| الوصف: |
Glacier deformation velocity is typically linked to ice thickness and is very sensitive to parameters of the Glen’s flow law (i.e. rate factor A and flow exponent n). Detailed observations of glacier deformation in the Greenland Ice Sheet (Ryser et al. 2014, Maier et al. 2019, 2021) reveal that the deformation profile close to the bed, where most of the deformation takes place, is not well represented by typical models and is heavily influenced by bed geometry. We present a similar study done on the ablation zone of Argentière Glacier, a temperate glacier in the french Alps. Glacier deformation was observed during 2020 with permanently installed inclinometers that record deformation at high temporal frequency. This data set is complemented with a dense network of GNSS sensors providing surface velocity. Combining the two sets of observations, we are able to reconstruct the deformation profile with depth and the sliding velocity. We show that the Shallow Ice Approximation strongly overestimates shear stress gradients at depth, such that it can lead to a wrong interpretation of deformation profiles. Instead, the use of a three-dimensional full-stokes model enables us to demonstrate that the yearly-averaged deformation profile is poorly sensitive to the flow law exponent n and instead mainly reflects an increase in the rate factor with depth. We interpret this rate factor increase by an associated increase in water content with depth, as expected from experimental data and field observations (Duval et al., 1977, Vallon et al., 1976). We further show that internal ice deformation also exhibits seasonal variability that is concomitant with and even stronger than surface velocity changes, such that basal sliding actually has to decrease in summer (at least locally) for our observations to be consistent. These results challenge the general view that glacier seasonal variability is mainly due to seasonal variability in sliding velocity. Further work remains to be conducted in order to fully determine the cause for internal deformation seasonality, which may be explained by changes in stress distribution, in water content or to other yet unidentified factors. |
