Abstract
Abstract. Basal ice motion is crucial to ice dynamics of ice sheets. The classic Weertman model for basal sliding over bedrock obstacles proposes that sliding velocity is controlled by pressure melting and/or ductile flow, whichever is the fastest; it further assumes that pressure melting is limited by heat flow through the obstacle and ductile flow is controlled by standard power-law creep. These last two assumptions, however, are not applicable if a substantial basal layer of temperate (T ∼ Tmelt) ice is present. In that case, frictional melting can produce excess basal meltwater and efficient water flow, leading to near-thermal equilibrium. High-temperature ice creep experiments have shown a sharp weakening of a factor 5–10 close to Tmelt, suggesting standard power-law creep does not operate due to a switch to melt-assisted creep with a possible component of grain boundary melting. Pressure melting is controlled by meltwater production, heat advection by flowing meltwater to the next obstacle and heat conduction through ice/rock over half the obstacle height. No heat flow through the obstacle is required. Ice streaming over a rough, hard bed, as possibly in the Northeast Greenland Ice Stream, may be explained by enhanced basal motion in a thick temperate ice layer.
Highlights
The manner in which ice deforms within an ice sheet and moves or slides over its base is critical to accurately model the dynamic past, present and future behaviour of such ice bodies (e.g. Marshall, 2005)
Evidence for palaeo-ice streaming on hard beds has been reported from the former Pleistocene Laurentide and British ice sheets and deglaciated parts of West Greenland
What are the rate-controlling factors for stoss-side pressure melting in a layer of temperate ice? In the conceptual model here (Fig. 5c), the temperate layer is thicker than the height of the obstacle
Summary
The manner in which ice deforms within an ice sheet and moves or slides over its base is critical to accurately model the dynamic past, present and future behaviour of such ice bodies (e.g. Marshall, 2005). Using an empirical drag factor is reasonable to describe and understand present-day, near-instantaneous ice-sheet behaviour but cannot reliably predict or reconstruct ice velocities if parameters such as ice thickness, driving forces and meltwater production change significantly This problem is acute for ice streams with poor topographic steering. Krabbendam: Sliding of temperate basal ice on a rough, hard bed (Smith, 1948; Stokes and Clark, 2003; Roberts and Long, 2005; Bradwell et al, 2008; Eyles, 2012; Bradwell, 2013; Eyles and Putkinen, 2014; Krabbendam et al, 2016) In these areas, the deforming-bed models cannot apply because little or no soft sediment is present.
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