segunda-feira, 4 de janeiro de 2016

T94. Radiocarbon dating

RADIOCARBON DATING by Fredric M. Menger, Emory Univ., Atlanta, GA

Radiocarbon dating (also referred to as carbon-14 dating) allows someone to tell the age of carbon-containing objects up to 50,000 years old. Its impact on archeology has been transformative, winning the developer, William Libby, a 1960 Nobel Prize. The method demonstrates how a great idea can be relatively simple in scope. The facts are these:

1) When atmospheric nitrogen-14 is hit by cosmic rays, radioactive carbon-14 is produced in tiny amounts. Thus, ratio of carbon-14 to non-radioactive carbon-12 (which is the ordinary carbon in Nature) is 1.3 x 10-12.

2) Now the half-life of carbon-14 is 5,730 years. That means if you have a gram of carbon -14, it will become half-a-gram in 5,730 years, a quarter-of-a–gram in 11,460 years etc. Now, and this is key, the carbon-14 level is assumed to remain constant over thousands of years because as carbon-14 decays, more of it is formed from the ever-present cosmic rays and nitrogen. In other words, an equilibrium is attained.

3) Carbon-14 is oxidized by atmospheric oxygen to carbon dioxide, CO2. The CO2, in turn, is incorporated into plants and, from there, into animal life. Thus, at the time that carbon-14 is taken in, the carbon-14/carbon-12 ratio in plants and animals equals the equilibrium value of 1.3 x 10-12.

4) As thousands of years pass by, the carbon-14 in the organisms decays, and the carbon-14/carbon-12 ratio decreases. For example, after 5,730 years following death of an organism, the carbon-14 content will decrease by one-half because no external carbon-14 is taken in during this time. Clearly, one can calculate the age of a carbon-bearing item from the intensity of the carbon-14 radioactivity relative to the equilibrium value.

Note: Atmospheric C-14/C-12 remains constant because as C-14 decays, it is replenished by cosmic rays.

Consider, for example, a fossil forest discovered in Wisconsin, USA. Its destruction is believed to have been caused by the southward movement of ice during the most recent ice age. Samples from the forest showed that the forest was destroyed between 13,730 and 13,550 years ago …… important numbers in our understanding of glaciation dates in United States.

Challenging thought question: Two human activities over past decades would cause small errors in radiocarbon dating if the current equilibrium carbon-14/carbon-12 ratio were used. What are they?

(1) The burning of oil and coal (materials sufficiently old that they contain little detectable carbon-14 as it has all decayed) reduces the carbon-14/carbon-12 ratio in the atmosphere.
(2) Above-ground nuclear testing creates carbon-14 (almost doubling the amount of carbon-14 in the atmosphere when such tests were finally banned after 1963).

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