A method of determining the temperatures during a past geological epoch. The ratio between the oxygen isotopes 16O and 18O that is found in marine carbonate shells, for example, is dependent upon prevailing temperatures. During an ice age, the lighter 16O is preferentially evaporated (and deposited as glacier ice), so oceanic water becomes enriched in the heavier 18O, this enrichment being reflected in the shells of marine organisms. Determination of oxygen-isotope ratios thus allows temperature variations during the various glacial and interglacial stages to be reconstructed.
Subjects: Meteorology and Climatology.