has 15 known isotopes, from 8C to 22C, of which 12C and 13C are stable. The longest-lived radioisotope is 14C, with a half-life of 5,730 years. This is also the only carbon radioisotope found in nature—trace quantities are formed cosmogenically by the reaction 14N + 1n → 14C + 1H. The most stable artificial radioisotope is 11C, which has a half-life of 20.364 minutes. All other radioisotopes have half-lives under 20 seconds, most less than 200 milliseconds. The least stable isotope is 8C, with a half-life of 2.0 x 10−21 s.
Carbon-11 or 11C is a radioactive isotope of carbon that decays to boron-11. This decay mainly occurs due to positron emission, with around 0.19–0.23% of decays instead occurring by electron capture. It has a half-life of 20.364 minutes. It is produced from nitrogen in a cyclotron by the reaction Carbon-11 is commonly used as a radioisotope for the radioactive labeling of molecules in positron emission tomography. Among the many molecules used in this context are the radioligands DASB|DASB and 25I-NBOMe|Cimbi-5.
12C and 13C are measured as the isotope ratio δ13C in benthicforaminifera and used as a proxy for nutrient cycling and the temperature dependent air-sea exchange of CO2 . Plants find it easier to use the lighter isotopes when they convert sunlight and carbon dioxide into food. So, for example, large blooms of plankton absorb large amounts of 12C from the oceans. Originally, the 12C was mostly incorporated into the seawater from the atmosphere. If the oceans that the plankton live in are stratified, then the surface water does not mix very much with the deeper waters, so that when the plankton dies, it sinks and takes away 12C from the surface, leaving the surface layers relatively rich in 13C. Where cold waters well up from the depths, the water carries 12C back up with it. So, when the ocean was less stratified than today, there was much more 12C in the skeletons of surface-dwelling species. Other indicators of past climate include the presence of tropical species, coral growths rings, etc.
The quantities of the different isotopes can be measured by mass spectrometry and compared to a standard; the result is expressed as parts per thousand : Stable carbon isotopes in carbon dioxide are utilized differentially by plants during photosynthesis. Grasses in temperate climates follow a C3 photosynthetic pathway that will yield δ13C values averaging about −26.5‰. Grasses in hot arid climates 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 eats only C3 plants, their δ13C values will be from −18.5 to −22.0‰ in their bone collagen and −14.5‰ in the hydroxylapatite of their teeth and bones. In contrast, C4 feeders will have bone collagen with a value of −7.5‰ and hydroxylapatite 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, or mixed plant and animal groups together.
