Abstract
Abstract. Impurities deposited in polar ice enable the reconstruction of the atmospheric aerosol concentration of the past. At the same time they impact the physical properties of the ice itself such as its deformation behaviour. Impurities are thought to enhance ice deformation, but observations are ambiguous due to a shortage of comprehensive microstructural analyses. For the first time, we systematically analyse micro-inclusions in polar fast flowing ice, i.e. from the East Greenland Ice Core Project ice core drilled through the Northeast Greenland Ice Stream. In direct relation to the inclusions we derive the crystal preferred orientation, fabric, grain size, and microstructural features at 10 depths, covering the Holocene and Late Glacial. We use optical microscopy to create microstructure maps to analyse the in situ locations of inclusions in the polycrystalline, solid ice samples. Micro-inclusions are more variable in spatial distribution than previously observed and show various distributional patterns ranging from centimetre-thick layers to clusters and solitary particles, independent of depth. In half of all samples, micro-inclusions are more often located at or close to the grain boundaries by a slight margin (in the areas occupied by grain boundaries). Throughout all samples we find strong indications of dynamic recrystallisation, such as grain islands, bulging grains, and different types of sub-grain boundaries. We discuss the spatial variability in micro-inclusions, the link between spatial variability and mineralogy, and possible effects on the microstructure and deformation behaviour of the ice. Our results emphasise the need for holistic approaches in future studies, combining microstructure and impurity analysis.
Highlights
Polar ice sheets are key elements of our climate system, and deep ice cores from these regions are used for, but not limited to, the reconstruction of the palaeoclimate (e.g. Lorius et al, 1985; Petit et al, 1999; Watanabe et al, 2003; EPICA Community Members, 2004; Dahl-Jensen et al, 2013) and the investigation of the dynamics of ice in ice sheets (e.g. Alley, 1988; Weikusat et al, 2017a)
East Greenland Ice Core Project (EGRIP) is an ice core drilling project located on Northeast Greenland Ice Stream (NEGIS), the largest ice stream in Greenland which terminates in three outlet glaciers (Nioghalvfjerds isstrømmen, Zachariae Isbræ, and Storsstrømmen) (Joughin et al, 2010; Vallelonga et al, 2014)
We derive a profile of the grain size with depth, displaying the grain size evolution of the upper 1340 m of the EGRIP ice core and microstructural data from the depth regimes analysed with optical microscopy (Fig. 1)
Summary
Polar ice sheets are key elements of our climate system, and deep ice cores from these regions are used for, but not limited to, the reconstruction of the palaeoclimate (e.g. Lorius et al, 1985; Petit et al, 1999; Watanabe et al, 2003; EPICA Community Members, 2004; Dahl-Jensen et al, 2013) and the investigation of the dynamics of ice in ice sheets (e.g. Alley, 1988; Weikusat et al, 2017a). Polar ice sheets are key elements of our climate system, and deep ice cores from these regions are used for, but not limited to, the reconstruction of the palaeoclimate Lorius et al, 1985; Petit et al, 1999; Watanabe et al, 2003; EPICA Community Members, 2004; Dahl-Jensen et al, 2013) and the investigation of the dynamics of ice in ice sheets As the snow transforms to ice, the deposited aerosols either dissolve in the ice structure or form micro-inclusions. With time they get transported into deeper regions in the ice sheet. Ice cores enable the gathering of information about the internal deformation of polar ice and its mechanisms as the localised characteristics determine the large-scale deformation behaviour
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