Stable Isotopes in Archaeology: C3, C4, and CAM Plants

Most plants fall into three groups, based on their chemical makeup: C₄, C₃, and CAM. 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%.

The third group of plants is called CAM for Crassulacean acid metabolism, which can use either pathway for photosynthesis, and so have biomass values that can look like C₃ or C₄ plants. To date, archaeologists have not used CAM plants for identification purposes.

There are other plant types, by the way; but they are not and never have been significant food resources for man or animal.

Sources

Michael DeNiro. 1987. Stable Isotopy and Archaeology. American Scientist 75: 182-191.

Nikolaas Van der Merwe. 1982. Carbon Isotopes, Photosynthesis and Archaeology. American Scientist 70:596-606.