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This seasonal oscillation can be measured down Greenland ice cores.
Furthermore the oxygen isotope ratio can provide a general location in the core for the ice formed during the Ice Age and the ice that fell afterwards.
These cores represent snowfall that has turned to ice.
During snowfall on top of the ice sheets, dust, air, acids, etc.
For practical purposes, the ratio between oxygen-18 and oxygen-16 is the variable used, oxygen-17 being too small to be useful.