New evidence on the microstructural localization of sulfur and chlorine in polar ice cores with implications for impurity diffusion

Aerosol-related impurities play an important part in the set of paleoclimate proxies obtained from polar ice cores. However, in order to avoid misinterpretation, post-depositional changes need to be carefully assessed, especially in deep ice. Na, S and Cl are among the relatively abundant impurity species in polar ice (albeit still at the low ppb level in bulk samples), with important applications to paleoclimate reconstructions and dating, e.g. via identification of volcanic eruptions. Especially S has been studied intensely with respect to peak broadening with depth/age related to diffusion, but the precise physical mechanisms remain unclear. Mapping the two-dimensional impurity distribution in ice with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has shown great potential for studying ice-impurity interactions, but the analytically more challenging elements S and Cl have not been targeted thus far. We show here that signals of S and Cl can be detected in Greenland and Antarctic ice by LA-ICP-MS mapping. In ice without evidence of volcanic activity, and unenhanced impurity concentrations, we obtain multi-elemental maps for Na, Cl and S at high resolution up to 10 mu m and also include some exemplarily high resolution maps with a spot size down to 1 mu m. We use Na as a previously investigated reference element and find a high level of localization of Na, S and Cl at grain boundaries but also some dispersed occurrence within grain interiors in dust-rich ice. The new maps support a view on diffusive transport not only through ice veins but also along grain boundaries. In the EPICA Dome C ice core samples we do not find any clear differences in impurity localization between samples from the Holocene and last glacial period. These results extend early studies targeting the localization of impurities, in particular through measuring S and Cl, and highlight the benefit of integrating such direct measurements with modelling efforts to determine the physical processes behind impurity diffusion.