δ18O

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The correct title of this article is δ18O. It features superscript or subscript characters that are substituted or omitted because of technical limitations.
Foraminifera samples.
Foraminifera samples.
δ18O ppm in deep sea sediment foraminifera carbonates over time from 0-600,000 years. Averaged over a large number of cores in order to isolate a global signal.
δ18O ppm in deep sea sediment foraminifera carbonates over time from 0-600,000 years. Averaged over a large number of cores in order to isolate a global signal.

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[(Rsample/Rstandard)-1], where Rx = (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.

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[edit] 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.

δ18O 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 δ18O 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.

[edit] Calculations

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 cooling.[1] Temperature can also be calculated using the equation:

T (°C) = 16.9 - 4.0(δ18Ocalcite - δ18Oseawater)

During the Pleistocene, a 0.11‰ δ18O signature correlates to 10m of sea level change as a consequence of changing ice volume.

[edit] See also

[edit] References

  1. ^ Visser, Katherine (2003). "Magnitude and timing of temperature change in the Indo-Pacific warm pool during deglaciation". Nature 421: 152. doi:10.1038/nature01297.