Research
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.
Project team
Coordinator:
Involved research group:
- Glaciology
lab-ULB
Tison Jean-Louis, Lelouchier Claire, El Amri Saïda - Center for Glaciology
University of Aberystwyth, Wales, UK - Department of Geography
University of Cambridge, UK - Department of Geography
and Geomatics
University of Glasgow, Scotland, UK - Department
of Earth and Atmospheric Sciences
Canadian Circumpolar Institute, Canada
Foreign partners: