Normalization is a correction for isotopic fractionation.
Belemnite is a calcareous Cretaceous fossil found in Pee Dee, South Carolina. 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.