Dating Techniques, Radiometric

Radiometric dating became a possibility with Becquerel's discovery in 1896 of natural radioactivity. Rutherford postulated that radioactivity could be used to determine the age of the Earth. His and Soddy's discovery (1902) of the transmutation of the atom became the basis for understanding exponential decay and the evolution of decay products (“daughter” elements). Age estimates for the Earth that had been determined by rate of heat loss (Lord Kelvin) now had to make allowances for the heat energy associated with radioactive decay. Thus, scholars were able to argue for great antiquity of the rocks on Earth. It was really with the advent of data collection technologies after World War II that the radiometric dating field began to develop with rapidity.

Radiometric dating must be viewed as having two forms: (1) techniques that rely on the decay of an isotope of an element, the production and decay of daughter decay products (radiocarbon dating, potassium-argon, argon-argon, and uranium-lead, uranium series) and (2) the techniques that rely on the crystal damage that is generated by the ionizing radiation generated by the decay of radioactive elements (thermoluminescence, electron spin resonance, and fission track).

All radiometric-dating determinations are a function of a statistical distribution of one or more sets of decay data that must be viewed as a probability result, approximating a particular age with an error attached to it. These errors in age determinations are usually expressed as standard deviations from a mean age value. These standard deviations are probability statements that a determined age actually will fall somewhere within the age distribution defined by the standard deviation. One standard deviation, often termed one sigma,means that 68% of the time, the determined age will fall between the range defined by that standard deviation. In the same manner, two-and three-sigma standard deviations mean the determined age will fall somewhere between the defined age range 95% and 99.6%,respectively. Therefore, it is immediately apparent that it would be a misnomer to suggest that these radiometric dating techniques were methods of absolute dating.

Radiocarbon-14 is the best known of the radiometric techniques and is in fact an established method that relies only on the decay of an isotope (14C) formed from the outer-atmosphere comicray-generated neutron bombardment of Nitrogen-14 (14N) without reference to daughter production. In this respect, it is the most straightforward of the radiometric dating techniques. Living organisms incorporate 14CO2 and maintain an equilibrium until death, at which time the radiocarbon clock begins to tick as the 14C decays exponentially with a rate known as theLibby half-life (5,568 years). A simple ratio measurement of the amount of 14C remaining versus the amount present when the organism left the living biomass yields a radiocarbon age, which can be converted to calendrical years with a dendrochronological curve that corrects for the cosmic ray fluctuations that have taken place in the past. This ratio of original-to-remaining 14C is obtained in one of two ways: (1) The direct decay of the 14C back to 14 yields a beta ray that can be detected in a shielded Geiger counter; and (2) the actual counting of the individual 14C atoms present in a sample compared with the stable 12C in that sample and an accompanying standard. The atom-counting method affords an advantage in some dating situations (for example, shorter counting times, smaller sample sizes, no cosmic ray backgrounds, and the extension of the age range from less than 40,000 years to 70,000 years). Potassium-argon (40K/40Ar) and argon-argon dating (40Ar/39Ar), uranium-lead dating (U/Pb), and uranium series dating are all dependant upon both the exponential decay of a “parent” radioactive isotope and the buildup of one or more “daughter” isotopes, which may or may not themselves be radioactive. The ratio of the “daughter” to the “parent” permits an age estimate to be made. These techniques tend to have an age range orders of magnitude greater than radiocarbon (for example, age of the Earth), because they use half-lives that are very long in comparison to radiocarbon (t½ for–40K: 1.28 × 109 yrs; 238U: 4.47 × 109 yrs; 235U:7.038 × 108 yrs; 232Th: 1.41 × 1,010 yrs), though uranium series disequilibrium dating has a dating range of a few days to about 20 million years.

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