Radiocarbon dating

Radiocarbon dating

Overview
Radiocarbon dating is a radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...

 method that uses the naturally occurring radioisotope carbon-14
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a 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 , to date archaeological, geological, and hydrogeological...

  to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present
Before Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...

" (BP), "Present" being defined as 1950. Such raw ages can be calibrated to give calendar dates. One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites.
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Encyclopedia
Radiocarbon dating is a radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...

 method that uses the naturally occurring radioisotope carbon-14
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a 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 , to date archaeological, geological, and hydrogeological...

  to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present
Before Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...

" (BP), "Present" being defined as 1950. Such raw ages can be calibrated to give calendar dates. One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites. When plants fix atmospheric carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

  into organic material during photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 they incorporate a quantity of that approximately matches the level of this isotope in the atmosphere (a small difference occurs because of isotope fractionation
Isotope fractionation
Isotope fractionation is the separation of a mixture of isotopes into its components.There are four types of isotope fractionation:* equilibrium fractionation* kinetic fractionation* transient kinetic isotope fractionation* mass-independent fractionation...

, but this is corrected after laboratory analysis). After plants die or they are consumed by other organisms (for example, by humans or other animals) the fraction of this organic material declines at a fixed exponential rate due to the radioactive decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...

 of . Comparing the remaining fraction of a sample to that expected from atmospheric allows the age of the sample to be estimated.

The technique of radiocarbon dating was developed by Willard Libby
Willard Libby
Willard Frank Libby was an American physical chemist noted for his role in the 1949 development of radiocarbon dating, a process which revolutionized archaeology....

 and his colleagues at the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...

 in 1949. Emilio Segrè asserted in his autobiography that Enrico Fermi
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...

 suggested the concept to Libby in a seminar at Chicago that year. Libby estimated that the steady state radioactivity concentration of exchangeable carbon-14 would be about 14 disintegrations per minute (dpm) per gram. In 1960, he was awarded the Nobel Prize in chemistry
Nobel Prize in Chemistry
The Nobel Prize in Chemistry is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in chemistry, physics, literature,...

 for this work. He demonstrated the accuracy of radiocarbon dating by accurately estimating the age of wood from a series of samples for which the age was known, including an ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...

ian royal barge of 1850 BC.

Basic physics


Carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 has two stable, nonradioactive isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

s: carbon-12
Carbon-12
Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....

 , and carbon-13
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...

 . In addition, there are trace amounts of the unstable isotope carbon-14 on Earth. Carbon-14 has a relatively short half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...

 of 5730 years, meaning that the amount of carbon-14 in a sample is halved over the course of 5730 years due to radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

. Carbon-14 would have long ago vanished from Earth were it not for the unremitting cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

 flux interactions with the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

, which create more of the isotope. The neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s resulting from the cosmic ray interactions participate in the following nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...

 on the atoms of nitrogen molecules in the atmosphere:


The highest rate of carbon-14 production takes place at altitudes of 9 to 15 km (30,000 to 50,000 ft), and at high geomagnetic latitudes, but the carbon-14 spreads evenly throughout the atmosphere and reacts with oxygen to form carbon dioxide. Carbon dioxide also permeates the ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s, dissolving in the water. For approximate analysis it is assumed that the cosmic ray flux is constant over long periods of time; thus carbon-14 is produced at a constant rate and the proportion of radioactive to non-radioactive carbon is constant: ca. 1 part per trillion (600 billion atoms/mole). In 1958 Hessel de Vries
Hessel de Vries
Hessel de Vries , was a Dutch physicist and professor at the University of Groningen who furthered the detection methods and applications of radiocarbon dating to a variety of sciences. But for his untimely death, he might have been a Nobel laureate...

 showed that the concentration of carbon-14 in the atmosphere varies with time and locality. For the most accurate work, these variations are compensated by means of calibration curve
Calibration curve
In analytical chemistry, a calibration curve is a general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples of known concentration...

s. When these curves are used, their accuracy and shape are the factors that determine the accuracy of the age obtained for a given sample. can also be produced at ground level at a rate of 1 x 10−4 g−1s−1, which is not considered significant enough to impact on dating without a known other source of neutrons.

Plants take up atmospheric carbon dioxide by photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, and are ingested by animals, so every living thing is constantly exchanging carbon-14 with its environment as long as it lives. Once it dies, however, this exchange stops, and the amount of carbon-14 gradually decreases through radioactive beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...

 with a half-life of 5,730 ± 40 years . Carbon-14 is stored in different amounts in different reservoirs because there is a dynamic equilibrium
Dynamic equilibrium
A dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants/products, but substances move between the chemicals at an equal rate, meaning there is no net change. It is a particular example of a system in a steady state...

 between production and decay.


Carbon-14 was discovered on February 27, 1940, by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory at Berkeley.

Computation of ages and dates


The number of decays per time is proportional to the current number of radioactive atoms. This is expressed by the following differential equation
Differential equation
A differential equation is a mathematical equation for an unknown function of one or several variables that relates the values of the function itself and its derivatives of various orders...

, where N is the number of radioactive atoms and λ is a positive number called the decay constant:
As the solution to this equation, the number of radioactive atoms N can be written as a function of time,
which describes an exponential decay over a timespan t with an initial condition of N0 radioactive atoms at t = 0.
Canonically, t is 0 when the decay started. In this case, N0 is the initial number of atoms when the decay started.

For radiocarbon dating a once living organism, the initial ratio of atoms to the sum of all other carbon atoms at the point of the organism's death and hence the point when the decay started, is approximately the ratio in the atmosphere.

Two characteristic times can be defined:
  • mean- or average-life: mean or average time each radiocarbon atom spends in a given sample until it decays.
  • half-life: time lapsed for half the number of radiocarbon atoms in a given sample, to decay,


It can be shown that:
= = radiocarbon mean- or average-life = 8033 years (Libby value)
= = radiocarbon half-life = 5568 years (Libby value)

Notice that dates are customarily given in years BP which implies t(BP) = –t because the time arrow for dates runs in reverse direction from the time arrow for the corresponding ages. From these considerations and the above equation, it results:

For a raw radiocarbon date:
and for a raw radiocarbon age:

After replacing values, the raw radiocarbon age becomes any of the following equivalent formulae:

using logs base e and the average life:

and

using logs base 2 and the half-life:

Wiggle matching
Wiggle matching
Wiggle matching, also known as carbon–14 wiggle-match dating uses the non-linear relationship between 14C age and calendar age to match the shape of a series of closely sequentially spaced 14C dates with the 14C calibration curve. A numerical approach to WMD allows one to assess the...

 uses the non-linear relationship between the age and calendar age to match the shape of a series of closely sequentially spaced dates with the calibration curve.

Measurements and scales



Measurements are traditionally made by counting the radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

 of individual carbon atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s by gas proportional counting
Proportional counter
A proportional counter is a measurement device to count particles of ionizing radiation and measure their energy.A proportional counter is a type of gaseous ionization detector. Its operation is similar to that of a Geiger-Müller counter, but uses a lower operating voltage. An inert gas is used to...

 or by liquid scintillation counting
Liquid scintillation counting
Liquid scintillation counting is a standard laboratory method in the life-sciences for measuring radiation from beta-emitting nuclides. Scintillating materials are also used in differently constructed "counters" in many other fields....

. For samples of sufficient size (several grams of carbon) this method is still widely used in the 2000s. Among others, all the tree ring samples used for the calibration curves (see below) were determined by these counting techniques. Such decay counting, however, is relatively insensitive and subject to large statistical uncertainties for small samples. When there is little carbon-14 to begin with, the long radiocarbon half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...

 means that very few of the carbon-14 atoms will decay during the time allotted for their detection, resulting in few disintegrations per minute.

The sensitivity of the method has been greatly increased by the use of accelerator mass spectrometry
Accelerator mass spectrometry
Accelerator mass spectrometry differs from other forms of mass spectrometry in that it accelerates ions to extraordinarily high kinetic energies before mass analysis. The special strength of AMS among the mass spectrometric methods is its power to separate a rare isotope from an abundant...

 (AMS). With this technique atoms can be detected and counted directly vs only detecting those atoms that decay during the time interval allotted for an analysis. AMS allows dating samples containing only a few milligrams of carbon.

Raw radiocarbon ages (i.e., those not calibrated) are usually reported in "years Before Present
Before Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...

" (BP). This is the number of radiocarbon years before 1950, based on a nominal (and assumed constant – see "calibration" below) level of carbon-14 in the atmosphere equal to the 1950 level. These raw dates are also based on a slightly-off historic value for the radiocarbon half-life. Such value is used for consistency with earlier published dates (see "Radiocarbon half-life" below). See the section on computation for the basis of the calculations.

Radiocarbon dating laboratories generally report an uncertainty for each date. For example, 3000 ± 30 BP indicates a standard deviation
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...

 of 30 radiocarbon years. Traditionally this included only the statistical counting uncertainty. However, some laboratories supplied an "error multiplier" that could be multiplied by the uncertainty to account for other sources of error in the measuring process. More recently, the laboratories try to quote the overall uncertainty, which is determined from control samples of known age and verified by international intercomparison exercises. In 2008, a typical uncertainty better than ±40 radiocarbon years can be expected for samples younger than 10,000 years. This, however, is only a small part of the uncertainty of the final age determination (see section Calibration below).

Samples older than the upper age-limit cannot be dated because the small number of remaining intrinsic atoms will be obscured by background atoms introduced into the samples while they still resided in the environment, during sample preparation, or in the detection instrument. , the limiting age for a 1 milligram sample of graphite is about ten half-lives, approximately 60,000 years. This age is derived from that of the calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....

 blanks used in an analysis, whose content is assumed to be the result of contamination during processing (as a result of this, some facilities will not report an age greater than 60,000 years for any sample).

A variety of sample processing and instrument-based constraints have been postulated to explain the upper age-limit. To examine instrument-based background activities in the AMS instrument of the W. M. Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory of the University of California, a set of natural diamonds were dated. Natural diamond samples from different sources within rock formations with standard geological ages in excess of 100 Ma yielded apparent ages 64,920 ± 430 BP to 80,000 ± 1100 BP as reported in 2007.

The need for calibration



Dates may be expressed as either uncalibrated or calibrated years. A raw BP date cannot be used directly as a calendar date, because the level of atmospheric has not been strictly constant during the span of time that can be radiocarbon dated. The level is affected by variations in the cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

 intensity which is in turn affected by variations in the Earth's magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

. In addition, there are substantial reservoirs of carbon in organic matter, the ocean, ocean sediments (see methane hydrate), and sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s. Changes in the Earth's climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

 can affect the carbon flows between these reservoirs and the atmosphere, leading to changes in the atmosphere's fraction. As the graph to the right shows, there is an overstatement of the age of the sample of nearly 1000 years in an uncalibrated dating of 7000 BP.

Aside from these changes due to natural processes, the level has also been affected by human activities. From the beginning of the industrial revolution
Industrial Revolution
The Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, transportation, and technology had a profound effect on the social, economic and cultural conditions of the times...

 in the 18th century to the 1950s, the fractional level of decreased because of the admixture of large quantities of into the atmosphere, due to the excavated oil reserves and combustion production of fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

. This decline is known as the Suess effect
Suess effect
The Suess effect is change in the ratio of the atmospheric concentrations of heavy isotopes of carbon 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...

, and also affects the isotope. However, atmospheric was almost doubled during the 1950s and 1960s due to atmospheric atomic bomb tests
Nuclear testing
Nuclear weapons tests are experiments carried out to determine the effectiveness, yield and explosive capability of nuclear weapons. Throughout the twentieth century, most nations that have developed nuclear weapons have tested them...

.

Calibration methods


The raw radiocarbon dates, in BP years, are calibrated to give calendar dates. Standard calibration curve
Calibration curve
In analytical chemistry, a calibration curve is a general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples of known concentration...

s are available, based on comparison of radiocarbon dates of samples that can be dated independently by other methods such as examination of tree growth rings (dendrochronology
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

), deep ocean sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 cores, lake sediment varve
Varve
A varve is an annual layer of sediment or sedimentary rock.The word 'varve' is derived from the Swedish word varv whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as Hvarfig lera on the first map produced by the Geological Survey of Sweden in...

s, coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...

 samples, and speleothem
Speleothem
A speleothem , commonly known as a cave formation, is a secondary mineral deposit formed in a cave. Speleothems are typically formed in limestone or dolostone solutional caves.-Origin and composition:...

s (cave deposits).

The calibration curves can vary significantly from a straight line, so comparison of uncalibrated radiocarbon dates (e.g., plotting them on a graph or subtracting dates to give elapsed time) is likely to give misleading results. There are also significant plateaus in the curves, such as the one from 11,000 to 10,000 radiocarbon years BP, which is believed to be associated with changing ocean circulation during the Younger Dryas
Younger Dryas
The Younger Dryas stadial, also referred to as the Big Freeze, was a geologically brief period of cold climatic conditions and drought between approximately 12.8 and 11.5 ka BP, or 12,800 and 11,500 years before present...

 period. Over the historical period from 0 to 10,000 years BP, the average width of the uncertainty of calibrated dates was found to be 335 years, although in well-behaved regions of the calibration curve the width decreased to about 113 years while in ill-behaved regions it increased to a maximum of 801 years. Significantly, in the ill-behaved regions of the calibration curve, increasing the precision of the measurements does not have a significant effect on increasing the accuracy of the dates.

The 2004 version of the calibration curve extends back quite accurately to 26,000 years BP. Any errors in the calibration curve do not contribute more than ±16 years to the measurement error during the historic and late prehistoric periods (0–6,000 yrs BP) and no more than ±163 years over the entire 26,000 years of the curve, although its shape can reduce the accuracy as mentioned above.

In late 2009, the journal Radiocarbon announced agreement on the INTCAL09 standard, which extends a more accurate calibration curve to 50,000 years.

History


Carbon dating was developed by American scientist Willard Libby
Willard Libby
Willard Frank Libby was an American physical chemist noted for his role in the 1949 development of radiocarbon dating, a process which revolutionized archaeology....

 and his team at the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...

. Libby calculated the half-life of carbon-14 as 5568 ± 30 years, a figure now known as the Libby half-life. Following a conference at the University of Cambridge
University of Cambridge
The University of Cambridge is a public research university located in Cambridge, United Kingdom. It is the second-oldest university in both the United Kingdom and the English-speaking world , and the seventh-oldest globally...

 in 1962, a more accurate figure of 5730 ± 40 years was agreed upon based on more recent experimental data (this figure is now known as the Cambridge half-life).

The chairman of the Cambridge conference, Harry Godwin
Harry Godwin
Sir Harry Godwin FRS, was a prominent English botanist and ecologist of the 20th century. He had a long association with Clare College, Cambridge.-Early life:...

, wrote to the scientific journal Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

, recommending that the Libby half-life continue to be used for the time being, as the Cambridge figure might itself be improved by future experiments. Laboratories today continue to use the Libby figure to avoid inconsistencies with earlier publications, although the Cambridge half-life is still the most accurate figure that is widely known and accepted. However, the inaccuracy of the Libby half-life is not relevant if calibration is applied: the mathematical term representing the half-life is canceled out as long as the same value is used throughout a calculation.

Carbon exchange reservoir


Libby's original exchange reservoir hypothesis assumes that the exchange reservoir is constant all over the world. The calibration method also assumes that the temporal variation in level is global, such that a small number of samples from a specific year are sufficient for calibration. However, since Libby's early work was published (1950 to 1958), latitudinal and continental variations in the carbon exchange reservoir have been observed by Hessel de Vries
Hessel de Vries
Hessel de Vries , was a Dutch physicist and professor at the University of Groningen who furthered the detection methods and applications of radiocarbon dating to a variety of sciences. But for his untimely death, he might have been a Nobel laureate...

 (1958; as reviewed by Lerman et al.). Subsequently, methods have been developed that allow the correction of these so-called reservoir effects, including:
  • When is transferred from the atmosphere to the oceans, it initially shares the concentration of the atmosphere. However, turnaround times of in the ocean are similar to the half-life of (making also a dating tool for ocean water). Marine organisms feed on this "old" carbon, and thus their radiocarbon age reflects the time of uptake by the ocean rather than the time of death of the organism. This marine reservoir effect is partly handled by a special marine calibration curve, but local deviations of several hundred years exist.

  • Erosion
    Erosion
    Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

     and immersion of carbonate rocks (which are generally older than 80,000 years and so shouldn't contain measurable ) causes an increase in and in the exchange reservoir, which depends on local weather conditions and can vary the ratio of carbon that living organisms incorporate. This is believed negligible for the atmosphere and atmosphere-derived carbon since most erosion will flow into the sea. The atmospheric concentration may differ substantially from the concentration in local water reservoirs. Eroded from CaCO3 or organic deposits, old carbon may be assimilated easily and provide diluted carbon into trophic chains. So the method is less reliable for such materials as well as for samples derived from animals with such plants in their food chain.

  • Volcanic eruptions
    Volcanic Eruptions
    Volcanic Eruptions is a company owned by Crispin Glover. The company produces and issues Glover's work: It has released two films to date, What Is It? and its sequel, It is Fine! EVERYTHING IS FINE . Its current plans include releasing the final film in the trilogy titled It Is Mine...

     eject large amount of carbon into the air, causing an increase in and in the exchange reservoir and can vary the exchange ratio locally. This explains the often irregular dating achieved in volcanic areas.

  • The earth is not affected evenly by cosmic radiation, the magnitude of the radiation depends on land altitude and earth's magnetic field strength at any given location, causing minor variation in the local production. This is accounted for by having calibration curves for different locations of the globe. However this could not always be performed, as tree rings for calibration were only recoverable from certain locations in 1958. The rebuttals by Münnich et al. and by Barker both maintain that while variations of carbon-14 exist, they are about an order of magnitude smaller than those implied by Crowe's calculations.


These effects were first confirmed when samples of wood from around the world, which all had the same age (based on tree ring analysis), showed deviations from the dendrochronological
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

 age. Calibration techniques based on tree-ring samples have contributed to increased accuracy since 1962, when they were accurate to 700 years at worst.

Speleothem studies extend calibration


Speleothem
Speleothem
A speleothem , commonly known as a cave formation, is a secondary mineral deposit formed in a cave. Speleothems are typically formed in limestone or dolostone solutional caves.-Origin and composition:...

s (such as stalagmite
Stalagmite
A stalagmite is a type of speleothem that rises from the floor of a limestone cave due to the dripping of mineralized solutions and the deposition of calcium carbonate. This stalagmite formation occurs only under certain pH conditions within the underground cavern. The corresponding formation on...

s) are calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 deposits that form from drips in limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 caves. Individual speleothems can be tens of thousands of years old. Scientists are attempting to extend the record of atmospheric carbon-14 by measuring radiocarbon in speleothems which have been independently dated using uranium-thorium dating
Uranium-thorium dating
Uranium-thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating technique commonly used to determine the age of calcium carbonate materials such as speleothem or coral...

. These results are improving the calibration for the radiocarbon technique and extending its usefulness to 45,000 years into the past. Initial results from a cave in the Bahamas suggested a peak in the amount of carbon-14 which was twice as high as modern levels. A recent study does not reproduce this extreme shift and suggests that analytical problems may have produced the anomalous result.

Examples

  • Ancient footprints of Acahualinca
    Ancient footprints of Acahualinca
    The Ancient footprints of Acahualinca exist in Managua, Nicaragua near the southern shore of Lake Managua . The region was once called "El Cauce"...

  • Chauvet Cave
    Chauvet Cave
    The Chauvet-Pont-d'Arc Cave is a cave in the Ardèche department of southern France that contains the earliest known cave paintings, as well as other evidence of Upper Paleolithic life. It is located near the commune of Vallon-Pont-d'Arc on a limestone cliff above the former bed of the Ardèche River...

  • Dolaucothi
  • Eve of Naharon
    Eve of Naharon
    Eve of Naharon is the skeleton of a 25 to 30 years old female found in the Naharon section of the underwater cave Sistema Naranjal in Mexico near the town of Tulum, around 80 miles south west of Cancún...

  • Haraldskær Woman
    Haraldskær Woman
    The Haraldskær Woman is an Iron Age bog body found naturally preserved in a bog in Jutland, Denmark. Labourers discovered the body in 1835 while excavating peat on the Haraldskær Estate...

  • Kennewick Man
    Kennewick Man
    Kennewick Man is the name for the skeletal remains of a prehistoric man found on a bank of the Columbia River in Kennewick, Washington, USA, on July 28, 1996...

  • Shroud of Turin
    Shroud of Turin
    The Shroud of Turin or Turin Shroud is a linen cloth bearing the image of a man who appears to have suffered physical trauma in a manner consistent with crucifixion. It is kept in the royal chapel of the Cathedral of Saint John the Baptist in Turin, northern Italy. The image on the shroud is...

  • Skeleton Lake
    Roopkund
    Roopkund is a glacial lake in Uttarakhand state of India famous due to more than five hundred human skeletons found at the edge of a lake...

  • Thera eruption
    Thera eruption
    The Minoan eruption of Thera, also referred to as the Thera eruption or Santorini eruption, was a major catastrophic volcanic eruption with a Volcanic Explosivity Index of 6 or 7 and a Dense-rock equivalent of , which is estimated to have occurred in the mid second millennium BCE. The eruption...

  • Vinland map
    Vinland map
    The Vinland map is claimed to be a 15th century mappa mundi with unique information about Norse exploration of America. It is very well known because of the publicity campaign which accompanied its revelation to the public as a "genuine" pre-Columbian map in 1965...


See also

  • Carbon sequestration
  • Cosmogenic isotopes
  • Environmental isotopes
    Environmental isotopes
    The environmental isotopes are a subset of the isotopes, both stable and radioactive, which are the object of Isotope geochemistry.The most used environmental isotopes are:* deuterium* tritium* carbon-13* carbon-14* nitrogen-15* oxygen-18...

  • Discussion of half-life and average-life or mean-lifetime
  • Arizona Accelerator Mass Spectrometry Laboratory

Further reading





External links