In the case of shellfish, for example, marine shells typically possess a δ13C value between -1 and 4 o/oo (per mille), whereas river shells possess a value of between -8 and -12 o/oo (per mille).Thus, in a case where the precise environment of the shell is not known, it is possible to determine the most likely environment by analysis of the δ13C result.This value is not reported but it is used to produce the correct “Conventional Radiocarbon Age”.Important: Reporting conventions using “Conventional Radiocarbon Age” terminology indicate the result has been corrected for isotopic fractionation.– Beta also measures a second δ13C value in an isotope ratio mass spectrometer (IRMS δ13C).Carbon-14 is continually formed in nature by the interaction of neutrons with nitrogen-14 in the Earth’s atmosphere; the neutrons required for this reaction are produced by cosmic rays interacting with the atmosphere.
Craig (1953) first identified that certain biochemical processes alter the equilibrium between the carbon isotopes.
The extent of isotopic fractionation on the 14C/12C ratio (which must be measured accurately) is approximately double that for the measured 13C/12C ratio.
If isotopic fractionation occurs in natural processes, a correction can be made by measuring the ratio of the isotope 13C to the isotope 12C in the sample being dated.
It is common practice in Carbon-14 laboratories to correct radiocarbon activities for sample fractionation.
The resultant ages are termed “normalized”, meaning the measured activity is modified with respect to -25 o/oo (per mille) with respect to VPBD.