Kinetic fractionation

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 isotopes, because "energy costs" are lower, resulting in a significant fractionation between the substrate (heavier) and the biologically mediated product (lighter). For example, photosynthesis preferentially takes up the light isotope of carbon ¹²C during assimilation of atmospheric CO₂. This kinetic isotope fractionation explains why plant material (and thus fossil fuels, which are derived from plants) is typically depleted in ¹³C by 25 per mil (2.5%) relative to most inorganic carbon on Earth.

A naturally occurring example of non-biological kinetic fractionation occurs during the evaporation of seawater to form clouds under conditions in which some part of the transport is unidirectional, such as evaporation into very dry air. In this case, lighter water molecules (i.e., those with ¹⁶O) evaporate slightly more easily than heavier water molecules with ¹⁸O; this difference will be greater than it would be if the evaporation was taking place under equilibrium conditions (with bidirectional transport).

During this process the oxygen isotopes are fractionated: the clouds become enriched with ¹⁶O, and the seawater becomes enriched in ¹⁸O. Whereas equilibrium fractionation makes the vapor about 10 per mil (1%) depleted in ¹⁸O relative to the liquid water, kinetic fractionation enhances this fractionation and often makes vapor that is about 15 per mil (1.5%) depleted. Condensation occurs almost exclusively by equilibrium processes, and so it enriches cloud droplets somewhat less than evaporation depletes the vapor. This explains part of the reason why rainwater is observed to be isotopically lighter than seawater.

The heavy isotope of hydrogen in water, deuterium (²H), is much less sensitive to kinetic fractionation than oxygen isotopes, relative to the very large equilibrium fractionation of deuterium. Therefore kinetic fractionation does not deplete ²H nearly as much, in a relative sense, as ¹⁸O. This gives rise to an excess of deuterium in vapor and rainfall, relative to seawater. The value of this "deuterium excess", as it is called, is about +10 per mil (1%) in most meteoric waters and its non-zero value is a direct manifestation of kinetic isotope fractionation.

A generalized treatment of kinetic isotopic effects is via the GEBIK and GEBIF equations describing transient kinetic isotope effects.

Other types of fractionation

• Equilibrium fractionation
• Mass-independent fractionation
• Transient kinetic isotope fractionation

See also

• Isotopic enrichment
• Isotopic ratio
• Kinetic isotope effect
• Hydrogen isotope biogeochemistry