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 A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...

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 The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at ...

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 Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

, 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 de ...

) , 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 Halo, halos or haloes usually refer to: * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head HALO, halo, halos or haloes may also refer to: Arts and entertainment Video games * ''Halo'' (franch ...

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 Positron emission, beta plus decay, or β+ decay is a subtype of radioactive decay called beta decay, in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron and an electron neutrino (). Positron ...

, 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 A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. Lawrence, Ernest O. ''Method and apparatus for the acceleration of ions'', filed: Jan ...

by the reaction : + → (122 seconds half-life); → + Carbon-11 is commonly used as a

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 metabolic processes, and in other physiological activities including blood flow, ...

. Among the many molecules used in this context are the

radioligand A radioligand is a radioactive biochemical substance (in particular, a ligand (biochemistry), ligand that is radioactive tracer, radiolabeled) that is used for diagnosis or for research-oriented study of the receptor (biochemistry), receptor system ...

s /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 Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...

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 " ...

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

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 Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

(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 Timothy Fridtjof Flannery (born 28 January 1956) is an Australian mammalogist, palaeontologist, environmentalist, conservationist, explorer, author, science communicator, activist and public scientist. He was awarded Australian of the Yea ...

''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 u ...

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

. Grasses in

temperate climate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout ...

s (

barley Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley p ...

rice Rice is the seed of the grass species '' Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and '' Porteresia'', both wild and domesticat ...

wheat Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeologi ...

, rye, and

oats The oat (''Avena sativa''), sometimes called the common oat, is a species of cereal grain grown for its seed, which is known by the same name (usually in the plural, unlike other cereals and pseudocereals). While oats are suitable for human co ...

, 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), ...

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 ...

tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word ...

es,

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,

rose A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...

s, and

Kentucky bluegrass ''Poa pratensis'', commonly known as Kentucky bluegrass (or blue grass), smooth meadow-grass, or common meadow-grass, is a perennial species of grass native to practically all of Europe, North Asia and the mountains of Algeria and Morocco. Altho ...

) follow a C3 photosynthetic pathway that will yield δ¹³C values averaging about −26.5‰. Grasses in hot

arid climate The desert climate or arid climate (in the Köppen climate classification ''BWh'' and ''BWk''), is a dry climate sub-type in which there is a severe excess of evaporation over precipitation. The typically bald, rocky, or sandy surfaces in deser ...

s (

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 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 a ...

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 other ...

sugar cane Sugarcane or sugar cane is a species of (often hybrid) tall, perennial grass (in the genus '' Saccharum'', tribe Andropogoneae) that is used for sugar production. The plants are 2–6 m (6–20 ft) tall with stout, jointed, fibrous stalk ...

, 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 whol ...

and −14.5‰ in the

hydroxylapatite Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. ...

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