Carbon has three naturally occurring
isotopes. About 99% of carbon on Earth is
carbon-12 (
12C), about 1% is carbon-13 (
13C), and a trace amount is carbon-14 (
14C). The 12C and 13C isotopes are stable, while 14C decays radioactively to
nitrogen-14 (
14N) with a
half-life of 5730 years. 14C on Earth is produced nearly exclusively by the interaction of cosmic radiation with the upper atmosphere. A 14C atom is created when a
thermal neutron displaces a
proton in 14N. Minuscule amounts of 14C are produced by other radioactive processes; a large amount was produced in the atmosphere during nuclear testing before the
Limited Test Ban Treaty. Natural 14C production and hence atmospheric concentration varies only slightly over time. Plants take up 14C by fixing atmospheric carbon through
photosynthesis. Animals then take 14C into their bodies when they consume plants (or consume other animals that consume plants). Thus, living plants and animals have nearly the same ratio of 14C to 12C as the atmospheric CO2. Once organisms die they stop exchanging carbon with the atmosphere and thus no longer take up new 14C. This effect is the basis of
radiocarbon dating, with the provison that mass-dependent fractionation and the decrease in 14C due to radioactive decay are accounted for.
Photosynthetically fixed carbon in terrestrial plants is depleted in 13C compared to atmospheric CO2. This
fractionation of carbon isotopes is caused by
kinetic isotope effects and mass dependence of CO2 diffusivity. The overall effect is slight in
C4 plants but much greater in
C3 plants which form the bulk of terrestrial biomass worldwide. Depletion in
CAM plants vary between the values observed for C3 and C4 plants. In addition, most
fossil fuels originate from C3 biological material produced tens to hundreds of millions of years ago. C4 plants did not become common until about 6 to 8 million years ago, and although CAM photosynthesis is present in
modern relatives of the
Lepidodendrales of the
Carboniferous lowland forests, even if these plants also had CAM photosynthesis they were not a major component of the total biomass. Fossil fuels such as
coal and
oil are made primarily of plant material that was deposited millions of years ago. This period of time equates to thousands of half-lives of 14C, so essentially all of the 14C in fossil fuels has decayed. Fossil fuels also are depleted in 13C relative to the atmosphere, because they were originally formed from living organisms. Therefore, the carbon from fossil fuels that is returned to the atmosphere through combustion is depleted in both 13C and 14C compared to atmospheric carbon dioxide. == See also ==