Abstract
ABSTRACTAlong the base of glaciers and ice sheets, the sliding of ice over till depends critically on water drainage. In locations where drainage occurs through Röthlisberger channels, the effective pressure along the base of the ice increases and can lead to a strengthening of the bed, which reduces glacier sliding. The formation of Röthlisberger channels depends on two competing effects: (1) melting from turbulent dissipation opens the channel walls and (2) creep flow driven by the weight of the overlying ice closes the channels radially inward. Variation in downstream ice velocity along the channel axis, referred to as an antiplane shear strain rate, decreases the effective viscosity. The softening of the ice increases creep closure velocities. In this way, even a modest addition of antiplane shear can double the size of the Röthlisberger channels for a fixed water pressure or allow channels of a fixed radius to operate at lower effective pressure, potentially decreasing the strength of the surrounding bed. Furthermore, we show that Röthlisberger channels can be deformed away from a circular cross section under applied antiplane shear. These results can have broad impacts on sliding velocities and potentially affect the total ice flux out of glaciers and ice streams.
Highlights
Observations show a definitive link between subglacial hydrology and glacier sliding (Iken and Bindschadler, 1986; Kamb, 1987; Fischer and Clarke, 1997)
There are generally thought to be two modes of drainage in subglacial hydrology: (1) concentrated channels such as Röthlisberger channels (R-channels) incised into the ice (Röthlisberger, 1972; Shreve, 1972; Weertman, 1972) or Nye channels eroded into hard bedrock (Weertman, 1972; Nye, 1973; Walder and Hallet, 1979), that operate at low water pressure relative to the overburden pressure of the overlying ice, (2) distributed water systems, including a network of linked cavities (Lliboutry, 1979; Anderson and others, 1982; Walder, 1986) or canals (Walder and Fowler, 1994; Fowler and Ng, 1996; Ng, 1998) connected by thin sheets of water at high pressure (Flowers and Clarke, 2002a; Creyts and Schoof, 2009; Hewitt, 2011)
We show that the amount of antiplane shear present in the ice can substantially increase the size of the R-channels or decrease the effective pressure, which can affect the duration of glacier surges and the location of ice-stream shear margins
Summary
Observations show a definitive link between subglacial hydrology and glacier sliding (Iken and Bindschadler, 1986; Kamb, 1987; Fischer and Clarke, 1997). High water content and high pore pressure, commonly underlies regions of fast flow of glaciers and ice sheets (Clarke and others, 1984; Blankenship and others, 1986; Kamb, 2001). Observations indicate that till typically behaves as a perfectly plastic material with a yield stress that depends on the effective pressure, the overburden pressure of the ice less the pore pressure (Kamb, 1991; Tulaczyk and others, 2000; Cuffey and Paterson, 2010). Water flow through subglacial hydrologic systems along the ice/till interface can locally depress the till yield stress, leading to till deformation and glacier sliding
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