Archaeology, carbon-14 dating

Carbon Dioxide. Carbon dioxide, also a colorless and odorless gas, makes up about 0.03% of dry air. Carbon dioxide is introduced into the atmosphere by several natural processes it is released from volcanoes, from burning organic matter, and from living animals as a byproduct of the respiration process. It is for this latter reason that carbon dioxide plays a vital role in the carbon cycle (see Fig. 62), which makes possible one of the more important scientific tools in archaeology, radiocarbon dating (see Textbox 52). 

Harrison, R. Frink, D.S. (2000) The OCR carbon dating procedure in Australia New dates from Wilinyjibari Rockshelter, southeastern Kimberley, Western Australia. Australian Archaeology 51, 6-15. 

Many archaeological artifacts are dated using carbon-dating methods. The ratio of carbon 14 to carbon 12 in wooden beams, tool and weapon handles, textiles, and charcoal from cooking fires can be used to estimate the ages of the artifacts. 

The extent of this process can be calculated from the content of the radioactive isotope C (half-life 5.5 X 10 years). The same concept is used in radio-carbon dating of archaeological or geological objects. 

Carbon has seven isotopes. In 1961 the International Union of Pure and Applied Chemistry adopted the isotope carbon-12 as the basis for atomic weights. Carbon-14, an isotope with a half-life of 5715 years, has been widely used to date such materials as wood, archaeological specimens, etc. 

A novel technique for dating archaeological samples called ammo acid racemiza tion (AAR) IS based on the stereochemistry of ammo acids Over time the configuration at the a carbon atom of a protein s ammo acids is lost m a reaction that follows first order kinetics When the a carbon is the only chirality center this process corresponds to racemization For an ammo acid with two chirality centers changing the configuration of the a carbon from L to D gives a diastereomer In the case of isoleucme for example the diastereomer is an ammo acid not normally present m proteins called alloisoleucme... 

The constant half-life of a nuclide is used to determine the ages of archaeological artifacts. In isotopic dating, we measure the activity of the radioactive isotopes that they contain. Isotopes used for dating objects include uranium-238, potassium-40, and tritium. However, the most important example is radiocarbon dating, which uses the decay of carbon-14, for which the half-life is 5730 a. 

Rink, W.J. and Schwarcz, H.P. 1995 Tests for diagenesis in tooth enamel ESR dating signals and carbonate contents. Journal of Archaeological Science 22 251-255. 

Some radioisotopes are continuously being produced by the bombardment of atoms on the surface of the earth or in its atmosphere with extraterrestrial particles or radiation. One of these is carbon-14, also known as radiocarbon, which is widely used for dating archaeological materials (see Textbox 55). Many radioisotopes that are not primordial or are not created by natural processes are now produced artificially using specialized equipment many of the "artificial" isotopes are of use for probing and analyzing materials. 

Frink, D. S. (1995), Application of the oxidizable carbon ratio (OCR) dating procedure and its implications for pedogenic research, in Pedological Perspectives in Archaeological Research, Soil Science Society Special Publication, Vol. 44, Soil Science Society of America, Madison, WI. 

Why then, is such a complicated and expensive set up necessary AMS combines mass spectrometric features with efficient discrimination of isobaric and molecular interferences. Therefore, it can detect and quantify atomic species of very low abundance. In the case of 14C dating, before AMS was utilized, about 1 g of carbon was needed to date an archaeological item. One gram of fresh carbon contains about 6 x 1010 14C atoms, of which 14 decay per minute. To get 0.5% statistical precision using decay counting, a 48 h acquisition time is necessary. The same result can be obtained with AMS in about 10 min and with only 1 mg of carbon. 

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