The abundance ratio between two of the three isotopes of oxygen. They have similar chemical properties because they have the same electronic structure, but because of the differences in mass between their nuclei they have different vibrational frequencies which cause them to behave slightly differently in physico-chemical reactions. These differences can provide information, e.g. regarding the source of water in a past environment or the temperature at which various interactions have taken place. For example, surface waters vary in their oxygen isotopes; light water (H₂¹⁶O) has a higher vapour pressure than H₂¹⁸O, and therefore is concentrated by evaporation so that fresh water and polar ice are light but sea water is heavy. CaCO₃ or SiO₂ are richer in the heavier isotope when precipitated from sea water than when precipitated from fresh water. Moreover, because of meteorological cycles of evaporation/condensation, there is a steady depletion of ¹⁸O in sea water towards the poles. See also isotope fractionation; oxygen-isotope analysis.