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oxygen isotopes There are three isotopes of oxygen, 16O, 17O, and 18O. The most important in geology are 16O and 18O, as both these isotopes are found together in carbonate rocks and minerals. The 18O: 16O ratio (see OXYGEN-ISOTOPE RATIO) varies with the temperature and chemical composition of the water in which shelly organisms grew, or of the subsurface waters from which diagenetic carbonate cements crystallized. The 18O: 16O ratio is therefore a valuable tool for palaeothermometry (see OXYGEN-ISOTOPE ANALYSIS) and for diagenetic studies (particularly when used together with a study of 13C: 12C isotope ratios), provided that there have not been subsequent changes to the isotopic composition. See DIAGENESIS.