Abstract:
The Greenland Ice Sheet and the Antarctic Ice Sheet are the largest reservoirs of
freshwater on our planet. They consist of ice which formed over thousands of years
out of the precipitation and due to natural densification of snow. As such, polar
ice represents a unique archive of the past climate. The large ice masses flow under
their own weight causing a transport of ice from the inlands towards the oceans. Numerical flow models are used to simulate the ice dynamics, i. a., in order to project
future contributions of the ice sheets to the rising sea level. The viscoplastic deformation of ice on the micro-scale involves similar mechanisms as the deformation
of other poly-crystalline materials. The shear of individual crystals is accompanied
by recrystallization and the development of characteristic microstructures. Thus,
structural-geological concepts and methods can be applied to natural ice samples in
order to study their deformation state and active physical processes.
Chemical impurities are deposited in snow and ice during the precipitation and
reflect the aerosol composition of the atmosphere. As such they play an important
role for the reconstructions of the climate of the past. Despite their extremely low
concentrations, impurities influence many physical properties of ice, in particular
also the deformation rate. The concentrations of the different impurities vary with
depth and these variations are correlated with heterogeneities in the flow-velocity
profile. This results in the development of localized shear zones in ice. A better
understanding of the mechanisms is necessary in order to implement the effect of
impurities into the flow models.
The presented thesis addresses the relationship between the chemical impurities
and the physical properties of polar ice. A central challenge of the project is to understand in which form impurities integrate in ice and where they are located in
the bulk, respectively how they interact with it during deformation and recrystallization. A combination of methods for the microstructural analysis was applied to
natural ice material from ice cores. The distribution and composition of impurities
was analyzed using a confocal cryo-Raman microscope. Within the scope of this
thesis, new routines for the acquisition and data processing were developed.
The results reveal a complex interplay between impurities, crystal structure and
localized deformation in ice. On the one hand, high impurity concentrations cause
higher strain rates occurring together with small grain sizes. On the other hand, the
deforming ice matrix affects the distribution of impurities and possibly also their
chemical composition. The portion of dissociated components in ice could be estimated only qualitatively and through the comparison to the chemical analysis of the
meltwater. However, the Raman-spectroscopy data clearly suggest that microscopic
inclusions of second phase are present in solid ice in significantly higher concentrations, compared to liquid water. Mixing and chemical reactions of impurities is
promoted through the deforming ice matrix. Our image of “ice as a frozen archive”
could be replaced by “ice as an effective reactor”, depending on the spatial scales
and time spans referred to. The resulting implication for the chrono-stratigraphic
integrity of ice-core records may still be positive, because the reaction products often posses lower diffusion rates. A universal mechanism for the impurity effect on
ice deformation could not be identified. The localized deformation seems to be in
fact an intrinsic property of ice produced by the mechanical anisotropy and triggered by the varying impurity concentrations. However, many questions regarding
the form and effect of impurities in ice remain open for future investigations.