Kinetic fractionation
Encyclopedia
Kinetic fractionation is a process that separates stable isotope
s from each other by their mass during unidirectional processes.
One naturally occurring example of kinetic fractionation is the evaporation of seawater
to form cloud
s. 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 the course of 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.
A generalized treatment of kinetic isotopic effects is via the GEBIK and GEBIF equations describing transient kinetic isotope effects
.
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s from each other by their mass during unidirectional processes.
One naturally occurring example of kinetic fractionation is the evaporation of seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
to form cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
s. In this instance, isotopically lighter water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
molecules (i.e., those with 16O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
) will evaporate slightly more easily than will the isotopically heavier water molecules with 18O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
.
During the course of this process the oxygen isotopes are fractionated
Fractionation
See also: Fractionated spacecraftFractionation is a separation process in which a certain quantity of a mixture is divided up in a number of smaller quantities in which the composition changes according to a gradient. Fractions are collected based on differences in a specific property of the...
: 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.
A generalized treatment of kinetic isotopic effects is via the GEBIK and GEBIF equations describing transient kinetic isotope effects
Transient kinetic isotope fractionation
Transient kinetic isotope effects occur when the reaction leading to isotope fractionation does not follow pure first-order kinetics and therefore isotopic effects cannot be described with the classical equilibrium fractionation equations or with steady-state kinetic fractionation equations...
.
See also
- equilibrium fractionationEquilibrium fractionationEquilibrium isotope fractionation is the partial separation of isotopes between two or more substances in chemical equilibrium. Equilibrium fractionation is strongest at low temperatures, and forms the basis of the most widely used isotopic paleothermometers : D/H and 18O/16O records from ice...
- mass-independent fractionationMass-independent fractionationMass-independent fractionation refers to any chemical or physical process that acts to separate isotopes, where the amount of separation does not scale in proportion with the difference in the masses of the isotopes...
- Transient kinetic isotope effectTransient kinetic isotope fractionationTransient kinetic isotope effects occur when the reaction leading to isotope fractionation does not follow pure first-order kinetics and therefore isotopic effects cannot be described with the classical equilibrium fractionation equations or with steady-state kinetic fractionation equations...
- Isotopic ratioTransient kinetic isotope fractionationTransient kinetic isotope effects occur when the reaction leading to isotope fractionation does not follow pure first-order kinetics and therefore isotopic effects cannot be described with the classical equilibrium fractionation equations or with steady-state kinetic fractionation equations...
- Isotopic compositionTransient kinetic isotope fractionationTransient kinetic isotope effects occur when the reaction leading to isotope fractionation does not follow pure first-order kinetics and therefore isotopic effects cannot be described with the classical equilibrium fractionation equations or with steady-state kinetic fractionation equations...
- Fractionation factorTransient kinetic isotope fractionationTransient kinetic isotope effects occur when the reaction leading to isotope fractionation does not follow pure first-order kinetics and therefore isotopic effects cannot be described with the classical equilibrium fractionation equations or with steady-state kinetic fractionation equations...
- Isotopic enrichmentTransient kinetic isotope fractionationTransient kinetic isotope effects occur when the reaction leading to isotope fractionation does not follow pure first-order kinetics and therefore isotopic effects cannot be described with the classical equilibrium fractionation equations or with steady-state kinetic fractionation equations...