Dating radiometric

Radiometric dating is a method that scientists use to measure the age of things. The most widely known form of radiometric dating is that based on carbon-14. It works like this. Carbon-12 is the most common form of carbon, and carbon-14 is what is called an isotope of it. (An isotope is a variation of the normal atom of an element, in that it has more or less neutrons than the standard atom.) Carbon-12 has six protons and six neutrons at its nucleus, and is therefore said to have an atomic weight of 12. The component of an atom that determines its character is the number of electrons in orbit around its nucleus. In carbon s case there are six. 

The standard measurement in radioactive decay is called a half-life. This is how long it would take a certain amount of a substance to decay to half its weight (a gram to half a gram, for instance). The half-life of carbon-14 is 5,730 years (plus or minus 40 years). To measure the age of things, scientists make the supposition that the ratio of carbon-12 to carbon-14 in the atmosphere has remained constant over time. 

That all sounds good in theory, but the problem lies in the original supposition that the ratio of carbon-12 to carbon-14 in the atmosphere has remained constant. This has not been the case. Remember that carbon-14 does not come from carbon-12, but rather from nitrogen in the atmosphere. With the advent of the Industrial Revolution and the burning of fossil 

C is absorbed by living things but lost after death. 

it has been discovered that plants discriminate against carbon dioxide containing carbon-14 atoms, so that the absorption rate is lower than for normal carbon-12. Additionally, the rate of carbon-14 formation in the atmosphere has fluctuated due to Earth s weakening magnetic field. And finally, the Hood would have made a huge difference in that a vast quantity of carbon, in the form of plant and animal life, was pulled out of the biosphere and buried under the sediment deposited as the waters receded. 

Because the half-life of any particular nuclide is constant, the half-life can serve as a nuclear clock to determine the age of objects. The method of dating objects based on their isotopes and isotope abundances is called radiometric dating. 

When carbon-14 is used in radiometric dating, the technique is known as radiocarbon dating. The procedure is based on the formation of carbon-14 as neutrons created by cosmic rays in the upper atmosphere convert nitrogen-14 into carbon-14 ( FIGURE 21.7). The reacts with oxygen to form C02 in the atmosphere, and this labeled CO2 is taken up by plants and introduced into the food chain through photosynthesis. This process provides a small but reasonably constant source of carbon-14, which is radioactive and undergoes beta decay with a half-life of 5715 yr  

Because a living plant or animal has a constant intake of carbon compounds, it is able to maintain a ratio of carbon-14 to carbon-12 that is nearly identical with that of the atmosphere. Once the organism dies, however, it no longer ingests carbon compounds to replenish the carbon-14 lost through radioactive decay. The ratio of carbon-14 to carbon-12 therefore decreases. By measuring this ratio and comparing it with that of the atmosphere, we can estimate the age of an object. For example, if the ratio diminishes to half that of the atmosphere, we can conclude that the object is one half-life, or 5715 yr old. 

This method cannot be used to date objects older than about 50,000 yr because after this length of time the radioactivity is too low to be measured accurately. 

Cosmic rays (lar gely protons) enter the atmosphere and collide with atoms, creating neutrons 

When carbon-14 is used in radiometric dating, the technique is known as radiocarbon dating. The procedure is based on the formation of carbon-14 as neutrons created by 

How does C02 become incorporated into the mammalian food chain  

Nitrogen atoms capture a neutron and emit a proton, forming 

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Moorbath S, Taylor PN, Orpen JL, Treloar P, Wilson JF (1987) First direct radiometric dating of Archaen stromatolitic limestone. Natnre 326 865-867... 

Bischoff JL, Shamp DD, Arambnm A, Arsuaga JL, Carbonell E, Bermudez de Castro JM (in press) The Sima de Los Hnesos Hominids date to beyond U/Th equilibrium (>350ky) and perhaps to 400-500 ky new radiometric dates. J Arch Sci... 

Figure 4. The decay schemes of 23SU and 235 U, showing the longer-lived isotopes used in radiometric dating of Pleistocene and Holocene deposits.

Radiocarbon dating is also called radiometric dating or radiochemical dating. ... 

Lead, Sr, C and O isotope compositions were measured in carbonates of the dolomitic unit and in chalcocite concentrate. U-Pb and K-Ar radiometric dates were obtained from basement rocks and mafic dykes. The analysis were undertaken and The University of Sao Paulo under supervision of Professor C. Tassinari (Tassinari 2002). 

Radioactive isotopes can be classified as being either artificial or natural. Only the latter are of interest in geology, because they are the basis for radiometric dating... 

One can apply the formalism discussed above to a wide variety of systems to produce a radiometric date. In this book, we will use the word date to mean the time calculated from the ratio of a radioactive isotope and its daughter isotope using the equation for radioactive decay. An age is the time between a natural event and the present. A date becomes a valid age when the conditions described in the previous paragraph are met. This terminology, suggested by Faure (1986), is not always used in the literature, where age and date are often used interchangeably. But there is value to the distinction because it helps a reader understand which numbers are significant. 

As with all radiometric dating systems, the K- Ar system will only produce a valid age if the basic assumptions of radiometric dating are satisfied. In particular, a valid age can only be obtained if the samples remained closed to diffusion of argon either into or out of the system. Two different approaches can be taken to date an object with the 40K-40Ar system. 

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