Oxygen has three stable isotopes, O, O, and O.  Oxygen ratios are measured relative to Vienna Standard Mean Ocean Water (VSMOW) or Vienna Pee Dee Belemnite (VPDB).  Variations in oxygen isotope ratios are used to track both water movement, paleoclimate, and atmospheric gases such as ozone and carbon dioxide.  Typically, the VPDB oxygen reference is used for paleoclimate, while VSMOW is used for most other applications.  Oxygen isotopes appear in anomalous ratios in atmospheric ozone, resulting from mass-independent fractionation.  Isotope ratios in fossilized foraminifera have been used to deduce the temperature of ancient seas.