The study of the geochemistry of stable isotopes in naturally occurring sediments and biological structures. Stable isotopes are atomic variations of elements that are stable over long periods of time, meaning they do not radioactively decay. Several elements, like oxygen and carbon, have several stable forms. Oxygen, for example, occurs in nature as 16O and 18O — these two forms are isotopes. They have different numbers of neutrons (16O has two more neutrons than 18O), and and is therefore heavier. Because the two isotopes have different masses, chemical and physical reactions like bonding, evaporation, and precipitation occur at different frequencies. The ratio of stable isotopes is preserved in chemical compounds like water, ice, and calcium carbonate and provides information on the environmental conditions at the time the compound formed. For example, the ratio of 18O/16O in an ice sample is linked to the water temperature of ancient oceans, which in turn reflects ancient climates.