Determination of the age of fossils
Age of fossils is determined by Radioactive Clock Method, or Radiometric dating, which was devised by Boltwood in 1907 and later by Rutherford in 1955. The rate of disintegration of radioactive material is always constant and is not affected by the environmental factors. The time taken by 50% of the radioactive material to disintegrate into stable element is known as its half-life. If we know the half-life of an element, then the age of the fossil can be calculated by finding out the ratio of radioactive element and its stable daughter element using scintillation counter or Geiger-Muller counter.
For example, half-life of Uranium238 is 4.5 billion years. With this rate of decay, one million grams of U238 would yield about 1.7400 grams of Pb206 in one year. OR, in one year one gram of U238 would yield 1/7,400,000,000 grams of Pb206. The radioactive isotope U238 disintegrates into its stable isotope, Pb206 by emitting 8 alpha particles and 6 beta emissions through a chain of daughter radioactive elements. This method is also explained in the following table.
Different radioactive materials have different half-life and, therefore, age of recent as well as very old fossils can be determined by selecting the appropriate radioactive element, provided that the element is present in that particular rock.
This method was devised by W.F. Libby in 1956 by recognising the fact that all living organisms have C14, the radioactive isotope of carbon in their bodies, which starts disintegrating after their death into C12 or N14 by the release of an electron, as it is no longer replenished after the death of animal or plant. This radioactive carbon is taken up by plants in the form of carbon dioxide, and animals get it into their bodies from plants.
Since after death, animals and plants do not replenish C14 through food, it only decays at a constant rate. The rate of decay is found out by counting the number of beta particles emitted by C14 per unit time, using Scintillation counter, Wilson-cloud chamber or Geiger-Muller counter (that use CO2 gas) and then feeding the data into a computer.
When there is water seepage through the bones, fluorine in water combines with calcium in bones to form fluorapatite, whose proportion in the bones can be detected to find out the age of the fossil.
This method works on collagen degradation in the bones and accumulation of other materials such as fluorine. How much nitrogen is retained in the bone gives an idea of the age of the fossil.
Bones buried in soil absorb uranium at a constant rate and longer they lay in those strata more uranium they will absorb. Since uranium is radioactive, its presence in the bones can be detected using scintillation counter.
Trace amount of Uranium238 impurities are commonly found in nature and spontaneous fission of this radioactive element produces small, permanent, microscopic damage trails in the insulating solids through long periods. Simple count of numbers of such trails gives the age of the fossils.
Potassium is abundant in nature and its isotope K39 comprises over 93% of the total, while K40 is only 0.0118% and K41 6.8%. K40 decays into its daughter isotopes, viz. Ca40 by beta decay and A40 by K capture, in a half life of 1.3 billion years. The ratio of the daughter isotopes can be detected in a mass spectrophotometer. This method is used to detect the age of very old fossils but theoretically potassium rich samples as young as 5,000 year old can also be dated.
Amino acid racemization dating
The method is based on the fact that all animals have L-amino acids in their proteins which after death transform by racemization into D-amino acids, whose proportion increases with time. The ratio of the two types of amino acids will give the age of the skeletal material in the fossil.
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