A new 3D approach to linking high-resolution analysis on impurities for application to deep ice

Ice that will be extracted from close to the bedrock of the Antarctic ice sheet during the Beyond EPICA Oldest Ice (BE-OI) project is expected to have more than 14,000 years of climatic information contained in a single vertical meter of ice. High-resolution analysis is required to extract meaningful climate signals from the impurities contained in this ice. This analysis should be comparable to the currently established continuous flow analysis (CFA) approach, which acquires a 1-dimensional impurity signal at approximately centimetre resolution. To date, it has been shown that smoothed high-resolution profiles taken using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) are comparable to CFA signals. However, the physical origin of this link needs to be better understood, especially in view of the imprint of the impurities on the ice crystal matrix recently revealed by 2D imaging on the micron scale. Here we present a framework to generate and explore 3-dimensional models mapping the location of soluble impurities within ice samples. This framework helps link experimentally acquired smoothed LA-ICP-MS profiles, 2-dimensional LA-ICP-MS maps of impurities, and CFA data. The conceptual step into 3-dimensions allows exploration of the distortions of the climate signal due to interactions of impurities with the ice matrix. In shallow ice with relatively small grains and well resolved stratigraphy, this distortion is likely not significant enough to compromise analysis that takes large sample volumes with large mixing, such as seen during CFA. In order to extract signals in deep ice with large crystal sizes and dense layering, where this distortion will be most relevant, we find that carefully designed LA-ICP-MS experiments coupled with post-processing upscaling techniques are required. For a test against experimental data, this work is now being applied to a comparative study involving Antarctic ice measured with both CFA and LA-ICP-MS systems to prove its application to shallower, better-understood ice intervals. Ultimately, the goal is to develop a combination of cm-scale CFA, micron-scale LA-ICP-MS imaging and 3D modelling that will provide key insight on the impurity-related climate signals in deep ice at the BEOI core and elsewhere.