Δ18O

In geochemistry, paleoclimatology and paleoceanography δ¹⁸O is a measure of the ratio of stable isotopes ¹⁸O:¹⁶O. The definition is δ¹⁸O (in per mil) = 10³ [(R_sample/R_standard)-1] , where R_x = (¹⁸O)/(¹⁶O) is the ratio of isotopic composition of a sample compared to that of an established standard, such as ocean water.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.

Mechanism

Foraminifera shells of calcium carbonate (CaCO₃), having oxygen in them, and being found in many common geological features, 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, as well as other factors such as the water's salinity, and the volume of water locked up in ice sheets.

d18O also reflects local evaporation and freshwater input, as rainwater is 16O enriched - a result of 16O's preferential evaporation 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 d18o signature than that at its margins, since heavier 18O precipitates first.

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

Calculations

If the signal can be attributed to temperature change alone, with the effects of salinity and ice volume change ignored, a d18O increase of 0.22‰ is equivalent to a 1oC cooling. [cite journal|doi=10.1038/nature01297|title=Magnitude and timing of temperature change in the Indo-Pacific warm pool during deglaciation|year=2003|author=Visser, Katherine|journal=Nature|volume=421|pages=152] Temperature can also be calculated using the equation:

T (°C) = 16.9 - 4.0(d18o_calcite - d18o_seawater)

During the Pleistocene, a 0.11‰ d18O signature correlates to 10m of sea level change as a consequence of changing ice volume.

ee also

*d13c|link

References

*Clark, Ian D. and Peter Fritz (1997) "Environmental Isotopes in Hydrogeology," CRC Press: 1997, ISBN 1566702496
*cite journal|author=Schmidt, G.A. |year=1999|url=http://www.agu.org/pubs/crossref/1999/1999PA900025.shtml |title=Forward Modeling of Carbonate Proxy Data from Planktonic Foraminifera Using Oxygen Isotope Tracers in a Global Ocean Model "Paleoceanography" 14, |pages=482-497