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oxygen-isotope curve A graphical plot of values for the relative proportions of two isotopes of oxygen. Oxygen can exist in several isotope forms, but only 16O and 18O are important in the analysis of oxygen isotopes (see OXYGEN-ISOTOPE RATIO). In nature, the present-day average ratio of 18O to 16O is about 1: 500, and measurements are made against this value as a standard. There is now evidence that this ratio changed in ocean waters in a cyclical fashion in succeeding glacial and interglacial periods (see OXYGEN-ISOTOPE ANALYSIS). At the height of the last (Devensian) glaciation it appears that deep-ocean waters may have been enriched in 18O by about 1.6 parts per thousand. This would be equivalent to a lowering of sea level of about 165 m compared to the present level.