Quantitative Insights on Impurities in Ice Cores at the Micro-Scale From Calibrated LA-ICP-MS Imaging

Understanding the microscopic variability of impurities in glacier ice on a quantitative level has importance for assessing the preservation of paleoclimatic signals and enables the study of macroscopic deformational as well as dielectric ice properties. Two-dimensional imaging via laser-ablation-inductively-coupled-plasma-mass-spectrometry (LA-ICP-MS) can provide key insight into the localization of impurities in the ice. So far, these findings are mostly qualitative and gaining quantitative insights remains challenging. Recent advances in LA-ICP-MS high-resolution imaging now allow ice grains and grain boundaries to be resolved individually. These resolutions require new adequate quantification strategies and, consequently, accurate calibration with matrix-matched standards. Here, we present three different quantification methods, which provide a high level of homogeneity at the scale of a few tens of microns and are dedicated to imaging applications of ice cores. One of the proposed methods has a second application, offering laboratory experiments to investigate the displacement of impurities by grain growth, with important future potential to study ice-impurity interactions. Standards were analyzed to enable absolute quantification of impurities in selected ice core samples. Calibrated LA-ICP-MS maps indicate similar spatial distributions of impurities in all samples, while impurity levels vary distinctly: Higher concentrations were detected in glacial periods and Greenland, and lower levels in interglacial periods and samples from central Antarctica. These results are consistent with ranges from complementary meltwater analysis. Further comparison with cm-scale melting techniques calls for a more sophisticated understanding of the ice chemistry across spatial scales, to which calibrated LA-ICP-MS maps now contribute quantitatively. Compared to the large amount of information relating to paleoclimate signals reconstructed from cm-scale impurity measurements on ice cores, knowledge about the spatial variability of impurities at the micro-scale is extremely sparse-and becomes even more rare once quantitative datasets are concerned. However, there is an increasing demand for quantitative data for assessing the preservation of paleoclimatic signals and for the study of macroscopic deformational as well as dielectric ice properties in ice flow modeling and remote sensing. Two-dimensional imaging via laser-ablation-inductively-coupled-plasma-mass-spectrometry (LA-ICP-MS) has shown great potential in this context, but so far, gaining reliable quantitative results for micro-scale imaging has not been possible. Here, we present new quantification strategies that finally allow accurate calibration using ice standards. We carefully discuss the pros and cons of each method, apply the calibration to different samples from Greenland and Antarctica, and deliver the first calibrated LA-ICP-MS impurity maps at 40 mu m resolution. Our results are consistent with bulk measurements performed on melted samples. The calibrated LA-ICP-MS maps will be essential for further comparison with bulk meltwater analysis, which may ultimately deliver an improved understanding of paleoclimate signals stored in deep ice. This study presents new quantification strategies for two-dimensional micro-scale impurity imaging on ice cores with laser-ablation-inductively-coupled-plasma-mass-spectrometry (LA-ICP-MS) Calibrated LA-ICP-MS maps reveal similar spatial distributions of impurities in all ice core samples, while concentrations vary distinctly We developed a method to investigate the displacement of impurities by grain growth and to study ice-impurity interactions in the laboratory