This diagram illustrates the path of stable isotope ratios as they travel through the food chain. As you can see, the ratio of carbon isotopes is different for C4 plants than C3 plants.
Most plants fall into groups based on their chemical makeup, two of which are of interest here, called C4 and C3. C4 plants are those that are subject to long growing seasons with lots of access to sun. They are called C4 because during the photosynthesis process, the plants convert atmospheric carbon into a chemical compound with four carbon atoms; biologists call these various conversion processes "pathways". In general, plants which follow a C4 pathway originated in subtropical areas. C4 plants of special interest to archaeologists include maize, sorghum, sugarcane, millet, fonio, tef, papyrus. C4 plants absorb carbon 13 far faster than carbon 12, and so the total biomass of C4 plants ranges between -9 to -16%, compared to the agreed-upon standard VPDB measurement, with a mean of -12.5%.
C3 plants are those that convert carbon to a compound containing three carbon atoms. They are found in a far broader range of environments. C3 plants which are of particular interest to archaeologists include rice, wheat, rye, barley, cassava, potatoes, algae, spinach, and yams. C3 plants are slower to take in carbon 13, so their total biomass ranges from -22% to -35%, with a mean of -26.5%.
When animals eat the plants, this ratio is taken into their systems virtually unchanged and recognizable.
