HR: 11:05h
AN: C52A-04    [Abstracts]
TI: Influence of a frozen glacier margin on sediment transport: measurements and modeling
AU: * Moore, P L
EM: pmoore@iastate.edu
AF: Iowa State University, Dept. Geological and Atmospheric Sciences 253 Science 1, Ames, IA 50011, United States
AU: Iverson, N R
EM: niverson@iastate.edu
AF: Iowa State University, Dept. Geological and Atmospheric Sciences 253 Science 1, Ames, IA 50011, United States
AU: Cohen, D
EM: dcohen@iastate.edu
AF: Iowa State University, Dept. Geological and Atmospheric Sciences 253 Science 1, Ames, IA 50011, United States
AU: Jansson, P
EM: peter.jansson@natgeo.su.se
AF: Stockholm University, Dept. Physical Geography and Quaternary Geology, Stockholm, 106 91, Sweden
AU: Brugger, K A
EM: bruggeka@morris.umn.edu
AF: University of Minnesota - Morris, Geology Discipline 600 E. 4th St., Madison, MN 56267, United States
AU: Hooyer, T S
EM: tshooyer@wisc.edu
AF: Wisconsin Geological and Natural History Survey, University of Wisconsin 3817 Mineral Point Rd., Madison, WI 53705, United States
AB: Broad belts of high-relief hummocky topography near the margins of former glaciers and ice sheets are commonly considered to be paleoclimatic indicators. These landscapes are thought by some to indicate a frozen bed at the glacier margin, thus requiring cold atmospheric temperatures at the time of formation. Such interpretations are usually justified by analogy with some modern polythermal glaciers where debris of apparent subglacial origin accumulates at the glacier surface in ridges and subsequently melts out over stagnating ice at the glacier terminus. In these modern glaciers, basal debris may be uplifted to the surface along shear zones or faults originating from the transition between warm-based ice upglacier and cold-based ice at the terminus. However, the relationship between supraglacial debris accumulation and the mechanics of ice flow at a frozen margin is not well established, so caution is needed in drawing paleoclimatic conclusions from these landscapes. Through a combination of field measurements and numerical modeling at Storglaciaren, Sweden, we attempt to better constrain this relationship. Storglaciaren is a small polythermal glacier with a cold surface layer that varies in thickness from 20 to 35 m. Borehole thermistor measurements and radar indicate that the zero-degree isotherm dips into the bed as the ice thins toward the terminus. Ice downglacier of this basal thermal transition (BTT) is frozen to the substrate, inhibiting basal slip whereas a short distance upglacier, borehole measurements (slidometers) indicate that the ice is slipping over the bed. Resulting longitudinal compression may result in uplift of basal ice near the BTT. Indeed, bands of sediment-rich ice, dipping steeply upglacier and apparently derived from the glacier bed, outcrop at the ice surface just downglacier from the BTT. Surface velocity measurements indicate that emergence velocity peaks just upglacier from the debris bands, but evidence for discrete shear across the debris bands is lacking. On the contrary, a finite element model of ice flow under similar conditions suggests that a slip/no-slip transition at the bed is sufficient to locally enhance emergence velocities without strongly localized shear and thereby convey basal ice to the glacier surface.
DE: 0768 Thermal regime
DE: 0776 Glaciology (1621, 1827, 1863)
DE: 0798 Modeling
DE: 1824 Geomorphology: general (1625)
DE: 8012 High strain deformation zones
SC: Cryosphere [C]
MN: 2007 Fall Meeting