Sliding of temperate basal ice on a rough, hard bed: creep mechanisms, pressure melting, and implications for ice streaming

摘要

Basal ice motion is crucial to ice dynamics of iceudsheets. The classic Weertman model for basal sliding overudbedrock obstacles proposes that sliding velocity is controlledudby pressure melting and/or ductile flow, whichever is theudfastest; it further assumes that pressure melting is limited byudheat flow through the obstacle and ductile flow is controlledudby standard power-law creep. These last two assumptions,udhowever, are not applicable if a substantial basal layer of temperateud(T � Tmelt/ ice is present. In that case, frictional meltingudcan produce excess basal meltwater and efficient waterudflow, leading to near-thermal equilibrium. High-temperatureudice creep experiments have shown a sharp weakening of audfactor 5–10 close to Tmelt, suggesting standard power-lawudcreep does not operate due to a switch to melt-assisted creepudwith a possible component of grain boundary melting. Pressureudmelting is controlled by meltwater production, heat advectionudby flowing meltwater to the next obstacle and heatudconduction through ice/rock over half the obstacle height. Noudheat flow through the obstacle is required. Ice streaming overuda rough, hard bed, as possibly in the Northeast GreenlandudIce Stream, may be explained by enhanced basal motion in audthick temperate ice layer.