  1.6 m  0.45 m) with extensive instrumentation was constructed in a trough blasted in the rock bed of Engabreen, a temperate glacier in Norway. Tunnels there provide access to the bed beneath 213m of ice. Pore-water pressure was regulated in the prism by pumping water to it. During experiments lasting 7–12 days, the glacier regelated downward into the prism to depths of 50– 80mm, accreting ice-infiltrated till at rates predicted by theory. During periods of sustained high pore- water pressure (70–100% of overburden), ice commonly slipped over the prism, due to a water layer at the prism surface. Deformation of the prism was activated when this layer thinned to a sub-millimeter thickness. Shear strain in the till was pervasive and decreased with depth. A model of slip by ploughing of ice-infiltrated till across the prism surface accounts for the slip that occurred when effective pressure was sufficiently low or high. Slip at low effective pressures resulted from water-layer thickening that increased non-linearly with decreasing effective pressure. If sufficiently widespread, such slip over soft glacier beds, which involves no viscous deformation resistance, may instigate abrupt increases in glacier velocity. 1. INTRODUCTION The subglacial setting in which temperate ice rests on a soft bed is most important for modulating fast glacier flow and mobilizing basal sediment (Clarke, 2005). Interactions among water, ice and sediment in this environment can result in abrupt temporal and spatial variations in glacier velocity. On large scales these variations may be mani- fested as surging (e.g. Clarke and others, 1984), spatio- temporal switching of ice-stream flow (e.g. Kamb, 2001) and lurching behavior of parts of ice sheets that has been measured directly (Bindschadler and others, 2003) and inferred from seismicity (Ekstro ¨ m and others, 2003, 2006). Also, very high rates of basal sediment transport have been postulated for glaciers that are soft-bedded and warm- based (Hooke and Elverhøi, 1996; Dowdeswell and Siegert, 1999). Moreover, under these conditions some of the most poorly understood of glacial landforms, such as drumlins (e.g. Boulton, 1987) and megascale lineations (e.g. Ottesen and Dowdeswell, 2006), are thought to have developed. The most detailed observations of soft-bed processes beneath glaciers have been made with instruments inserted through boreholes drilled to the bed, from either ice tunnels (e.g. Boulton and Hindmarsh, 1987) or glacier surfaces (e.g. Blake and others, 1992; Kamb, 2001). These observations, although illuminating, have been limited by the restricted range of instruments that can be inserted through boreholes. Moreover, interpretations are compli- cated by ambiguities regarding instrument position, bed geometry and bed disturbance by hot-water drilling. Our goal was to avoid these difficulties by building and studying a 1.3 m 3 prism of simulated till beneath Enga- breen, Norway, (Iverson and others, 2003) where tunnels through the subglacial rock provide access to the bed. Our results provide a field demonstration of regelation infiltra- tion of ice into a till bed (e.g. Iverson and Semmens, 1995; Clarke, 2005; Christoffersen and others, 2006), help illustrate the relationship between basal water pressure and the coupling between ice and till (e.g. Fischer and Clarke, 2001) and provide new insights into controls on the vertical distribution of bed deformation (e.g. Boulton and others, 2001). Associated analysis indicates that regelation infiltration occurred at rates in agreement with theory, and a new model of ploughing clarifies how changes in the thickness of a water layer at the bed surface control the basal flow mechanism. 2. BACKGROUND 2.1. Regelation infiltration In a process that Clarke (2005) calls regelation infiltration, temperate ice may move downward into the pores of subglacial sediment by melting and refreezing. Evidence for this process comes from laboratory experiments (Iverson, 1993; Iverson and Semmens, 1995). Its rate is controlled by the bed-normal pressure gradient across the sediment layer that has been entrained, which depends on the effective pressure at the bed and the thickness of the layer. Although the isotopic composition (Iverson and Souchez, 1996) and structure (Truffer and others, 1999; Christoffersen Journal of Glaciology, Vol. 53, No. 182, 2007 323