Carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

(₆C) has 14 known

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

s, from to as well as , of which only and are stable. The longest-lived radioisotope is , with a

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

of years. This is also the only carbon radioisotope found in nature, as 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 . Light isotopes tend to decay into isotopes of boron and heavy ones tend to decay into isotopes of nitrogen.

List of isotopes

, -id=Carbon-8 , , style="text-align:right" , 6 , style="text-align:right" , 2 , ,
[] , proton emission, 2p , Also immediately emits two protons for the net reaction of → + 4 , 0+ , , , -id=Carbon-9 , rowspan=3, , rowspan=3 style="text-align:right" , 6 , rowspan=3 style="text-align:right" , 3 , rowspan=3, , rowspan=3, , β⁺ () , All three product nuclei are unbound and so the net reaction in each case is → 2 + + , rowspan=3, 3/2− , rowspan=3, , rowspan=3, , - , β⁺α () , , - , β⁺p () , , -id=Carbon-10 , , 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 , , -id=Carbon-11m , style="text-indent:1em" , , colspan=3 style="text-indent:2em" , , , p? , ? , 1/2+ , , , -id=Carbon-12 , , style="text-align:right" , 6 , style="text-align:right" , 6 , 12 exactlyThe dalton 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"> , -id=Carbon-13 , Ratio of ¹²C to ¹³C used to measure biological productivity in ancient times and differing types of

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

, style="text-align:right" , 6 , style="text-align:right" , 7 , , colspan=3 align=center, Stable , 1/2− , ref name="Atomic Weight of Carbon"/> , -id=Carbon-14 , 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 , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

s striking atoms of ( + → + ) , < 10⁻¹² , -id=Carbon-14m , style="text-indent:1em" , , colspan="3" style="text-indent:2em" , , , IT , , (2−) , , , -id=Carbon-15 , , style="text-align:right" , 6 , style="text-align:right" , 9 , , , β⁻ , , 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, , β⁻ n () , , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , β⁻ () , , -id=Carbon-17 , rowspan=2, , rowspan=2 style="text-align:right" , 6 , rowspan=2 style="text-align:right" , 11 , rowspan=2, , rowspan=2, , β⁻ () , , rowspan=2, 3/2+ , rowspan=2, , rowspan=2, , - , β⁻n () , , -id=Carbon-18 , rowspan=2, , rowspan=2 style="text-align:right" , 6 , rowspan=2 style="text-align:right" , 12 , rowspan=2, , rowspan=2, , β⁻ () , , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , β⁻n () , , -id=Carbon-19 , 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 () , , -id=Carbon-20 , 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⁻ (> ) , , -id=Carbon-22 , 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⁻ (> ) ,

Carbon-11

Carbon-11 or is a radioactive isotope of

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

that decays to boron-11. 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 emi ...

, with around 0.19–0.23% of decays instead occurring by electron capture. It has a

of . : → + + + : + → + + It is produced by hitting nitrogen with protons of around 16.5 MeV in a cyclotron. The causes the endothermic reaction : + → + − 2.92 MeV 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 /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 δ¹³C in benthic foraminifera and used as a proxy for nutrient cycling and the temperature dependent air–sea exchange of CO₂ (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 Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

(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 Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for ...

), 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. 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 (‰) divergence from the ratio of a standard: :

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

‰ The usual standard is Peedee Belemnite, abbreviated "PDB", a fossil belemnite. Due to shortage of the original PDB sample, artificial "virtual PDB", or "VPDB", is generally used today. Stable carbon isotopes in

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

are utilized differentially by plants during

. Grasses in temperate climates (

barley Barley (), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains; it was domesticated in the Fertile Crescent around 9000 BC, giving it nonshattering spikele ...

rice Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...

wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...

, rye, and oats, plus sunflower, potato,

tomato The tomato (, ), ''Solanum lycopersicum'', is a plant whose fruit is an edible Berry (botany), berry that is eaten as a vegetable. The tomato is a member of the nightshade family that includes tobacco, potato, and chili peppers. It originate ...

es,

peanut The peanut (''Arachis hypogaea''), also known as the groundnut, goober (US), goober pea, 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 by small and large ...

cotton Cotton (), first recorded in ancient India, 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 ...

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

, and most trees and their nuts or fruits,

rose A rose is either a woody perennial plant, perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred Rose species, species and Garden roses, tens of thousands of cultivar ...

s, 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''), also known as corn in North American English, is a tall stout grass that produces cereal grain. It was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. Native American ...

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 millets belong to the tribe Paniceae. Millets are important crops in the Semi-arid climate, ...

sorghum ''Sorghum bicolor'', commonly called sorghum () and also known as great millet, broomcorn, guinea corn, durra, imphee, jowar, or milo, is a species in the Poaceae, grass genus ''Sorghum (genus), Sorghum'' cultivated for its grain. The grain i ...

, 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 of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...

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