Abstract
Impurities in polar ice cores have been studied so far mainly for the purpose of reconstructions of past atmospheric aerosol concentrations. However, impurities also critically influence physical properties of the ice matrix itself. To improve the data basis regarding the in-situ form of incorporation and spatial distribution of impurities in ice we used micro-cryo-Raman spectroscopy to identify the location, phase and composition of micrometer-sized inclusions in natural ice samples around the transition from marine isotope stage (MIS) 6 into 5e in the EDML ice core. The combination of Raman results with ice-microsctructure measurements and complementary impurity data provided by the standard analytical methods (IC, CFA and DEP) allows for a more interdisciplinary approach interconnecting ice core chemistry and ice core physics. While the interglacial samples were dominated by sulfate salts - mainly gypsum, sodium sulfate (possibly thenardite) and iron-potassium sulfate (likely jarosite) - the glacial ice contained high numbers of mineral dust particles - in particular quartz, mica, feldspar, anatase, hematite and carbonaceous particles (black carbon). We cannot confirm cumulation of impurities in the grain boundary network as reported by other studies, neither micro-particles being dragged by migrating grain boundaries nor in form of liquid veins in triple junctions. We argue that mixing of impurities on millimeter scale and chemical reactions are facilitated by the deforming ice matrix. We review possible effects of impurities on physical properties of ice, however the ultimate identification of the deformation agent and the mechanism behind remains challenging.
Highlights
Polar ice sheets represent the most direct archives of the past atmosphere
The Continuous Flow Analysis (CFA) signal shows strong peaks on the millimeter to centimeter scale which are not reflected in the Dielectric profiling (DEP) signal
The total acquisition time per micro-inclusion was limited by the cumulative heating of the particles, which would in extreme cases cause local melting or even destruction of the particles
Summary
Polar ice sheets represent the most direct archives of the past atmosphere. The ice as well as its gas and impurity content originates from snow which once precipitated at the surface, was compacted due to accumulation of new layers, transformed into ice, and further traveled through the ice column. Ice sheets are subject to continuous deformation and flow, which - in case of Impurities and Microstructure Using Cryo-Raman Microscopy equilibrium - balances the mass accumulation on the surface with mass loss on the borders. They represent highly dynamic systems which react sensitively to changes in boundary conditions, e.g., temperature, accumulation rate etc. Up to now, neither are the relevant processes understood nor the acting species identified
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