Impurity Analysis and Microstructure Along the Climatic Transition From MIS 6 Into 5e in the EDML Ice Core Using Cryo-Raman Microscopy

Abstract

Impurities in polar ice cores have been studied so far mainlyfor the purpose ofreconstructions of past atmospheric aerosol concentrations. However, impurities alsocritically influence physical properties of the ice matrix itself. To improve the data basisregarding thein-situform of incorporation and spatial distribution of impurities in ice weused micro-cryo-Raman spectroscopy to identify the location, phase and compositionof micrometer-sized inclusions in natural ice samples around the transition from marineisotope stage (MIS) 6 into 5e in the EDML ice core. The combination of Raman resultswith ice-microsctructure measurements and complementaryimpurity data provided bythe standard analytical methods (IC, CFA, and DEP) allows for a more interdisciplinaryapproach interconnecting ice core chemistry and ice core physics. While the interglacialsamples were dominated by sulfate salts—mainly gypsum, sodium sulfate (possiblythenardite) and iron–potassium sulfate (likely jarosite)—the glacial ice contained highnumbers of mineral dust particles— in particular quartz, mica, feldspar, anatase, hematiteand carbonaceous particles (black carbon). We cannot confirm cumulation of impuritiesin the grain boundary network as reported by other studies, neither micro-particles beingdragged by migrating grain boundaries nor in form of liquid veins in triple junctions. Weargue that mixing of impurities on millimeter scale and chemical reactions are facilitated bythe deforming ice matrix. We review possible effects of impurities on physical propertiesof ice, however the ultimate identification of the deformation agent and the mechanismbehind remains challenging.