Carbon (₆C) has 15 known isotopes, from to , of which and are

stable A stable is a building in which livestock, especially horses, are kept. It most commonly means a building that is divided into separate stalls for individual animals and livestock. There are many different types of stables in use today; the ...

. The longest-lived radioisotope is , with a half-life of years. This is also the only carbon radioisotope found in nature—trace quantities are formed cosmogenically by the reaction + → + . The most stable artificial radioisotope is , which has a half-life of . All other radioisotopes have half-lives under 20 seconds, most less than 200 milliseconds. The least stable isotope is , with a half-life of .

List of isotopes

, - , , style="text-align:right" , 6 , style="text-align:right" , 2 , ,
[] , proton emission, 2p , Subsequently decays by double proton emission to for a net reaction of → + 4 , 0+ , , , - , rowspan=3, , rowspan=3 style="text-align:right" , 6 , rowspan=3 style="text-align:right" , 3 , rowspan=3, , rowspan=3, , β⁺ () , , rowspan=3, 3/2− , rowspan=3, , rowspan=3, , - , β⁺α () , Immediately decays by proton emission to for a net reaction of → 2 + + , - , β⁺p () , Immediately decays into two atoms for a net reaction of → 2 + + , - , , style="text-align:right" , 6 , style="text-align:right" , 4 , , , β⁺ , , 0+ , , , - , rowspan=1, Used for labeling molecules in PET scans , rowspan=1 style="text-align:right" , 6 , rowspan=1 style="text-align:right" , 5 , rowspan=1 , , rowspan=1 , , β⁺ , , rowspan=1 , 3/2− , rowspan=1 , , rowspan=1 , , - , style="text-indent:1em" , , colspan=3 style="text-indent:2em" , , , p ?Decay mode shown is energetically allowed, but has not been experimentally observed to occur in this nuclide. , ? , 1/2+ , , , - , , style="text-align:right" , 6 , style="text-align:right" , 6 , 12 exactlyThe unified atomic mass unit is defined as 1/12 of the mass of an unbound atom of carbon-12 in its ground state. , colspan=3 align=center, Stable , 0+ , ref name="Atomic Weight of Carbon"> , - , Ratio of ¹²C to ¹³C used to measure biological productivity in ancient times and differing types of photosynthesis , style="text-align:right" , 6 , style="text-align:right" , 7 , , colspan=3 align=center, Stable , 1/2− , ref name="Atomic Weight of Carbon"/> , - , Has an important use in radiodating (see

carbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was dev ...

) , style="text-align:right" , 6 , style="text-align:right" , 8 , , , β⁻ , , 0+ , TracePrimarily cosmogenic, produced by neutrons striking atoms of ( + → + ) , < 10⁻¹² , - , style="text-indent:1em" , , colspan="3" style="text-indent:2em" , , , IT , , (2−) , , , - , , style="text-align:right" , 6 , style="text-align:right" , 9 , , , β⁻ , , 1/2+ , , , - , rowspan=2, , rowspan=2 style="text-align:right" , 6 , rowspan=2 style="text-align:right" , 10 , rowspan=2, , rowspan=2, , β⁻ n () , , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , β⁻ () , , - , rowspan=3, , rowspan=3 style="text-align:right" , 6 , rowspan=3 style="text-align:right" , 11 , rowspan=3, , rowspan=3, , β⁻ () , , rowspan=3, 3/2+ , rowspan=3, , rowspan=3, , - , β⁻n () , , - , β⁻2n ? , ? , - , rowspan=3, , rowspan=3 style="text-align:right" , 6 , rowspan=3 style="text-align:right" , 12 , rowspan=3, , rowspan=3, , β⁻ () , , rowspan=3, 0+ , rowspan=3, , rowspan=3, , - , β⁻n () , , - , β⁻2n ? , ? , - , rowspan=3, Has 1 halo neutron , rowspan=3 style="text-align:right" , 6 , rowspan=3 style="text-align:right" , 13 , rowspan=3, , rowspan=3, , β⁻n () , , rowspan=3, 1/2+ , rowspan=3, , rowspan=3, , - , β⁻ () , , - , β⁻2n () , , - , rowspan=3, , rowspan=3 style="text-align:right" , 6 , rowspan=3 style="text-align:right" , 14 , rowspan=3, , rowspan=3, , β⁻n () , , rowspan=3, 0+ , rowspan=3, , rowspan=3, , - , β⁻2n (< ) , , - , β⁻ (> ) , , - , ?This isotope has not yet been observed; given data is inferred or estimated from periodic trends. , style="text-align:right" , 6 , style="text-align:right" , 15 , # , < , n ? , ? , 1/2+# , , , - , rowspan=3, Has 2 halo neutrons , rowspan=3 style="text-align:right" , 6 , rowspan=3 style="text-align:right" , 16 , rowspan=3, , rowspan=3, , β⁻n () , , rowspan=3, 0+ , rowspan=3, , rowspan=3, , - , β⁻2n (< ) , , - , β⁻ (> ) , , - , ? , style="text-align:right" , 6 , style="text-align:right" , 17 , # , , n ? , ? , 3/2+# , ,

Carbon-11

Carbon-11 or is a radioactive isotope of carbon that decays to

boron-11 Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has thr ...

. 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 . : → + + + : + → + + It is produced from nitrogen in a cyclotron by the reaction : + → (122 seconds half-life); → + Carbon-11 is commonly used as a radioisotope for the radioactive labeling of molecules in

positron emission tomography Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in Metabolism, metabolic processes, and in other physiological activities including bl ...

. Among the many molecules used in this context are the radioligands /a>ASB and /a>imbi-5.

Natural isotopes

There are three naturally occurring isotopes of carbon: 12, 13, and 14. and are stable, occurring in a natural proportion of approximately 93:1. is produced by thermal neutrons from cosmic radiation in the upper atmosphere, and is transported down to earth to be absorbed by living biological material. Isotopically, constitutes a negligible part; but, since it is radioactive with a half-life of years, it is radiometrically detectable. Since dead tissue does not absorb , the amount of is one of the methods used within the field of archeology for radiometric dating of biological material.

Paleoclimate

and are measured as the

isotope ratio The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

δ¹³C in

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

foraminifera and used as a proxy for

nutrient cycling A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cycli ...

and the temperature dependent air–sea exchange of CO₂ (ventilation). 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 (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. So, 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, coral growths rings, etc. Tim Flannery ''The weather makers: the history & future of climate change'', The Text Publishing Company, Melbourne, Australia.

Tracing food sources and diets

The quantities of the different isotopes can be measured by

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

and compared to a standard; the result (e.g. the delta of the = δ) is expressed as parts per thousand (‰): :

\delta \ce = \left( \frac - 1 \right) \times 1000

‰ Stable carbon isotopes in carbon dioxide are utilized differentially by plants during photosynthesis. Grasses in temperate climates ( barley, rice, wheat,

rye Rye (''Secale cereale'') is a grass grown extensively as a grain, a cover crop and a forage crop. It is a member of the wheat tribe (Triticeae) and is closely related to both wheat (''Triticum'') and barley (genus ''Hordeum''). Rye grain is u ...

, and oats, plus

sunflower The common sunflower (''Helianthus annuus'') is a large annual forb of the genus ''Helianthus'' grown as a crop for its edible oily seeds. Apart from cooking oil production, it is also used as livestock forage (as a meal or a silage plant), as ...

, potato, tomatoes, peanuts, cotton,

sugar beet A sugar beet is a plant whose root contains a high concentration of sucrose and which is grown commercially for sugar production. In plant breeding, it is known as the Altissima cultivar group of the common beet (''Beta vulgaris''). Together wi ...

, and most trees and their nuts or fruits, roses, and Kentucky bluegrass) follow a C3 photosynthetic pathway that will yield δ¹³C values averaging about −26.5‰. Grasses in hot arid climates ( maize in particular, but also

millet Millets () are a highly varied group of small-seeded grasses, widely grown around the world as cereal crops or grains for fodder and human food. Most species generally referred to as millets belong to the tribe Paniceae, but some millets al ...

sorghum ''Sorghum'' () is a genus of about 25 species of flowering plants in the grass family (Poaceae). Some of these species are grown as cereals for human consumption and some in pastures for animals. One species is grown for grain, while many othe ...

, sugar cane, and crabgrass) 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 (or human) eats only C3 plants, their δ¹³C values will be from −18.5 to −22.0‰ in their bone

collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...

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 (northern Chinese historically subsisted on wheat and millet), or mixed plant and animal groups together (for example, southeastern Chinese subsisting on rice and fish).

References