1990, without a decay correction Fractionation: The change in isotopic ratio that can occur when a material undergoes a chemical reaction or certain types of physical processes.
Although the different isotopes of an element are said to have identical chemical properties, the rate at which they take part in a chemical reaction depends on their mass.
Chemical reactions are not the only causes of fractionation.
Evaporation, condensation, diffusion or the passage of vapour through a small aperture can also give rise to isotopic fractionation.
Conventional Radiocarbon Age (CRA): The age obtained from a radiocarbon measurement using conventions set out in a paper by Stuiver & Polach (1977).
The processes that alter the stable isotope ratio will also affect the 14C, and it is necessary to distinguish between the change in 14C due to radioactive decay (which is the basis of radiocarbon dating) and that due to chemical fractionation effects.
It has since decreased to about 10% above "pre-bomb" levels, due mainly to the exchange of CO2 between the atmosphere and the oceans. Radiocarbon ages are conventionally specified relative to the year 1950, defined as "present". This is accomplished by comparing the measured CRA with a table of radiocarbon ages measured on samples of wood whose chronological ages are known exactly by tree-ring counting.