Effects of basal sliding on isochrones and flow near an ice divide

Abstract

If an ice sheet is frozen to its bed, deep ice directly under a divide experiences low deviatoric stress and is relatively hard, because the rheology of polar ice is described by a power-law constitutive relation. In steady state, stratigraphic layers tend to form an arch (“Raymond bump”) in this region. However, when the basal ice can slide, the stresses are redistributed, and longitudinal extension due to sliding is associated with increased deviatoric stress in the deep ice under the divide. This increased deviatoric stress weakens the tendency to form a Raymond bump. To find a realistic spatial distribution of sliding under an ice divide, we incorporate a thin layer of viscous till in a finite-element plane-strain flow model. The resulting basal “sliding” velocity varies approximately linearly with distance from the ice divide. By varying the till viscosity, we can adjust the amount of basal motion. We find that the Raymond bump decays exponentially with the fraction of total ice flux carried by sliding: the arch is 50% smaller when the sliding flux is only 7% of the total ice flux. This implies that the possibility of a wet bed must be considered when inferring past ice-divide locations from radar internal layering.

On the effects of divide migration, along‐ridge flow, and basal sliding on isochrones near an ice divide

“…In the thermomechanical experiments (section 4), we set the upper surface temperature θ s to −30°C and the geothermal heat flux Q G = 60 mW m −2 . For ice and rock, the thermal conductivity is K = 2.10 W m −1 K −1 and the specific heat capacity is c = 2009.0 J kg −1 K −1 (the values of thermal conductivity and diffusivity for ice fall within the typical range for sedimentary rocks [e.g., Pettit et al , 2003]).…”

“…In many places in Antarctica during the glacial period, ice was much thicker, which normally leads to warming at the base of the ice through the greater insulating effect. Pettit et al [2003] have shown that when a linear relation in the basal sliding law is considered, the Raymond effect is sharply reduced. The implication of this is that the onset of operation of the Raymond effect may date the time of freezing of the base of the ice rather than some other flow change.…”

Dating ice flow change near the flow divide at Roosevelt Island, Antarctica, by using a thermomechanical model to predict radar stratigraphy

“…Our preliminary testing suggested only a limited effect from basal sliding, which was restricted to cases of extreme values of model parameters and extremely high initial ice thickness. Nevertheless, the alternative interpretation by Pettit et al [2003] of Conway et al [1999] dating that prior to 3200 years B.P. Roosevelt Island could have supported and ice divide over a wet bed that allowed sliding, is sufficient to imply that this subject deserves some further attention.…”

Evidence from Keewatin (Central Nunavut) for Paleo-Ice Divide Migration*

“…Wilch and Hughes, 2000), flowlines probably extended nearly to the ice drainage divide, as recorded by striation limits and to some extent by corresponding landforms. Although basal shear stresses are known to be small near ice divides, Pettit et al (2003) recently suggested longitudinal stresses (pulling effect of down-stream ice) in present-day warm-based ice can be significant and can control basal sliding velocity in these areas. Therefore, although we observed old glacial landforms preserved beneath the divide zone, we argue that the Keewatin Ice Divide Zone 1 of is a reasonable approximation of the last position of the central axis of the divide, as opposed to the "general position throughout the last glaciation" (Aylsworth and Shilts, 1989: p. 3).…”