Hydrogen and oxygen fractionation in the water cycle
Hydrogen atoms occur in two stable isotope forms, a common light form (1H) and a rare heavy form (2H). The element oxygen (O) also occurs naturally in light and heavy isotope forms: 16O and 18O.
Hydrogen and oxygen can combine to form water; a body of
water contains a mixture of light and heavy H and O stable isotopes.
|
|
Ocean water is the largest body of water on the planet and has a defined
stable isotope ratio of 0‰ for both H and O. As water vapor forms over
the oceans, the mass differences between water molecules formed with
various isotopes of H and O cause lighter water molecules to evaporate
first. Thus, the water within a cloud is isotopically lighter than ocean
water itself. As the cloud moves inland from the
ocean, heavier water molecules condense first and precipitation along
the coast is isotopically heavier than the water vapor remaining in the
cloud. However, the precipitation is isotopically lighter than ocean
water.
|
|
The continual evaporation and condensation fractionation processes
within a moving cloud create a geographic pattern of isotope ratios
within water across landscapes. Plants and animals
incorporate H and O isotopes directly from their local water sources.
Thus, the stable isotope analysis of collected organic materials may
reflect geographic isotope differences and be useful in determining a
sample’s region-of-origin.
|
|
