Abstract
AbstractThe contribution of glaciers to sea-level rise and their effects on landscape evolution depend on the poorly known relationship between sliding speed and drag at the ice/bed interface. Results from experiments with a new rotary laboratory device demonstrate empirically for the first time a double-valued drag relationship like that suggested by some sliding theories: steady drag on a rigid, sinusoidal bed increases, peaks and declines at progressively higher sliding speeds due to growth of cavities in the lee sides of bed undulations. Drag decreases with increased sliding speed if cavities extend beyond the inflection points of up-glacier facing surfaces, so that adverse bed slopes in contact with ice diminish with further cavity growth. These results indicate that shear tractions on glacier beds can potentially decrease due to increases in sliding speed driven by weather or climate variability, promoting even more rapid glacier motion by requiring greater strain rates to produce resistive stresses. Although a double-valued drag relationship has not yet been demonstrated for the complicated geometries of real glacier beds, both its potential major implications and the characteristically convex stoss surfaces of bumps on real glacier beds provide stimulus for exploring the effects of this relationship in ice-sheet models.
Highlights
Increases in the speeds of some outlet glaciers in Greenland and West Antarctica in response to recent atmospheric and oceanic warming highlight the important role of glacier speed-ups in future sea-level rise (e.g. Stocker and others, 2013)
Central to recent theoretical efforts for the case of glacier sliding over a rigid bed is separation of ice from leeward surfaces of bed undulations
For sinusoidal beds commonly considered in theories (Lliboutry, 1968, 1979, 1987; Fowler, 1986, 1987; Kamb, 1987; Schoof, 2005), the point of separation occurs at the inflection point of the leeward bed surface (Lliboutry, 1987)
Summary
Increases in the speeds of some outlet glaciers in Greenland and West Antarctica in response to recent atmospheric and oceanic warming highlight the important role of glacier speed-ups in future sea-level rise (e.g. Stocker and others, 2013). Central to recent theoretical efforts for the case of glacier sliding over a rigid bed is separation of ice from leeward surfaces of bed undulations. The maximum drag that the bed can support – commonly called Iken’s bound (Fowler, 1986, 1987; Schoof, 2005) – is predicted to decrease if sliding speed and cavity size increase further because the maximum drag that a stoss segment of the bed can support is proportional to its maximum slope in contact with ice (Iken, 1981; Schoof, 2005). For consistency with many past studies, we call this a ‘double-valued’ drag relationship, bearing in mind, that drag is double-valued only when it, rather than sliding speed, is considered to be the independent variable
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