Glaciers form as layers of snow accumulate on top of each other. Each layer of snow is different in chemistry and texture, summer snow differing from winter snow. Over time, the buried snow compresses under the weight of the snow above it, forming ice.
Four environmental characteristics are encoded in these gas properties.
A variety of methods are used to date an ice core. The most direct method is to count annual layers in much the same way that tree-rings can be counted to determine the age of the tree. However, the layers in ice cores are not generally visible in the ice. They only become apparent when the core is analysed for a chemical signal that varies with the seasons, which most signals do, to some extent.
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