HR: 17:45h
AN: C54A-08    [Abstracts]
TI: Stabilizing feedbacks in-glacier bed erosion: Constraints from numerical simulations
AU: * Creyts, T T
EM: tcreyts@eps.berkeley.edu
AF: Earth and Planetary Science University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720-4767, United States
AU: Clarke, G K
EM: clarke@eos.ubc.ca
AF: Earth and Ocean Sciences University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada
AB: Recently, the idea of a stabilizing feedback in subglacial erosion was introduced using a simple thermodynamic balance (Alley and others, 2003). The hypothesis states that glaciers with an active basal hydrology will modify the bed slope so that it is adverse to and approximately 20--70% steeper than the glacier surface slope based on a thermodynamically-controlled supercooling threshold. When water flow or sediment supply changes, bed slopes will be driven back to this threshold slope. In the ablation zone where sufficient surface meltwater reaches the ice--bed interface, water flowing up a bed slope adverse to ice flow can supercool and freeze to the glacier sole. Because the density of ice is greater than water, the remaining water pressurizes and sediments will not be mobilized effectively. Glacier beds will thus tend to be at the slope where supercooling occurs subglacially. In order to test this hypothesis, we construct a numerical model of one dimensional water flow based on the balances of mass, momentum, and enthalpy. These balances quantify water, ice, and sediment movement at the base of the glacier. Upstream conditions are simplified to recreate a melt season with diurnal meltwater fluctuations. The model is fully transient and does not rely on steady state or other similar assumptions. Simulations reveal behavior that cannot be inferred from simplified models. For example, while total simulated ice accretion is comparable to field estimates, locations of simulated ice accretion along the ice--bed interface conflict with steady-state models, which tend to overpredict accretion amounts. Simulations also indicate that much sediment deposition occurs prior to water being supercooled. As a result, sediment deposition and supercooling are largely decoupled in simulations and may be either partially or largely decoupled in the field. In addition, sediment deposition tends to smooth subglacial topography rather than enhance it. This result stems from the rate of change of sediment transport being determined by the curvature of the water flowpath. A prominent result is that glaciers will tend to smooth out their beds rather than enhance them. This result runs contrary to the hypothesis proposed for stabilizing feedbacks in glacier-bed erosion. We discuss additional results and implications of these results and encourage dialogue on stabilizing feedbacks.
DE: 0720 Glaciers
DE: 0766 Thermodynamics (1011, 3611, 8411)
DE: 0776 Glaciology (1621, 1827, 1863)
DE: 0798 Modeling
DE: 1824 Geomorphology: general (1625)
SC: Cryosphere [C]
MN: 2007 Fall Meeting