All the elements in the periodic table have stable isotopes that have slightly different atomic masses but react chemically in the same way. Carbon, for example, consists of about 99 percent 12C and 1 percent 13C; the latter has an extra neutron in the nucleus. (The radioactive isotope 14C, which provides the basis for radiocarbon dating, is present in such small amounts that it cannot be detected in the mass spectrometers that are normally used to measure 13C/12C ratios.)
The two stable isotopes react chemically in the same way (e.g. C + O2 = CO2, regardless of the isotopes involved), but do so at different RATES, because the atoms are of different size. When carbon is burned in a stream of oxygen, the carbon dioxide that is produced is initially all 12CO2; as the reaction is driven to completion, 13CO2 is produced.
This fractionation of the isotopes of carbon is particularly noticeable when atmospheric carbon dioxide is converted to plant matter during photosynthesis. The C3 photosynthetic pathway (used by all trees and bushes, and by grasses from temperate and shaded forest environments) discriminates strongly against 13C. The C4 photosynthetic pathway (used by grasses of subtropical environments) does not discriminate as strongly. In the North American Woodlands, for example, all the plants use the C3 pathway. Maize, a subtropical grass domesticated in Mexico, is a C4 plant. The result is that people from the Woodlands who ate maize had higher 13C contents than people who did not. The amount of maize in their diets can be measured by analysing their bones. Likewise, the bush/grass ratio in the diets of savanna herbivores can be measured; this ratio is carried along the food chain to carnivores.
