Isotopes of carbon

Carbon (6C) has 15 known isotopes, from 8C to 22C, of which 12C and 13C are stable. The longest-lived radioisotope is 14C, with a half-life of 5,700 years. This is also the only carbon radioisotope found in nature—trace quantities are formed cosmogenically by the reaction 14N + 1n → 14C + 1H. The most stable artificial radioisotope is 11C, which has a half-life of 20.334 minutes. All other radioisotopes have half-lives under 20 seconds, most less than 200 milliseconds. The least stable isotope is 8C, with a half-life of 2.0 x 10−21 s.

Main isotopes of carbon (6C)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
11C syn 20 min β+ 11B
12C 98.9% stable
13C 1.1% stable
14C 1 ppt 5730 y β 14N
Standard atomic weight Ar, standard(C)
  • [12.0096, 12.0116][1]
  • Conventional: 12.011

Carbon-11

Carbon-11 or 11C is a radioactive isotope of carbon that decays to boron-11. This decay mainly occurs due to positron emission; however, around 0.19–0.23% of the time, it is a result of electron capture.[2][3] It has a half-life of 20.334 minutes.

11
C
11
B
+
e+
+
ν
e
+ 0.96 MeV
11
C
+
e
11
B
+
ν
e
+ 1.98 MeV

It is produced from nitrogen in a cyclotron by the reaction

14
N
+
p
11
C
+ 4
He

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 [11
C
]DASB
and [11C]Cimbi-5.

Natural isotopes

There are three naturally occurring isotopes of carbon: 12, 13, and 14. 12C and 13C are stable, occurring in a natural proportion of approximately 93:1. 14C 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, 14C constitutes a negligible part; but, since it is radioactive with a half-life of 5,700 years, it is radiometrically detectable. Since dead tissue doesn't absorb 14C, the amount of 14C is one of the methods used within the field of archeology for radiometric dating of biological material.

Paleoclimate

12C and 13C 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) (Lynch-Stieglitz et al., 1995). Plants find it easier to use the lighter isotopes (12C) when they convert sunlight and carbon dioxide into food. So, for example, large blooms of plankton (free-floating organisms) absorb large amounts of 12C from the oceans. Originally, the 12C 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 12C from the surface, leaving the surface layers relatively rich in 13C. Where cold waters well up from the depths (such as in the North Atlantic), the water carries 12C back up with it. So, when the ocean was less stratified than today, there was much more 12C in the skeletons of surface-dwelling species. Other indicators of past climate include the presence of tropical species, coral growths rings, etc.[4]

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 13C = δ13C) is expressed as parts per thousand (‰):[5]

Stable carbon isotopes in carbon dioxide are utilized differentially by plants during photosynthesis. Grasses in temperate climates (barley, rice, wheat, rye and oats, plus sunflower, potato, tomatoes, peanuts, cotton, sugar beet, and most trees and their nuts/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, 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.[6]

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).[7]

List of isotopes

nuclide
symbol
Z(p) N(n)  
isotopic mass (u)
 
half-life decay mode(s)[8] daughter
isotope(s)[n 1]
nuclear
spin and
parity
representative
isotopic
composition
(mole fraction)
range of natural
variation
(mole fraction)
8C 6 2 8.037675(25) 2.0(4) × 10−21 s
[230(50) keV]
2p 6
Be
[n 2]
0+
9C 6 3 9.0310367(23) 126.5(9) ms β+ (60%) 9
B
[n 3]
(3/2−)
β+, p (23%) 8
Be
[n 4]
β+, α (17%) 5
Li
[n 5]
10C 6 4 10.0168532(4) 19.290(12) s β+ 10
B
0+
11C[n 6] 6 5 11.0114336(10) 20.334(24) min β+ (99.79%) 11
B
3/2−
EC (0.21%)[2][3] 11
B
12C 6 6 12 exactly[n 7] Stable 0+ 0.9893(8) 0.98853–0.99037
13C[n 8] 6 7 13.0033548378(10) Stable 1/2− 0.0107(8) 0.00963–0.01147
14C[n 9] 6 8 14.003241989(4) 5,730 years β 14
N
0+ Trace[n 10] <10−12
15C 6 9 15.0105993(9) 2.449(5) s β 15
N
1/2+
16C 6 10 16.014701(4) 0.747(8) s β, n (97.9%) 15
N
0+
β (2.1%) 16
N
17C 6 11 17.022586(19) 193(5) ms β (71.59%) 17
N
(3/2+)
β, n (28.41%) 16
N
18C 6 12 18.02676(3) 92(2) ms β (68.5%) 18
N
0+
β, n (31.5%) 17
N
19C[n 11] 6 13 19.03481(11) 46.2(23) ms β, n (47.0%) 18
N
(1/2+)
β (46.0%) 19
N
β, 2n (7%) 17
N
20C 6 14 20.04032(26) 16(3) ms
[14(+6-5) ms]
β, n (72.0%) 19
N
0+
β (28.0%) 20
N
21C 6 15 21.04934(54)# <30 ns n 20
C
(1/2+)#
22C[n 12] 6 16 22.05720(97)# 6.2(13) ms
[6.1(+14-12) ms]
β 22
N
0+
  1. ^ Bold for stable isotopes
  2. ^ Subsequently decays by double proton emission to 4He for a net reaction of 8C → 4He + 41H
  3. ^ Immediately decays by proton emission to 8Be, which immediately decays to two 4He atoms for a net reaction of 9C → 24He + 1H + e+
  4. ^ Immediately decays into two 4He atoms for a net reaction of 9C → 24He + 1H + e+
  5. ^ Immediately decays by proton emission to 4He for a net reaction of 9C → 24He + 1H + e+
  6. ^ Used for labeling molecules in PET scans
  7. ^ The unified atomic mass unit is defined as 1/12 the mass of an unbound atom of carbon-12 at ground state
  8. ^ Ratio of 12C to 13C used to measure biological productivity in ancient times and differing types of photosynthesis
  9. ^ Has an important use in radiodating (see carbon dating)
  10. ^ Primarily cosmogenic, produced by neutrons striking atoms of 14N (14N + 1n → 14C + 1H)
  11. ^ Has 1 halo neutron
  12. ^ Has 2 halo neutrons

Notes

  • The precision of the isotope abundances and atomic mass is limited through variations. The given ranges should be applicable to any normal terrestrial material.
  • Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
  • Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.
  • The carbon-12 nuclide is of particular importance as it is used as the standard from which atomic masses of all nuclides are expressed: its atomic mass is by definition 12 Da.
  • Nuclide masses are given by IUPAP Commission on Symbols, Units, Nomenclature, Atomic Masses and Fundamental Constants (SUNAMCO).
  • Isotope abundances are given by IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW).

See also

References

  1. ^ Meija, J.; et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry. 88 (3): 265–91. doi:10.1515/pac-2015-0305.
  2. ^ a b Scobie, J.; Lewis, G. M. (1 September 1957). "K-capture in carbon 11". Philosophical Magazine. 2 (21): 1089–1099. Bibcode:1957PMag....2.1089S. doi:10.1080/14786435708242737.
  3. ^ a b Campbell, J. L.; Leiper, W.; Ledingham, K. W. D.; Drever, R. W. P. (1967-04-11). "The ratio of K-capture to positron emission in the decay of 11C". Nuclear Physics A. 96 (2): 279–287. Bibcode:1967NuPhA..96..279C. doi:10.1016/0375-9474(67)90712-9. Retrieved 27 March 2012.
  4. ^ Tim Flannery The weather makers: the history & future of climate change, The Text Publishing Company, Melbourne, Australia. ISBN 1-920885-84-6
  5. ^ Miller, Charles B.; Wheeler, Patricia (2012). Biological oceanography (2nd ed.). Chichester, West Sussex: John Wiley & Sons, Ltd. p. 186. ISBN 9781444333022. OCLC 794619582.
  6. ^ Tycot, R. H. (2004). M. Martini; M. Milazzo; M. Piacentini, eds. "Stable isotopes and diet: you are what you eat" (PDF). Proceedings of the International School of Physics 'Enrico Fermi' Course CLIV.
  7. ^ Hedges Richard (2006). "Where does our protein come from?". British Journal of Nutrition. 95 (6): 1031–2. doi:10.1079/bjn20061782.
  8. ^ "Universal Nuclide Chart". nucleonica. (Registration required (help)).
Acanthochronology

Acanthochronology is the interdisciplinary study of cactus spines or Euphorbia thorns grown in time ordered sequence (i.e. in series). Physical, morphological or chemical characteristics and information about the relative order or absolute age of the spines or thorns is used to study past climate or plant physiology.

For example, columnar cactus spines grow from the apex of the plant. After several weeks the spines stop growing and have been moved to the side of the stem. The old spines remain in place for decades as new spines are created at the continually growing apex. The result is that along each external "rib" of the cactus is a series of spines arranged in the order they grew in – the oldest spines are at the bottom and the youngest spines are at the top. These spines can be dated using bomb-spike Carbon-14 and isotopes of carbon (Carbon-13) and oxygen (Oxygen-18) may be used to infer past climate (e.g. precipitation or temperature), plant stem growth or plant physiology (e.g. photosynthetic processes). Alternatively, the width of small transverse bands in the spine may be used to infer daily information about cloud cover or plant productivity, although this remains to be tested. It has also been shown that regular waxy banding on the sides of a Costa Rican cactus (Lemaireocereus aragonii) indicate annual growth and can be used as temporal chronometers.

This sub-discipline of paleoclimatology and ecophysiology is relatively new. Acanthochronology is closely related to dendrochronology, dendroclimatology and isotope geochemistry and borrows many of the methods and techniques from these sub-disciplines of the Earth Sciences. It also draws heavily from the field of ecophysiology, a branch of Biology, to ascribe spine or thorn characteristics to particular environmental or physiological variables.The first peer-reviewed article to present and explain an isotope spine series was from a saguaro cactus in Tucson, Arizona. This and other work shows that radiocarbon and isotope time-series derived from spines can be used for demographic or palaeoclimate studies.

Atomic mass unit

The unified atomic mass unit or dalton (symbol: u, or Da or AMU) is a standard unit of mass that quantifies mass on an atomic or molecular scale (atomic mass). One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol. It is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest, and has a value of 1.660539040(20)×10−27 kg, or approximately 1.66 yoctograms. The CIPM has categorised it as a non-SI unit accepted for use with the SI, and whose value in SI units must be obtained experimentally.The atomic mass unit (amu) without the "unified" prefix is technically an obsolete unit based on oxygen, which was replaced in 1961. However, many sources still use the term amu but now define it in the same way as u (i.e., based on carbon-12). In this sense, most uses of the terms atomic mass units and amu, today, actually refer to unified atomic mass unit. For standardization, a specific atomic nucleus (carbon-12 vs. oxygen-16) had to be chosen because the average mass of a nucleon depends on the count of the nucleons in the atomic nucleus due to mass defect. This is also why the mass of a proton or neutron by itself is more than (and not equal to) 1 u.

The atomic mass unit is not the unit of mass in the atomic units system, which is rather the electron rest mass (me).

Until the 2019 redefinition of SI base units, the number of daltons in a gram is exactly the Avogadro number by definition, or equivalently, a dalton is exactly equivalent to 1 gram/mol. Thereafter, these relationships will no longer be exact, but they will still be extremely accurate approximations.

Carbon

Carbon (from Latin: carbo "coal") is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. Carbon's abundance, its unique diversity of organic compounds, and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables this element to serve as a common element of all known life. It is the second most abundant element in the human body by mass (about 18.5%) after oxygen.The atoms of carbon can bond together in different ways, termed allotropes of carbon. The best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form. For example, graphite is opaque and black while diamond is highly transparent. Graphite is soft enough to form a streak on paper (hence its name, from the Greek verb "γράφειν" which means "to write"), while diamond is the hardest naturally occurring material known. Graphite is a good electrical conductor while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials. All carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form at standard temperature and pressure. They are chemically resistant and require high temperature to react even with oxygen.

The most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of inorganic carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil, and methane clathrates. Carbon forms a vast number of compounds, more than any other element, with almost ten million compounds described to date, and yet that number is but a fraction of the number of theoretically possible compounds under standard conditions. For this reason, carbon has often been referred to as the "king of the elements".

Carbon-12

Carbon-12 (12C) is the more abundant of the two stable isotopes of carbon (Carbon-13 being the other), amounting to 98.93% of the element carbon; its abundance is due to the triple-alpha process by which it is created in stars. Carbon-12 is of particular importance in its use as the standard from which atomic masses of all nuclides are measured, thus, its atomic mass is exactly 12 daltons by definition. Carbon-12 is composed of 6 protons 6 neutrons and 6 electrons.

Carbon-13

Carbon-13 (13C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes, it makes up about 1.1% of all natural carbon on Earth.

Carbon-14

Carbon-14, (14C), or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues (1949) to date archaeological, geological and hydrogeological samples. Carbon-14 was discovered on February 27, 1940, by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory in Berkeley, California. Its existence had been suggested by Franz Kurie in 1934.There are three naturally occurring isotopes of carbon on Earth: carbon-12, which makes up 99% of all carbon on Earth; carbon-13, which makes up 1%; and carbon-14, which occurs in trace amounts, making up about 1 or 1.5 atoms per 1012 atoms of carbon in the atmosphere. Carbon-12 and carbon-13 are both stable, while carbon-14 is unstable and has a half-life of 5,730±40 years. Carbon-14 decays into nitrogen-14 through beta decay. A gram of carbon containing 1 atom of carbon-14 per 1012 atoms will emit ~0.2 beta particles per second. The primary natural source of carbon-14 on Earth is cosmic ray action on nitrogen in the atmosphere, and it is therefore a cosmogenic nuclide. However, open-air nuclear testing between 1955–1980 contributed to this pool.

The different isotopes of carbon do not differ appreciably in their chemical properties. This resemblance is used in chemical and biological research, in a technique called carbon labeling: carbon-14 atoms can be used to replace nonradioactive carbon, in order to trace chemical and biochemical reactions involving carbon atoms from any given organic compound.

Charles Cantor

Charles Cantor (born 1942) is an American molecular geneticist who, in conjunction with David Schwartz, developed pulse field gel electrophoresis for very large DNA molecules. Cantor's three-volume book, Biophysical Chemistry co-authored with Paul Schimmel, was an influential textbook in the 1980s and 1990s.

Charles Cantor is Director of the Center for Advanced Biotechnology at Boston University. He is currently on a two-year sabbatical acting as Chief Scientific Officer at Sequenom, Inc. However, his research laboratory at Boston University continues to be active, and he works there frequently. He is also a co-founder and Director of Retrotope, a US-based company using heavier isotopes of carbon (C13) and hydrogen (deuterium) to stabilize essential compounds like amino acids, nucleic acids and lipids to target age-related diseases.Cantor held positions at Columbia University and the University of California, Berkeley.

Cantor’s laboratory at Boston University has developed methods for separating large DNA molecules, for studying structural relationships in complex proteins and nucleic acids, and for sensitive detection of proteins and nucleic acids in a variety of settings.

Professor Cantor has been director of the Department of Energy Human Genome Project and Chairman of the Department of Biomedical Engineering at Boston University.

Cantor is a consultant to more than 16 biotech firms, has published more than 400 peer reviewed articles, been granted 54 US patents, and co-authored a three-volume textbook on Biophysical Chemistry.

Fractionation of carbon isotopes in oxygenic photosynthesis

Photosynthesis converts carbon dioxide to carbohydrates via several metabolic pathways that provide energy to an organism and preferentially react with certain stable isotopes of carbon. The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated with among oxygenic phototrophs. The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism. Understanding these variations in carbon fractionation across species is useful for biogeochemical studies, including the reconstruction of paleoecology, plant evolution, and the characterization of food chains.

Oxygenic photosynthesis is a metabolic pathway facilitated by autotrophs, including plants, algae, and cyanobacteria. This pathway converts inorganic carbon dioxide from the atmosphere or aquatic environment into carbohydrates, using water and energy from light, then releases molecular oxygen as a product. Organic carbon contains less of the stable isotope Carbon-13, or 13C, relative to the initial inorganic carbon from the atmosphere or water because photosynthetic carbon fixation involves several fractionating reactions with kinetic isotope effects. These reactions undergo a kinetic isotope effect because they are limited by overcoming an activation energy barrier. The lighter isotope has a higher energy state in the quantum well of a chemical bond, allowing it to be preferentially formed into products. Different organisms fix carbon through different mechanisms, which are reflected in the varying isotope compositions across photosynthetic pathways (see table below, and explanation of notation in "Carbon Isotope Measurement" section). The following sections will outline the different oxygenic photosynthetic pathways and what contributes to their associated delta values.

Isotope fractionation

Isotope fractionation describes processes that affect the relative abundance of isotopes, often used in isotope geochemistry. Normally, the focus is on stable isotopes of the same element. Isotopic fractionation in the natural environment can be measured by isotope analysis, using isotope-ratio mass spectrometry, to separate different element isotopes on the basis of their mass-to-charge ratio, an important tool to understand natural systems. For example, in biochemistry processes cause a fluctuation in the amount of isotopes of carbon ratios incorporated into a biological being. The difference between the true amount of carbon and the amount in the plant is known as isotope fractionation.

Isotopes of neon

Neon (10Ne) possesses three stable isotopes, 20Ne, 21Ne, and 22Ne. In addition, 16 radioactive isotopes have been discovered ranging from 16Ne to 34Ne, all short-lived. The longest-lived is 24Ne with a half-life of 3.38 minutes. All others are under a minute, most under a second. The least stable is 16Ne with a half-life of 9×10−21 s. See isotopes of carbon for notes about the measurement.

Isotopes of nitrogen

Natural nitrogen (7N) consists of two stable isotopes, nitrogen-14, which makes up the vast majority of naturally occurring nitrogen, and nitrogen-15, which is less common. Fourteen radioactive isotopes (radioisotopes) have also been found so far, with atomic masses ranging from 10 to 25, and one nuclear isomer, 11mN. All of these radioisotopes are short-lived, with the longest-lived one being nitrogen-13 with a half-life of 9.965 minutes. All of the others have half-lives below 7.15 seconds, with most of these being below five-eighths of a second. Most of the isotopes with atomic mass numbers below 14 decay to isotopes of carbon, while most of the isotopes with masses above 15 decay to isotopes of oxygen. The shortest-lived known isotope is nitrogen-10, with a half-life of about 200 yoctoseconds.

Isotopically pure diamond

An isotopical pure diamond is a type of diamond that is composed entirely of one isotope of carbon. Isotopically pure diamonds have been manufactured from either the more common carbon isotope with mass number 12 (abbreviated as 12C) or the less common 13C isotope. Compared to natural diamonds that are composed of a mixture of 12C and 13C isotopes, isotopically pure diamonds possess improved characteristics such as increased thermal conductivity. Thermal conductivity of diamonds is at a minimum when 12C and 13C are in a ratio of 1:1 and reaches a maximum when the composition is 100% 12C or 100% 13C.

Phil Ineson

Professor Phil Ineson is a Chair in Global Change Ecology at the University of York. Ineson is particularly noted for his work with stable isotopes (and was the first to grow C3 plants on C4 soil).

Ineson received his BSc from Manchester Polytechnic in 1982, receiving a Ph.D. from the University of Liverpool in 1986. He was then a NERC Post-Doctoral Research Assistant at the University of Exeter until 1989. NERC Research Fellow at ITE Merlewood and later Senior Scientific Officer (SSO). Between 1996 and 1999 he was at the Centre for Ecology and Hydrology, (CEH) Merlewood. Between 1998 and 2000 he was Visiting Professor at Lancaster University. He was made Chair in Global Change Ecology at York in 2000.

Ineson et al. (1996) were able to track the movement of carbon through a plant by using the stable isotopes of carbon, namely 12C and 13C. To obtain soil with a different isotope ratio to normal, they obtained soil from North America on which C4 plants had been grown, giving it a different signature to soil on which C3 plants had been grown. Comparisons of the signatures allowed the turnover of carbon to be measured. This is now a commonly used technique (see e.g. Pataki et al. (2003)) particularly useful in light of elevated carbon dioxide levels due to atmospheric pollution.

Radiocarbon 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 developed in the late 1940s by Willard Libby, who received the Nobel Prize in Chemistry for his work in 1960. It is based on the fact that radiocarbon (14C) is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting 14C combines with atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into plants by photosynthesis; animals then acquire 14C by eating the plants. When the animal or plant dies, it stops exchanging carbon with its environment, and from that point onwards the amount of 14C it contains begins to decrease as the 14C undergoes radioactive decay. Measuring the amount of 14C in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died. The older a sample is, the less 14C there is to be detected, and because the half-life of 14C (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by this process date to around 50,000 years ago, although special preparation methods occasionally permit accurate analysis of older samples.

Research has been ongoing since the 1960s to determine what the proportion of 14C in the atmosphere has been over the past fifty thousand years. The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age. Other corrections must be made to account for the proportion of 14C in different types of organisms (fractionation), and the varying levels of 14C throughout the biosphere (reservoir effects). Additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the 1950s and 1960s. Because the time it takes to convert biological materials to fossil fuels is substantially longer than the time it takes for its 14C to decay below detectable levels, fossil fuels contain almost no 14C, and as a result there was a noticeable drop in the proportion of 14C in the atmosphere beginning in the late 19th century. Conversely, nuclear testing increased the amount of 14C in the atmosphere, which attained a maximum in about 1965 of almost twice what it had been before the testing began.

Measurement of radiocarbon was originally done by beta-counting devices, which counted the amount of beta radiation emitted by decaying 14C atoms in a sample. More recently, accelerator mass spectrometry has become the method of choice; it counts all the 14C atoms in the sample and not just the few that happen to decay during the measurements; it can therefore be used with much smaller samples (as small as individual plant seeds), and gives results much more quickly. The development of radiocarbon dating has had a profound impact on archaeology. In addition to permitting more accurate dating within archaeological sites than previous methods, it allows comparison of dates of events across great distances. Histories of archaeology often refer to its impact as the "radiocarbon revolution". Radiocarbon dating has allowed key transitions in prehistory to be dated, such as the end of the last ice age, and the beginning of the Neolithic and Bronze Age in different regions.

Suess

Suess may refer to:

Süß, a German surname transliterated as Suess

Eduard Suess (1831–1914), an Austrian geologist

Suess (lunar crater), named for the geologist

Suess (Martian crater), named for the geologist

Suess Glacier, a glacier in Canada named for the geologist

Hans Suess (1909-1993), an Austrian born American physical chemist, nuclear physicist and grandson of the geologist Eduard Suess

Suess effect, a change in the ratio of the atmospheric concentrations of heavy isotopes of carbon noted by the chemist

Suess effect

The Suess effect is a change in the ratio of the atmospheric concentrations of heavy isotopes of carbon (13C and 14C) by the admixture of large amounts of fossil-fuel derived CO2, which is depleted in 13CO2 and contains no 14CO2. It is named for the Austrian chemist Hans Suess, who noted the influence of this effect on the accuracy of radiocarbon dating. More recently, the Suess effect has been used in studies of climate change. The term originally referred only to dilution of atmospheric 14CO2. The concept was later extended to dilution of 13CO2 and to other reservoirs of carbon such as the oceans and soils.

Δ13C

In geochemistry, paleoclimatology and paleoceanography δ13C (pronounced "delta c thirteen") is an isotopic signature, a measure of the ratio of stable isotopes 13C : 12C, reported in parts per thousand (per mil, ‰).

The definition is, in per mil:

where the standard is an established reference material.

δ13C varies in time as a function of productivity, the signature of the inorganic source, organic carbon burial and vegetation type. Biological processes preferentially take up the lower mass isotope through kinetic fractionation. However some abiotic processes do the same, methane from hydrothermal vents can be depleted by up to 50%.

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