Basics of Radiocarbon Dating

Kumud Kumari and Brajesh Kumar
Patna, India.


The term “radiocarbon” is commonly used to denote 14c, an isotope of carbon which is radioactive with a half-life of about 5730 years. 14 is produced by cosmic rays in the stratosphere and upper troposphere. It is then distributed throughout the rest of the troposphere, the oceans, and Earth’s other exchangeable carbon reservoirs. In the surface atmosphere, about one part per trillion (ppt) of carbon is 14C.
All organisms absorb carbon from their environment. Those that absorb their carbon directly or indirectly from the surface atmosphere have about 1 ppt of their carbon content as 14. Such organisms comprise almost all land dwelling plants and animals. (Other organisms—e.g. fish—have slightly less of their carbon as 14; this affects how radiocarbon dating works, and there are methods of adjusting for it.) When an organism dies, carbon stops being absorbed. Hence after 5730 years, about half of its 14 will have radioactively decayed (to nitrogen): only about 0.5 ppt of the carbon of the organism’s remains will be 14. And if the carbon of the remains is found to be 0.25 ppt 14, then the organism would be assumed to have died about 11 460 years ago. Thus, a simple calculation can find the age, since death, from any 14c concentration. (Remains older than about 50 000 years, however, have a 14C concentration that is in practice too small to measure; so they cannot be dated via 14C..) Ages are conventionally reported together with the standard deviation of the laboratory 14C. measurement, e.g. 900±25 14C. BP ( 14C. -dated, years Before Present). This should be doubled to obtain a 95%-confidence interval, e.g. 850–950 14C. BP. (The true range of 95%-confidence, though, will often be larger than this, due to non laboratory sources of error—e.g. the admixture of impurities with the remains.) Although a tree may live for hundreds, even thousands, of years, each ring of a tree absorbs carbon only during the year in which it grows. The year in which a ring was grown can be determined exactly (by counting); so radiocarbon dating can be tested by measuring the 14C. concentrations in old tree rings. Such testing found errors of up to several centuries. It turns out that the concentration of 14C. in the surface atmosphere has not been a constant 1 ppt, but has varied with time. Thus the simple calculation of age from 14C concentration is atom itself. Eventually, a particle is emitted from the carbon 14 atom, and carbon 14 disappears. Most of the carbon on Earth exists in a slightly different atomic form, although it is chemically speaking, identical to all carbon.
In the 1940s, scientists succeeded in finding out how long it takes for radiocarbon to disappear, or decay, from a sample of carbon from a dead plant or animal. Willard Libby, the principal scientist, had worked in the team making the nuclear bomb during World War 2, so he was an expert in nuclear and atomic chemistry. After the war he became very interested in peaceful applications of atomic science. He and two students first measured the "half-life" of radiocarbon. The half-life refers to the amount of time it takes for half the radiocarbon in a sample of bone or shell or any carbon sample to disappear. Libby found that it took 5568 years for half the radiocarbon to decay. After twice that time (about 11000 years), another half of that remaining amount will have disappeared. After another 5568 years, again another half will have disappeared. You can work out that after about 50 000 years of time, all the radiocarbon will have gone. Therefore, radiocarbon dating is not able to date anything older than 60 or 70 000 years old. The job of a radiocarbon laboratory is to measure the remaining amounts of radiocarbon in a carbon sample. This is very difficult and requires a lot of careful work to produce reliable dates.
  unreliable. Tree rings, though, also provide a solution to this problem. The concentration of 14C in an organism’s remains can be compared with the concentrations in tree rings; the tree rings that match, within confidence limits, give the years in which the organism could have plausibly died. Ages determined this way are called “calibrated 14C ages”; others are called “uncalibrated 14C ages”, or simply “14C ages”, and continue to be reported as “14C BP”. (Calibration via tree rings, though, does not extend for 50000 years, only several thousand. Other ways of calibrating are therefore being developed.) Calibrated 14C ages are generally greater than uncalibrated 14C ages, with the differences increasing with age.

How was radiocarbon dating developed? The radiocarbon method was developed by a team of scientists led by the late Professor Willard F. Libby of the University of Chicago after the end of World War 2. Libby later received the Nobel Prize in Chemistry in 1960 for the radiocarbon discovery. Today, there are over 130 radiocarbon dating laboratories around the world producing radiocarbon dates for the scientific community. The C14 method has been and continues to be applied and used in many, many different fields including hydrology, atmospheric science, oceanography, geology, palaeoclimatology, archaeology and biomedicine.


How does radiocarbon dating work?

All plants and animals on Earth are made principally of carbon. During the period of a plant's life, the plant is taking in carbon dioxide through photosynthesis, which is how the plant makes energy and grows. Animals eat plants, and some eat other animals in the food chain. Carbon follows this pathway through the food chain on Earth so that all living things are using carbon, building their bodies until they die.

A tiny part of the carbon on the Earth is called Carbon-14 (C14), or radiocarbon. It is called 'radio'-carbon, because it is 'radioactive'. This means that its atomic structure is not stable and there is an uneasy relationship between the particles in the nucleus of the What kind of things can you date using radiocarbon? Because carbon is very common on Earth, there are alot of different types of material which can be dated by scientists. Below is a list of the different kinds of materials which can be dated:

• Charcoal, wood, twigs and seeds.
• Bone.
• Marine, estuarine and riverine shell.
• Leather.
• Peat
• Coprolites (samples of preserved faeces).
• Lake muds (gyttja) and sediments.
• Soil.
• Ice cores.
• Pollen.
• Hair.
• Pottery.
• Metal casting ores.
• Wall paintings and rock art works.
• Iron and meteorites.
• Bird eggshell.
• Corals and foraminifera.
• Blood residues.
• Textiles and fabrics.
• Paper and parchment.
• Fish remains.
• Insect remains.
• Resins and glues.
• Antler and horn.
• Water.
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