Carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

(₆C) has 15 known

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...

s, 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 Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ...

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) , style="text-align:right" , 6 , style="text-align:right" , 8 , , , β⁻ , , 0+ , TracePrimarily cosmogenic, produced by

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...

s 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 Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

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 Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. ...

. It has a

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

Natural isotopes

There are three naturally occurring

s 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 foraminifera and used as a

proxy Proxy may refer to: * Proxy or agent (law), a substitute authorized to act for another entity or a document which authorizes the agent so to act * Proxy (climate), a measured variable used to infer the value of a variable of interest in climate re ...

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 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 Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

are utilized differentially by plants during photosynthesis. Grasses in temperate climates ( barley, rice, wheat, rye, and oats, plus sunflower,

potato The potato is a starchy food, a tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial in the nightshade family Solanaceae. Wild potato species can be found from the southern Uni ...

, tomatoes,

peanut The peanut (''Arachis hypogaea''), also known as the groundnut, goober (US), pindar (US) or monkey nut (UK), is a legume crop grown mainly for its edible seeds. It is widely grown in the tropics and subtropics, important to both small an ...

cotton Cotton is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus '' Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose, and can contain minor pe ...

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 ...

, 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 Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American English, North American and Australian English), is a cereal grain first domesticated by indigenous peoples of Mexico, indigenous ...

in particular, but also millet, sorghum, 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 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