Radiometric Methods of Dating Fossils

Abstract

Today, most of the methods utilized for chronometric dating of fossils are radiometric. Radiometric dating, in general, refers to the dating of material by using the known rate at which certain radioactive isotopes decay, or at what rate there are collective changes due to radioactivity. Even though isotopes of an element can be different when it comes to atomic mass, the atomic number of the isotope is always the same.

Radioactive elements decay at unique rates, dependant on the isotope. This rate of decay is known as half-lives, it is the time necessary for ½ of the atoms to decay in a particular element. The decay follows a geometric scale, in that in the first half-life of an element, ½ of the atoms decay, yet in the second half-life, ½ of those remaining decay, meaning a ¼ of the original atoms decay, and so forth. By measuring this decay, and knowing the half life of an element, scientists can date a sample.

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Radiometric Methods of Dating Fossils

Introduction:

Today, most of the methods utilized for chronometric dating of fossils are radiometric. Radiometric dating, in general, refers to the dating of material by using the known rate at which certain radioactive isotopes decay, or at what rate there are collective changes due to radioactivity. Even though isotopes of an element can be different when it comes to atomic mass, the atomic number of the isotope is always the same.

Radioactive elements decay at unique rates, dependant on the isotope. This rate of decay is known as half-lives, it is the time necessary for ½ of the atoms to decay in a particular element. The decay follows a geometric scale, in that in the first half-life of an element, ½ of the atoms decay, yet in the second half-life, ½ of those remaining decay, meaning a ¼ of the original atoms decay, and so forth. By measuring this decay, and knowing the half life of an element, scientists can date a sample.

Carbon-14 Dating:

One of the most common forms of radiometric dating applied today is radiocarbon, or Carbon-14 dating. This method is used in the dating of organic materials. Cosmic radiation is constantly assailing the Earth’s atmosphere, and when it hits an atom of nitrogen, it alters the nucleus, changing the atom into hydrogen and Carbon-14. These two atoms then bond with oxygen and form carbon dioxide, which is utilized by plants in photosynthesis. Animals then eat these plants, then other animals eat these animals, and the spread of Carbon-14 through all living things is completed.

When an organism is alive, it consumes Carbon-14 at the same ratio as what is in the Earth’s atmosphere. However, once an organism dies, they no longer consume Carbon-14, and the Carbon-14 that was in their body begins to decay into the more stable Carbon-12. It is this rate of decay that is measured and used to date the organism.

In the conventional testing of Carbon-14, the process involved the scientist burning a sample of the material being tested in a closed vessel, with nothing else but pure oxygen. Most of the carbon from the sample combines with the oxygen, during the combustion process, and

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forms carbon dioxide, which is cooled to a liquid state, and then placed into a lead shielded box. A Geiger counter is then used to measure the radioactivity of the Carbon-14 atoms. The release of beta particles from the Carbon-14 decay is measured over a specified period of time. As older samples have less Carbon-14, less beta particles will be measured.

In addition to this combustion method of testing, scientists have recently utilized an accelerator mass spectrometer in radiocarbon dating. An accelerator mass spectrometer is typically used by physicist to study matter and energy, however, in this case, the instrument is utilized to actually count the individual Carbon-14 atoms. With a half-life of approximately 5,720 years for Carbon-14, the traditional combustion method of dating is usually only used for samples younger than 50,000 years old, however, with the use of an accelerator mass spectrometer it is possible to date organic materials as old as 100,000 years. (“Radiocarbon”, 2004)

In addition to the limitation on age, due to Carbon-14’s fairly short half-life, there are other limitations as well. First is the potential for contamination of the sample that will give an inaccurate dating. This contamination can occur from substances as ubiquitous as exhaust emissions from vehicles in the air, or the burning of other fossil fuels. This type of contamination can result in an erroneously older date. Oils from the hands of someone who has handled the sample or even tobacco smoke can result in an erroneously younger date. (“Radiocarbon”, 2004)

Another limitation is the adjustment needed due to the fact that cosmic radiation has not entered the Earth’s atmosphere at a constant rate, therefore, levels of Carbon-14 have risen and fallen accordingly. Although some adjustments have been made using items such as seeds in historically dated tombs, there is still chance for error. In an effort to calibrate Carbon-14 dating to take this into account, “more than 250 carbon-14 accelerator mass spectrometry dates of terrestrial macrofossils from annually laminated sediments from Lake Suigetsu (Japan) provided a first atmospheric calibration for almost the total range of the radiocarbon method (45,000 years before the present).” (Kitagawa, 1998)

However, other items need to be taken into account. There is also a question of whether or not all plants take in Carbon-14 at the same rate, which would require additional calibration.

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