How does natural variation in stable isotopes come about?

Variations in the abundances of stable isotopes among different compounds arise because the chemical bonding is stronger in molecules containing heavier isotopic forms, making it more difficult to break up the molecule in a chemical reaction (often termed kinetic fractionation), or because of differences in the physical properties of molecules containing heavier isotopic forms (often termed diffusive and equilibrium fractionation).

With kinetic fractionation, the rate of an enzymatic reaction is faster with substrates that contain the lighter isotopic form than in reactions involving the heavier isotopic form. As a consequence, there will be differences in the abundances of the stable isotopes between substrate and product. Such differences will occur unless, of course, all of the substrate were consumed, in which case there would be no difference in the isotopic composition of substrate and product. Expression of a significant kinetic fractionation in most biological reactions involves substrates at branch points in metabolism, such as the initial fixation of CO2 in photosynthesis.

Equilibrium fractionation events reflect the observation that during equilibrium reactions, such as the equilibration of liquid and gaseous water, molecules with the heavier isotopic species are typically more abundant in the lower energy state phase.

Diffusive fractionation events reflect the observation that heavier isotopic forms diffusive more slowly than lighter isotopic forms.

What is the natural range of isotopic variation in nature?

The natural variations in isotopic abundance can be large, including the that found for materials frequently of interest in global changes studies: waters, greenhouse gases, and biological materials. As a starting point, note that some atmospheric gases, such as CO2, N2, and O2, exhibit limited variation, while N2O and CH4 exhibit wide isotopic variation. The larger isotopic ranges in the latter two gases reflects both significant isotopic fractionation by microbes as well as different biological substrates which are used to produce these gases.


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