Salt inclusions in polar ice core: Location and chemical form of water-soluble impurities

Ice cores retrieved from ice sheets provide the most detailed records of climate over the past several hundred thousand years. For example, water-soluble impurities have been used as proxies for atmospheric chemistry. However, numerical models warned that the distributions of the trace constituents are significantly altered by rapid diffusion through liquid veins along triple junctions during ice sheet flow. Although these calculations assumed that the trace constituents are preferentially confined to grain boundaries and form a vein network, this assumption has not been confirmed experimentally. Using micro-Raman spectroscopy, here we show that most of the soluble impurities are confined as salt inclusions within grains. These inclusions are mainly composed of the sulfate salts Na2SO4·10H2O, MgSO4·12H2O, and CaSO4·2H2O. We also found that methane-sulfonate salts and other complex salts exist only in the glacial-period ice. Our findings imply that these salts form within 185 m of the ice surface and then preserve their composition as the ice is pushed to greater depths. This means that depth profiles of the trace constituents should be preserved in deep ice cores as reliable proxies for past climate.