[A Conventional Radiocarbon Age or CRA, does not take into account specific differences between the activity of different carbon reservoirs.A CRA is derived using an age calculation based upon the decay corrected activity of the absolute radiocarbon standard (1890 AD wood) which is in equilibrium with atmospheric radiocarbon levels (as mentioned previously, 1890 wood is no longer used as the primary radiocarbon standard, instead Oxalic Acid standards I and II were correlated with the activity of the original standard).A d13C value, then, represents the per mille (part per thousand) deviation from the PDB standard.
The isotopic composition of the sample being measured is expressed as delta13C which represents the parts per thousand difference (per mille) between the sample carbon 13 content and the content of the international PDB standard carbonate (Keith et al., 1964; Aitken, 1990).
The extent of isotopic fractionation on the 14C/12C ratio which radiocarbon daters are seeking to measure 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.
One of the most commonly referenced reservoir effects concerns the ocean.
The average difference between a radiocarbon date of a terrestrial sample such as a tree, and a shell from the marine environment is about 400 radiocarbon years (see Stuiver and Braziunas, 1993).
Correction factors are available in most C14 texts.