Carbon Dating
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A method of estimating the ages of archaeological specimens
of biological origin. As a result of cosmic radiation a small
number of atmospheric nitrogen nuclei are continuously being
transformed by neutron bombardment into radioactive nuclei of
carbon-14:
147N + n ->146C + P
Some of these radiocarbon atoms find their way into living trees and other
plants in the form of carbon dioxide, as a result of photosynthesis. When
the tree is cut down photosynthesis stops and the ratio of radiocarbon
atoms to stable carbon atoms begins to fall as the radiocarbon decays. The
ratio of 14C/12C in the specimen can be measured and enables the time that has
elapsed since the tree was cut down to be calculated. The method has been
shown to give consistent results for specimens up to some 40,000 years old,
though its accuracy depends upon assumptions concerning the past
intensity of the cosmic radiation. The technique was developed by Willard
F. Libby (1908-80) and his coworkers in 1946-47.
Has the level of background cosmic radiation varied significantly over the last 40,000 years? If the cosmic radiation level had been significantly higher, more carbon-14 would have been produced, and test objects would look younger than they actually were. On the other hand, if the level of cosmic background radiation were less, objects would look older than they actually were because less carbon-14 would have been produced for ingestion by biological entities.
The major source of cosmic radiation that impinges on the earth is the sun. The question now becomes, has the sun shown any significant variation in energy output over the last 40,000 years? Model calculations conclude that
the Sun becomes 10 percent brighter every billion years, but such a change is insignificant over a mere 40,000 year timescale (0.0016% increase). Other than that the Sun is a very stable source of energy; its radiative output, called the solar constant, is 137
ergs per square metre per second (ergs/m /sec), or 1.98 calories per square centimeter per
minute (cal/cm /min), at the Earth and varies by no more than 0.1 percent.
Carbon dating, or radiocarbon dating, is a widely used method for determining the age of an object containing organic material by measuring the amount of carbon-14 it contains. However, several issues and limitations are associated with this technique:
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Contamination:
- Modern Contamination: Introduction of modern carbon (e.g., from handling or storage) can skew results to appear younger.
- Ancient Contamination: Addition of old carbon (e.g., from groundwater or geological processes) can make samples appear older.
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Calibration:
- The amount of carbon-14 in the atmosphere has not been constant over time due to factors such as solar activity and human activities (e.g., burning fossil fuels, nuclear tests). Calibration curves, which are developed using dendrochronology (tree rings) and other methods, are needed to adjust raw radiocarbon dates to calendar dates.
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Sample Size and Quality:
- Smaller samples or degraded samples can yield less accurate results. The amount of carbon-14 that can be measured diminishes over time, making it harder to date very old samples accurately.
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Assumptions of Initial Conditions:
- Radiocarbon dating assumes that the initial ratio of carbon-14 to carbon-12 in the organism was the same as the current ratio in the atmosphere. This may not always be accurate due to local variations or changes in atmospheric composition over time.
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Limitations of Age Range:
- The method is most effective for dating objects up to about 50,000 years old. Beyond this range, the amount of carbon-14 remaining is often too small to measure accurately.
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Reservoir Effects:
- Marine organisms or those that consume significant amounts of seafood can appear older than they are because the carbon in the oceans is older than atmospheric carbon (marine reservoir effect).
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Isotope Fractionation:
- Different forms of carbon (isotopes) are absorbed at different rates by different materials and organisms. Corrections must be made for these fractionation effects to ensure accurate dating.
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Environmental Factors:
- Natural phenomena such as volcanic eruptions, which release large amounts of carbon dioxide (devoid of carbon-14), can dilute the carbon-14 concentration in the atmosphere, affecting the dating accuracy.
Addressing these problems typically involves cross-checking carbon dating results with other dating methods and using calibration curves to correct for variations in atmospheric carbon-14 levels. Despite these challenges, radiocarbon dating remains a valuable tool in archaeology, geology, and other fields when used carefully and with an understanding of its limitations.
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Potassium-Argon Dating
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A dating technique for certain rocks that depends on the decay of the radioisotope potassium-40 to argon-40,
a process with a half-life of 1.27 x 1010 years. It assumes that all the argon-40 formed in the potassium-bearing mineral accumulates within it and that all the argon present is formed by the decay of potassium-40. The mass of argon-40 and potassium-40 in the sample is estimated and the sample is then dated from the equation:
40Ar = 0.1102 40K(ect-1)
where c is the decay constant and t is the time in years since the mineral cooled to about 300Cwhen the 40Ar became trapped in the crystal lattice. The method id effective for micas, feldspar, and some other minerals.
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