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Non-belspo (1997-2007)

BIFTeG - Basal Ice Facies of Temperate Glaciers

Project description

This project is probably the longest-lived of our laboratory. It has been looking at the basal layer of various types of alpine glaciers from the Swiss-French-Italian Alps, using the multiparametric approach of ice properties (ice fabrics and textures, ice chemistry, stable isotopes, solid impurity content a.s.o). to decipher boundary conditions at the ice-bedrock interface. We are looking at an array of topics such as: subglacial hydrology, glacial erosion processes, chemical sorting effect in sediment loaded ice, debris incorporation a the glacier bed, carbonate precipitates on subglacial floors, ice deformation around obstacles, inherited characteristics of proglacial streams…

Recent work has focused on the Tsanfleuron Glacier, a plateau glacier, showing a recent important retreat, located in the Swiss Alps. Eight 45 meters ice cores (including three down to the bedrock in the ablation zone) have led to the identification of three distinctive internal zones. Variation in the bulk ionic chemistry of these zones has been used to approximate their relative liquid water concentrations and ice viscosities. Results suggest that relative bulk water concentration and ice softness vary by an order of magnitude between these zones, not taking into account, however, other potential factors affecting the ice rheology. Implications of this variability for predictions of the glacier's response to climate change have been evaluated by incorporating these relative softnesses into a multi-layered (2-d) model of ice flow. Model output is compared with that from an identical model constrained with a spatially-uniform ice viscosity under advance and retreat modelling scenarios. The former scenario is used to tune viscosity by growing a glacier to its present long-section geometry - resulting in best-fit ice hardness values of 1.2 a-l bar-3 for the englacial ice in the multi- layered model and 7.0 a-l bar-3 for all of the ice in the single-layered model. Both result in close approximations to the current long-profile. ln contrast, a sing1e- layered model constrained with a hardness of 1.2 a-l bar-3 overestimates the current measured long-section area by 31 %. Under the retreat modelling scenario, which gauges the response of the glacier to an imposed 75 m rise in ELA, the multi-layered model predicts a long-section area reduction of 78%. This contrasts with a reduction of 64% for the single layered model (hardness = 7.0 a-l bar-3) and 85% for the single-layered model (hardness = 1.2 a-l bar-3). These results point to the potential impact of observed variations in impurity content of temperate ice for the model predictions of the response of temperate ice masses to future climate change.
Detailed crystallographic studies of the same eight cores at Tsanfleuron Glacier have also shown that fabric and texture contrasts will equally need to be incorporated in future models of temperate ice flow. Results indicate the presence of four crystallographic units. Unit 1 composed of homogeneous, fine-grained ice with a uniform fabric, is located within c. 20 m of the ice surface in the accumulation area of the glacier. Crystal growth within this unit occurs in the absence of significant stresses, and its rate is closely described by an Arrhenius- type relationship. Unit 2 ice, characterized by the local development of coarser crystals, forms after some decades of Arrhenius growth, marking the initial influence of processes of dynamic recrystallisation. Unit 3 ice, characterized by an abrupt increase in minimum crystal sire, occurs at a depth of c. 33m throughout the glacier. In the accumulation area, this increase coincides with the first evidence of systematic fabric enhancement, interpreted in terms of dynamic recrystallisation. Unit 4 ice, characterized by large, interlocking grains with a multi-modal girdle fabric, develops within c.10m of the glacier bed. Here, the measured minimum crystal size is consistent with a steady-state balance between Arrhenius processes of grain growth and strain-related processes of grain-sire reduction. These changes are interpreted in terms of the effects of intense, continuous deformation in this basal zone.

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