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‰. This would be equivalent to a lowering of sea level of about 165 m compared to the present level.