Absolute dating is a dating method that allows the assignment of a specific date to an archaeological or palaeontological site or artifact. Most absolute dating techniques utilize predetermined rates of radioactive decay to calculate the elapsed period of time.
In contrast to relative dating techniques (i.e. stratigraphy), absolute dating is a quantitative measurement allowing determination of a specific time, rather than relative. Absolute dating provides a numerical age for the material tested, while relative dating can only provide a sequence of age.
Radioactive decay of uranium was first discovered in 1896 by Henry Becquerel, a French physicist. By 1905, the British physicist Lord Rutherford made the first clear suggestion for using radioactivity as a tool for measuring geologic time directly; shortly thereafter, in 1907, Professor B. B. Boltwood, radiochemist of Yale Uniyersity, published a list of geologic ages based on radioactivity. Although Boltwood's ages have since been revised, they did show correctly that the duration of geologic time would be measured in terms of hundreds-to-thousands of millions of years.
The next 40 years was a period of expanding research on the nature and behavior of atoms, leading to the development of nuclear fission and fusion as energy sources. A byproduct of this atomic research has been the development and continuing refinement of the various methods and techniques used to measure the age of Earth materials. Precise dating has been accomplished since 1950.
- Main Article: Radiometric dating
A chemical element consists of atoms with a specific number of protons in their nuclei but different atomic weights owing to variations in the number of neutrons. Atoms of the same element with differing atomic weights are called isotopes. Radioactive decay is a spontaneous process in which an isotope (the parent) loses particles from its nucleus to form an isotope of a new element (the daughter). The rate of decay is conveniently expressed in terms of an isotope's half-life, or the time it takes for one-half of a particular radioactive isotope in a sample to decay. Most radioactive isotopes have rapid rates of decay (that is, short half-lives) and lose their radioactivity within a few days or years. Some isotopes, however, decay slowly, and several of these are used as geologic clocks.
The parent isotopes and corresponding daughter products most commonly used to determine the ages of ancient rocks are listed below:
|Parent Isotope||Stable Daughter Product||Currently Accepted Half-Life Values|
|Uranium-238||Lead-206||4.5 billion years|
|Uranium-235||Lead-207||704 million years|
|Thorium-232||Lead-208||14.0 billion years|
|Rubidium-87||Strontium-87||48.8 billion years|
|Potassium-40||Argon-40||1.25 billion years|
|Samarium-147||Neodymium-143||106 billion years|
Uniformitarian geologists consider this form of dating strong evidence that the Earth is billions of years old. However, research by creationists has revealed a large number of problems with radiometric dating. In some cases such as Carbon-14 dating, radioactive dating actually gives strong evidence for a young Earth, while other methods such as K-Ar dating and Isochron dating are based on faulty assumptions and are so unreliable as to be useless.
- Main Article: Carbon-14 dating
Carbon-14 dating is a radiometric dating technique used to deduce the approximate age of organic remains by measuring the quantity of the isotope ¹⁴C in the sample and comparing it with the current atmospheric level. The usual isotope of carbon found in living organisms, ¹²C, is stable, while ¹⁴C is not stable. It is formed when cosmic radiation strikes ¹⁴N (Nitrogen), converting it into ¹⁴C, and it decays back into ¹⁴N, with a half-life of 5730 years.
- Main Article: Isochron dating
Scientists have realized that there are difficulties in dealing with the assumptions of radiometric dating. Isochron dating has been developed in an attempt to solve such problems. According to theory, the sample starts out with daughter isotopes present at constant ratios in relation to one another, but with the parent isotope, the ratio is arbitrary. As a result it forms a straight horizontal line on a graph. As the parent decays to daughter, the ratios change and the straight line remains but becomes angled. The slope of the line equals the number of half-lives the parent isotope has passed since solidification.
- Main Article: Concordia dating
Concordia dating is a form of uranium/lead dating that uses a concordia diagram. The theory is that when zircons crystallize they lose all of their lead and as long as the crystal remains closed its lead/uranium ratios should follow a predictable trend. It is further theorized that since all isotopes of the same element are chemically identical, they should be removed in proportional amounts, forming a straight line on the concordia diagram, that crosses the concordia curve at both the crystallization and the contamination date. Loss of uranium moves the point up and to the right, while a loss of lead moves the point down and to the left.
- Main Article: Fission-track dating
Fission-track dating involves counting the damage tracks left by fragments of the spontaneous fission of uranium-238. The spontaneous fission of ²³⁸U has a known rate, and as such the number of tracks is theoretically related to the age of the sample. Because fission-track dating requires a manual count of the fission tracks, the process is more prone to human error and bias than other radiometric dating methods. This problem is made worse because other types of crystal defects can easily be counted as fission tracks.
- Radiometric Time Scale by the U.S. Geological Survey