Carbon (<sub>6</sub>C) has 14 known isotopes, from to as well as , of which only and are stable. The longest-lived radioisotope is , with a half-life of 5700 years. This is also the only carbon radioisotope found in nature, as trace quantities are formed cosmogenically by the reaction + â + . The most stable artificial radioisotope is , which has a half-life of 20.34 minutes. All other radioisotopes have half-lives under 20 seconds, most less than 200 milliseconds. Lighter isotopes exhibit beta-plus decay into isotopes of boron and heavier ones beta-minus decay into isotopes of nitrogen, though at the limits particle emission occurs as well. The two lightest isotopes decay into helium via short-lived isotopes of lithium, beryllium and boron.
|-id=Carbon-8 | | style="text-align:right" | 6 | style="text-align:right" | 2 | | style="text-align:center" | 1974 | <br/>[] | 2p | | 0+ | |-id=Carbon-9 | rowspan=3| | rowspan=3 style="text-align:right" | 6 | rowspan=3 style="text-align:right" | 3 | rowspan=3| | rowspan=3 style="text-align:center" | 1964 | rowspan=3| | ò<sup>+</sup> () | | rowspan=3|3/2â | rowspan=3| |- | ò<sup>+</sup>ñ () | |- | ò<sup>+</sup>p () | |-id=Carbon-10 | | style="text-align:right" | 6 | style="text-align:right" | 4 | | style="text-align:center" | 1949 | | ò<sup>+</sup> | | 0+ | |- | rowspan=1| | rowspan=1 style="text-align:right" | 6 | rowspan=1 style="text-align:right" | 5 | rowspan=1 | | style="text-align:center" | 1934 | rowspan=1 | | ò<sup>+</sup> | | rowspan=1 |3/2â | rowspan=1 |
|-id=Carbon-12 | | style="text-align:right" | 6 | style="text-align:right" | 6 | 12 exactly | style="text-align:center" | 1919 | colspan=3 align=center|Stable | 0+ | [, ] |-id=Carbon-13 | | style="text-align:right" | 6 | style="text-align:right" | 7 | | style="text-align:center" | 1929 | colspan=3 align=center|Stable | 1/2â | [, ] |-id=Carbon-14 | | style="text-align:right" | 6 | style="text-align:right" | 8 | | style="text-align:center" | 1936 | | ò<sup>âÂÂ</sup> | | 0+ | Trace
|-id=Carbon-15 | | style="text-align:right" | 6 | style="text-align:right" | 9 | | style="text-align:center" | 1950 | | ò<sup>âÂÂ</sup> | | 1/2+ | |-id=Carbon-16 | rowspan=2| | rowspan=2 style="text-align:right" | 6 | rowspan=2 style="text-align:right" | 10 | rowspan=2| | rowspan=2 style="text-align:center" | 1961 | rowspan=2| | ò<sup>âÂÂ</sup>n () | | rowspan=2|0+ | rowspan=2| |- | ò<sup>âÂÂ</sup> () | |-id=Carbon-17 | rowspan=3| | rowspan=3 style="text-align:right" | 6 | rowspan=3 style="text-align:right" | 11 | rowspan=3| | rowspan=3 style="text-align:center" | 1968 | rowspan=3| | ò<sup>âÂÂ</sup> () | | rowspan=3|3/2+ | rowspan=3| |- | ò<sup>âÂÂ</sup>n () | |- | ò<sup>âÂÂ</sup>2n ? | ? |-id=Carbon-18 | rowspan=3| | rowspan=3 style="text-align:right" | 6 | rowspan=3 style="text-align:right" | 12 | rowspan=3| | rowspan=3 style="text-align:center" | 1969 | rowspan=3| | ò<sup>âÂÂ</sup> () | | rowspan=3|0+ | rowspan=3| |- | ò<sup>âÂÂ</sup>n () | |- | ò<sup>âÂÂ</sup>2n ? | ? |-id=Carbon-19 | rowspan=3| | rowspan=3 style="text-align:right" | 6 | rowspan=3 style="text-align:right" | 13 | rowspan=3| | rowspan=3 style="text-align:center" | 1974 | rowspan=3| | ò<sup>âÂÂ</sup>n () | | rowspan=3|1/2+ | rowspan=3| |- | ò<sup>âÂÂ</sup> () | |- | ò<sup>âÂÂ</sup>2n () | |-id=Carbon-20 | rowspan=3| | rowspan=3 style="text-align:right" | 6 | rowspan=3 style="text-align:right" | 14 | rowspan=3| | rowspan=3 style="text-align:center" | 1981 | rowspan=3| | ò<sup>âÂÂ</sup>n () | | rowspan=3|0+ | rowspan=3| |- | ò<sup>âÂÂ</sup>2n (< ) | |- | ò<sup>âÂÂ</sup> (> ) |
|-id=Carbon-22 | rowspan=3| | rowspan=3 style="text-align:right" | 6 | rowspan=3 style="text-align:right" | 16 | rowspan=3| | rowspan=3 style="text-align:center" | 1986 | rowspan=3| | ò<sup>âÂÂ</sup>n () | | rowspan=3|0+ | rowspan=3| |- | ò<sup>âÂÂ</sup>2n (< ) | |- | ò<sup>âÂÂ</sup> (> ) |
Carbon-11 or is a radioactive isotope of carbon that decays to boron-11 with a half-life to 20.34 minutes. This decay mainly occurs due to positron emission, with around 0.19âÂÂ0.23% of decays instead occurring by electron capture.
It is produced by hitting nitrogen with protons of around 16.5 MeV in a cyclotron. The causes the endothermic reaction
It can also be produced by fragmentation of by shooting high-energy at a target.
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 and []Cimbi-5.
Carbon-12 and carbon-13 account for approximately 98.9% and 1.1% (respectively) of the naturally occurring carbon on Earth. However, the ratio of stable <sup>13</sup>C and <sup>12</sup>C in a material can vary due to differences in precursor source and isotopic fractionation induced by a variety of biogeochemical processes. The quantities of the different isotopes are commonly measured via isotope ratio mass spectrometry and expressed as parts per thousand (â° or "per mille") divergence from the ratio of a standard:
Peedee Belemnite ("PDB"), a fossil belemnite, was the original reference standard used for standardizing isotope ratio values. Due to the depletion of the original PDB, artificial "Vienna PDB", or "VPDB", is generally used today.
and are measured as the isotope ratio ô<sup>13</sup>C in benthic foraminifera and used as a proxy for nutrient cycling and the temperature dependent airâÂÂsea exchange of CO<sub>2</sub> (ventilation). Plants find it easier to use the lighter isotope () when they convert sunlight and carbon dioxide into food. For example, large blooms of plankton (free-floating organisms) absorb large amounts of from the oceans. Originally, the was mostly incorporated into the seawater from the atmosphere. If the oceans that the plankton live in are stratified (meaning that there are layers of warm water near the top, and colder water deeper down), then the surface water does not mix very much with the deeper waters, so that when the plankton dies, it sinks and takes away from the surface, leaving the surface layers relatively rich in . Where cold waters well up from the depths (such as in the North Atlantic), the water carries back up with it; when the ocean was less stratified than today, there was much more in the skeletons of surface-dwelling species. Other indicators of past climate include the presence of tropical species and coral growth rings.
Different photosynthetic pathways preferentially select for the lighter , but their selectivity differs. Grasses in temperate climates (barley, rice, wheat, rye, and oats, plus sunflower, potato, tomatoes, peanuts, cotton, sugar beet, and most trees and their nuts or fruits, roses, and Kentucky bluegrass) follow a C<sub>3</sub> photosynthetic pathway that will yield ô<sup>13</sup>C values averaging about âÂÂ26.5â°. Grasses in hot arid climates (maize in particular, but also millet, sorghum, sugar cane, and crabgrass) follow a C<sub>4</sub> photosynthetic pathway that produces ô<sup>13</sup>C values averaging about âÂÂ12.5â°.
It follows that eating these different plants will affect the ô<sup>13</sup>C values in the consumer's body tissues. If an animal (or human) eats only C<sub>3</sub> plants, their ô<sup>13</sup>C values will be from âÂÂ18.5 to âÂÂ22.0â° in their bone collagen and âÂÂ14.5â° in the hydroxylapatite of their teeth and bones.
In contrast, C4 feeders will have bone collagen with a value of âÂÂ7.5â° and hydroxylapatite value of âÂÂ0.5â°.
In 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 C<sub>3</sub> and C<sub>4</sub> 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).
Carbon-14 (also called radiocarbon) occurs in trace amounts and has a half-life of 5700 years. The primary source of on Earth is the reaction of with thermal neutrons from cosmic radiation in the upper atmosphere; this mixes throughout the atmosphere, and biological processes such as photosynthesis incorporate the into living organisms. Since organisms stop absorbing upon dying, measurement of the amount of in a sample may be used to estimate its age. This technique is called radiocarbon dating and is one of the principal methods of radiometric dating in the field of archaeology.
Daughter products other than carbon