Isotope analysis looking at the O18/O16 ratio in materials. The method can be used to classify glass types and to analyze mollusk shells in order to try and reconstruct their original environment and thus the source. It is also used to interpret deep sea cores. The basis for this technique is the fact that the ratio of two of the stable isotopes of oxygen varies according to the material in which it is found. The oxygen is released from the sample, and is converted to carbon dioxide; the oxygen isotopic ratio is determined after ionization in a mass spectrometer. Variations in the isotopic ratios for the raw materials can lead to a classification of types and even, in some cases, the suggestion of a source for the raw materials. The technique is also used to analyze mollusk shells in an attempt to reconstruct the original aquatic environment. Because temperature variations are correlated with changes in atmospheric O18/O16 ratios, oxygen isotope analysis has also been used to identify seasonal changes in ice cores, interpret temperature variations during speleothem precipitation, and examine isotopic variations in tree ring climates. Foraminifera sampled from deep sea cores have revealed fluctuations in the O18/O16 ratio. These present evidence for glacial-interglacial cycles in the form of continental ice volume change.