In geochemistry, paleoclimatology and paleoceanography δ18O or delta-O-18 is a measure of the ratio of stable isotopes 18O:16O (oxygen-18:oxygen-16). It is commonly used as a measure of the temperature of precipitation, as a measure of groundwater/mineral interactions, and as an indicator of processes that show isotopic fractionation, like methanogenesis. In paleosciences, 18O:16O data from corals, foraminifera and ice cores are used as a proxy for temperature. The definition is, in "per mil" (‰, parts per thousand):
Foraminifera shells are composed of calcium carbonate (CaCO3) and are found in many common geological environments. The ratio of 18O to 16O in the shell is used to indirectly determine the temperature of the surrounding water at the time the shell was formed. The ratio varies slightly depending on the temperature of the surrounding water, as well as other factors such as the water's salinity, and the volume of water locked up in ice sheets.
δ18O also reflects local evaporation and freshwater input, as rainwater is 16O enriched—a result of the preferential evaporation of the lighter 16O from seawater. Consequently, the surface ocean contains greater amounts of 18O around the subtropics and tropics where there is more evaporation, and lesser amounts of 18O in the mid-latitudes where it rains more.
Similarly, when water vapor condenses, heavier water molecules holding 18O atoms tend to condense and precipitate first. The water vapor gradient heading from the tropics to the poles gradually becomes more and more depleted of 18O. Snow falling in Canada has much less H218O than rain in Florida; similarly, snow falling in the center of ice sheets has a lighter δ18O signature than that at its margins, since heavier 18O precipitates first.
Changes in climate that alter global patterns of evaporation and precipitation therefore change the background δ18O ratio.
If the signal can be attributed to temperature change alone, with the effects of salinity and ice volume change ignored, a δ18O increase of 0.22‰ is equivalent to a 1 °C (1.8 °F) cooling. Temperature can also be calculated using the equation:
During the Pleistocene, a 0.11‰ δ18O signature correlates to 10 m of sea level change as a consequence of changing ice volume (higher δ18O correlates with a lower mean temperature and lower sea level).
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