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Kinetic fractionation is an isotopic fractionation process that separates stable isotopes from each other by their mass during unidirectional processes. Biological processes are generally unidirectional and are very good examples of "kinetic" isotope reactions. All organisms preferentially use lighter isotopic species, because "energy costs" are lower, resulting in a significant fractionation between the substrate (heavier) and the biologically mediated product (lighter). As an example, evaporation can take place under equilibrium conditions (Equilibrium fractionation)(i.e., 100% humidity, the air is still), but more typically the products become partially isolated from the reactants (e.g., the resultant vapor is blown downwind). Under these conditions, the isotopic compositions of the water and vapor are affected by an additional kinetic isotope fractionation of variable magnitude.
A naturally occurring example of kinetic fractionation is the evaporation of seawater to form clouds. In this instance, isotopically lighter water molecules (i.e., those with 16O) will evaporate slightly more easily than will the isotopically heavier water molecules with 18O.
During this process the oxygen isotopes are fractionated: the clouds become enriched with 16O, the seawater becomes enriched in 18O. Thus, rainwater is observed to be isotopically lighter than seawater.
Heavier isotopes favor the less energetic liquid phase of water during evaporation and condensation. Water vapor is enriched with light isotopes relative to sea water. Clouds are depleted of light isotopes relative to water vapor. This results in higher latitude waters being isotopically "light". As water vapor is driven poleward by Earth's energy budget and rotating Hadley cells, the heavy isotopes are left behind as clouds and rain.
Other types of fractionation
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