Isotopic Signature - Carbon Isotopes

Carbon Isotopes

For example, different sources and sinks of methane have different affinity for the 12C and 13C isotopes, which allows distinguishing between different sources by the 13C/12C ratio in methane in the air. In geochemistry, paleoclimatology and paleoceanography this ratio is called δ13C.

Similarly, carbon in inorganic carbonates shows little isotopic fractionation, while carbon in materials originated by photosynthesis is depleted of the heavier isotopes. In addition, there are two types of plants with different biochemical pathways; the C3 carbon fixation, where the isotope separation effect is more pronounced, C4 carbon fixation, where the heavier 13C is less depleted, and Crassulacean Acid Metabolism (CAM) plants, where the effect is similar but less pronounced than with C₄ plants. The different isotope ratios for the two kinds of plants propagate through the food chain, thus it is possible to determine if the principal diet of a human or an animal consists primarily of C₃ plants (rice, wheat, soybeans, potatoes) or C₄ plants (corn, or corn-fed beef) by isotope analysis of their flesh and bone collagen. Similarly, marine fish contain more 13C than freshwater fish, with values approximating the C₄ and C₃ plants respectively.

The ratio of carbon-13 and carbon-12 isotopes in these types of plants is as follows:

• C₄ plants: -16 to -10 ‰
• CAM plants: -20 to -10 ‰
• C₃ plants: -33 to -24 ‰

Limestones formed by precipitation in seas from the atmospheric carbon dioxide contain normal proportion of 13C. Conversely, calcite found in salt domes originates from carbon dioxide formed by oxidation of petroleum, which due to its plant origin is 13C-depleted.

The 14C isotope is important in distinguishing biosynthetized materials from man-made ones. Biogenic chemicals are derived from biospheric carbon, which contains 14C. Carbon in artificially made chemicals is usually derived from fossil fuels like coal or petroleum, where the 14C originally present has decayed below detectable limits. The amount of 14C currently present in a sample therefore indicates the proportion of carbon of biogenic origin.