Normalization is a correction for isotopic fractionation. It is based on the ratio between C-12 and C-13, called del C-13, which is expressed in parts per mil (parts per thousand) with respect to a standard known as Pee Dee Belemnite (PDB). Belemnite is a calcareous Cretaceous fossil found in Pee Dee, South Carolina. Most organic materials contain less C-13 than PDB, yielding negative values for del C-13. For example, most C3 plants have C-13 ratios near -25 parts per mil, whereas C-13 ratios in C4 plants are in the range of -10 to -12.5 parts per mil. Herbivores are less selective against the heavier isotopes, and their bone collagen is enriched by 5 parts per mil in relation to their diet. Yet another change occurs in carnivores whose bone collagen is enriched by an additional 1 part per mil. Marine plants are similar to C3 plants, but they obtain their carbon from dissolved oceanic bicarbonates that differ from the atmosphere in their isotope ratios, and this difference is passed up the marine food chain.

Radiocarbon dates can be normalized to any chosen value, and the value chosen by international convention is -25 parts per mil based on an internationally accepted oak standard. Every part per mil difference from -25 is equivalent to 16 years. For example, bone collagen from marine mammals commonly has a C-13 ratio of -15 parts per mil. That difference of 10 parts per mil from the oak standard means that the age of the marine mammal bone can be normalized by adding 160 years to its measured age.

Further reading:

Stuiver, M. and Polach, H.A.
1977 Reporting of 14C data. Radiocarbon 19(3): 355-363.