Oxygen Isotopes - proxy for glacial ice volume

The water molecule is composed of two hydrogens and one oxygen atom. Because the Oxygen-16 isotope is lighter than the Oxygen-18 isotope (both common in natural waters), oxygen-16 evaporates more readily. For the same reason, clouds rain out the heavier isotope more readily. Therefore clouds become enriched in the lighter isotope of oxygen. By the time the clouds reach the polar ice caps, they are very enriched in the light isotope form of water and the snow captured by glaciers locks up this form. The oceans on the other hand, become enriched in the heavy isotope of oxygen. Calcite shells of plankton are made of CaC03 and reflect in some way changing concentrations of the oxygen isotopes. Therefore as glaciers grow and subside, the changing volumes of ice (hence light oxygen isotopes), are recorded in the sediments of the world's ocean.