A Review of the Microstructural Location of Impurities in Polar Ice and Their Impacts on Deformation

Abstract

Insoluble and soluble impurities, enclosed in polar ice sheets, have a major impact on the deformation behaviour of the ice. Macro- and Micro-scale deformation observed in ice sheets and ice cores has been retraced to chemical loads in the ice, even though the absolute concentration is negligible. And therefore the exact location of the impurities matters: Allocating impurities to specific locations inside the ice microstructure inherently determines the physical explanation of the observed interaction between chemical load and the deformational behaviour. Both, soluble and non-soluble impurities were located in grain boundaries, triple junctions or in the grain interior, using different methods, samples and theoretical approaches. While each of the observations is adding to the growing understanding of the effect of impurities in polar ice, the growing number of ambiguous results calls for a dedicated and holistic approach in assessing the findings. Thus, we here aim to give a state of the art overview of the development in microstructural impurity research over the last 20 years. We evaluate the used methods, discuss proposed deformation mechanisms and identify two main reasons for the observed ambiguity: 1) limitations and biases of measurement techniques and 2) the physical state of the analysed impurity. To overcome these obstacles we suggest possible approaches, such as the continuous analysis of impurities in deep ice cores with complementary methods, the implementation of these analyses into established in-situ ice core processing routines, a more holistic analysis of the microstructural location of impurities, and an enhanced knowledge-transfer via an open access data base.

Microstructure, Micro-inclusions and Mineralogy along the EGRIP ice core – Part 1: Localisation of inclusions and deformation patterns

“…Our higher value can be explained by sample selection from depths with high dust content as shown in a companion paper. Our results indicate that the spatial variability of micro-inclusions within Holocene ice is larger than previously thought emphasising the difficulty in generalising spatial patterns as suggested by Stoll et al (2021).…”

“…Impurities affect the physical and mechanical properties of ice, especially the deformation and the flow of ice (Paterson, 1991;Cullen and Baker, 2001), and most studies show that impurity-rich ice is easier to deform than impurity-poor ice (e.g., Fisher and Koerner, 1986;Paterson, 1991;Cuffey et al, 2000). The potential influence of impurities on other, partly related, ice properties, such as CPO and grain size, are complex and manifold (Stoll et al, 2021). We discuss potential processes involving micro-inclusions and observed microstructural features and their implications for deformation via dominant mechanisms, and dynamic recrystallization.…”

“…Direct indications would be e.g., a sharp edge at a second phase particle in an otherwise curved grain boundary segment or a bulging grain boundary restricted by a second phase particle (e.g., Fig. 2 in Stoll et al (2021)) (Passchier and Trouw, 2005). This agrees with other studies (Ohno et al, 2005;Faria et al, 2010;Eichler et al, 2017Eichler et al, , 2019 and indicates that the grain evolution is not affected strongly by the strict Zener pining process of micro-inclusions as proposed by Alley et al (1986b), but if effected via reduced grain growth at all, it may rather be effected by a reduced grain boundary mobility.…”

Microstructure, Micro-inclusions and Mineralogy along the EGRIP ice core – Part 2: Implications for paleo-mineralogy

“…Over the last decades different ice cores from Greenland and Antarctica were analysed regarding the chemistry and location of impurities and the main results were recently summarised by Stoll et al (2021). While studies using optical microscopy 2 https://doi.org/10.5194/tc-2021-190 Preprint.…”

“…All these studies are demanding in time and costly and thus often rely on a small number of samples taken at arbitrary depths. This results in a poor understanding of the underlying processes and in a lack of identified species involved hampering the development of realistic generalisations of impurities for polar ice (Stoll et al, 2021).…”