Isotopes of Carbon - Tracing Food Sources and Diets

Tracing Food Sources and Diets

The quantities of the different isotopes can be measured by mass spectrometry and compared to a standard; the result (e.g. the delta of the 13C = δ13C) is expressed as parts per thousand (‰).

Stable carbon isotopes in carbon dioxide are utilized differentially by plants during photosynthesis. Grasses in temperate environments (barley, rice, wheat, rye and oats, plus sunflower, potato, tomatoes, peanuts, cotton, sugar beet, and most trees and their nuts/fruits, roses and Kentucky bluegrass) follow a C3 photosynthetic pathway that will yield δ13C values averaging about −26.5‰. Grasses in hot arid environments (maize in particular, but also millet, sorghum, sugar cane and crabgrass) follow a C4 photosynthetic pathway that produces δ13C values averaging about −12.5‰.

It follows that eating these different plants will affect the δ13C values in the consumer’s body tissues. If an animal (or human) eats only C3 plants, their δ13C values will be −12.5‰ in their bone collagen and −14.5‰ in their apatite.

In contrast, C4 feeders will have bone collagen with a value of −7.5‰ and apatite value of −0.5‰.

In actual case studies, millet and maize eaters can easily be distinguished from rice and wheat eaters. Studying how these dietary preferences are distributed geographically through time can illuminate migration paths of people and dispersal paths of different agricultural crops. However, human groups have often mixed C3 and C4 plants (northern Chinese historically subsisted on wheat and millet), or mixed plant and animal groups together (for example, southeastern Chinese subsisting on rice and fish).