OVERDEEPENINGS, SUPERCOOLING, AND GLACIAL GEOMORPHOLOGY: DON J. EASTERBROOK DISTINGUISHED SCIENTIST AWARD

Graded subaerial fluvial systems are stabilized in part by feedbacks between erosion/deposition and the hydrologic potential gradient arising from slope (Easterbrook Sfc Proc & Landforms 1999). Subglacial streams fed by surface melt typically dominate glacial sediment budgets (Hunter et al 1996 JGlac) and cause faster geomorphic change than do subaerial fluvial systems (Hallet et al 1996 Global Planet Change). Subglacial-stream potential gradient is largely controlled by the ice-air surface slope with only indirect stabilizing feedbacks. We hypothesize that the most important stabilizing feedback for such subglacial systems arises from glaciohydraulic supercooling of water flowing through glacially produced overdeepenings. This process grows ice in stream channels when erosion causes adverse slopes to become too steep, thereby reducing erosion and sediment transport (Lawson et al 1998 JGlac).

Plucking likely controls subglacial bedrock erosion, and increases with sliding speed (Hallet 1996 Ann Glac) and with water-pressure fluctuations that are linked to water influx (Alley et al 1999 GSA Spec Pap 337). Localization of water input by crevassing over bed convexities is hypothesized to form overdeepenings in bedrock (Hooke 1991 GSA Bull), with erosion limited by supercooling that damps pressure fluctuations if adverse slopes of overdeepenings become too steep. Sediments overridden by advancing glaciers are primarily recycled by subglacial streams at rates also hypothesized to be limited by supercooling. Ice accretion during supercooling traps mainly fine-grained sediment, subsequently producing debris-flow and meltout diamictons when released at the glacier margin (Lawson 1979 CRREL Rept), while bedload in subglacial channels is trapped subglacially (Pearce et al 2001 AGU Fall Meet), contributing to distinct deposits and landforms as well as reducing slopes to feed back on supercooling and sediment transport.

Early studies on glacial erosion and often-overdeepened fjords by John Andrews and coworkers (Andrews et al, and Dowdeswell and Andrews, in JT Andrews ed 1985 Quaternary Environments) contributed to progress in these areas.