text: For example, different sources and sinks of methane have different affinity for the C and C isotopes, which allows distinguishing between different sources by the C/C ratio in methane in the air. In geochemistry, paleoclimatology and paleoceanography this ratio is called δC. The ratio is calculated with respect to Pee Dee Belemnite (PDB) standard:
  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 C is less depleted, and Crassulacean Acid Metabolism (CAM) plants, where the effect is similar but less pronounced than with C plants. Isotopic fractionation in plants is caused by physical (slower diffusion of C in plant tissues due to increased atomic weight) and biochemical (preference of C by two enzymes: RuBisCO and phosphoenolpyruvate carboxylase) factors. 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 C plants (rice, wheat, soybeans, potatoes) or C plants (corn, or corn-fed beef) by isotope analysis of their flesh and bone collagen (however, to obtain more accurate determinations, carbon isotopic fractionation must be also taken into account, since several studies have reported significant C discrimination during biodegradation of simple and complex substrates).
  Within C3 plants processes regulating changes in δC are well understood, particularly at the leaf level, but also during wood formation. Many recent studies combine leaf level isotopic fractionation with annual patterns of wood formation (i.e. tree ring δC) to quantify the impacts of climatic variations and atmospheric composition on physiological processes of individual trees and forest stands. The next phase of understanding, in terrestrial ecosystems at least, seems to be the combination of multiple isotopic proxies to decipher interactions between plants, soils and the atmosphere, and predict how changes in land use will affect climate change.
  Similarly, marine fish contain more C than freshwater fish, with values approximating the C and C plants respectively.
  The ratio of carbon-13 and carbon-12 isotopes in these types of plants is as follows:
  * C plants: -16 to -10 ‰
  * CAM plants: -20 to -10 ‰
  * C plants: -33 to -24 ‰
  Limestones formed by precipitation in seas from the atmospheric carbon dioxide contain normal proportion of C. Conversely, calcite found in salt domes originates from carbon dioxide formed by oxidation of petroleum, which due to its plant origin is C-depleted. The layer of limestone deposited at the Permian extinction 252 Mya can be identified by the 1% drop in C/C.
  The C isotope is important in distinguishing biosynthetized materials from man-made ones. Biogenic chemicals are derived from biospheric carbon, which contains C. Carbon in artificially made chemicals is usually derived from fossil fuels like coal or petroleum, where the C originally present has decayed below detectable limits. The amount of C currently present in a sample therefore indicates the proportion of carbon of biogenic origin.