Δ18O

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In geochemistry, paleoclimatology and paleoceanography δ¹⁸O is the ratio of stable isotopes ¹⁸O:¹⁶O. It is commonly used as a measure of the temperature of precipitation, as a measure of groundwater/mineral interactions, as an indicator of processes that show isotopic fractionation, like methanogenesis. In paleosciences, foraminifera and ice core data ¹⁸O:¹⁶O is used as a proxy for temperature.

Detail

Foraminifera shells of calcium carbonate (CaCO₃), having oxygen in them, and being found in many common geological features, such as the White Cliffs of Dover that were once at the bottom of the ocean, are most commonly used to do tests on. The ratio of ¹⁸O to ¹⁶O is used to tell the temperature of the surrounding water of the time solidified, indirectly. The ratio varies slightly depending on the temperature of the surrounding water. Although the change is only about 0.2 parts per thousand decrease for each degree Celsius of temperature increase, this is sufficient to estimate the temperature of the water in which the foraminifera lived in millions of years ago.

δ¹⁸O value of the shells depends on the δ¹⁸O in the surrounding water, which can be as variable as its temperature. This variability arises because of when water evaporates the lighter molecules of water—those with ¹⁶O atoms instead of ¹⁸O—tend to evaporate first, because of their lower mass. This makes the water vapor more depleted of ¹⁸O than the ocean from which it evaporates. The 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.

Changes in climate alter global patterns of evaporation and precipitation therefore change the background δ¹⁸O ratio.

References

• Clark, Ian D. and Peter Fritz (1997) Environmental Isotopes in Hydrogeology, CRC Press: 1997, ISBN 1566702496
• Schmidt, G.A. (1999). Forward modeling and interpretation of carbonate proxy data using oxygen isotope tracers in a global ocean model. Paleoceanography 14, 482-497.
• Schmidt, G.A. (1999). Cold climates, warm climates: How can we tell past temperatures? NASA. Retrieved May 26, 2006, from http://www.giss.nasa.gov/research/briefs/schmidt_01