Radiocarbon Dating -----Natural Clock Yanhua li. “seldom has a single discovery in chemistry had...
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Radiocarbon Dating -----Natural Clock
Yanhua li
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• “seldom has a single discovery in chemistry had such an impact on the thinking of so many fields of human endeavor. Seldom has a single discovery generated such wide public interest”-----
one of the scientists who proposed Libby for the Nobel laureate characterized the significance of the C14 method.
W.F.Libby, Nobel Prize for Chemistry in 1960
Radiocarbon after four decades, Springer-Verlag,1992
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Outline
• Principles
• Sample and measurement
• Application
• Advantages and limitations
• Future developments
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Principle • First developed by J. R. Arnold and W. F.
Libby in 1949;• In laboratory, many studies of neutrons
produced by cosmic radiation on all the ordinary elements and especially on nitrogen and oxygen ( the constituents of the air)
• Oxygen is extraordinarily inert, nitrogen is reactive; dominant reaction
• N14 +n=C14+H1
Radiocarbon dating by W.F.Libby,1955
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Url: http://id-archserve.ucsb.edu/Anth3/Courseware/Chronology/Movies/Reaction.html
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Url: http://id-archserve.ucsb.edu/Anth3/Courseware/Chronology/Movies/Reaction.html
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http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/carbondating.htm
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• The unstable isotope C-14 is brought to Earth. • attached to complex organic molecules through
photosynthesis in plants and becomes part of their molecular makeup.
• Animals eating those plants in turn absorb Carbon-14 as well as the stable isotopes.
• This process of ingesting C-14 continues as long as the plant or animal remains alive.
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• The C-14 within an organism is continually decaying into stable carbon isotopes, but since the organism is absorbing more C-14 during its life, the ratio of C-14 to C-12 remains about the same as the ratio in the atmosphere.
• When the organism dies, the ratio of C-14 within its carcass begins to gradually decrease. The rate of decrease is 1/2 the quantity at death every 5,730 years, which is the half-life of C-14.
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Radiocarbon dating by W.F.Libby,1955
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• Samples and measurements Sampling order:
1.Charcoal or charred organic material such as heavily burned bone;
2. Well-preserved wood;
3.Grasses, cloth, and peat;
4. Well-preserved antler and similar hairy structures
5. Well-preserved shell;
Sample should contain the original carbon atoms at the
time it died. large molecular structures, such as cellulose molecules, resist putrefaction and chemical alteration well.
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1) Conventional measuring
CO2 gas proportional counting
organic material is converted to CO2 by combustion, electrons originating from the decay of radiocarbon are collected and counted at the anode.
C14=β-+N14
Liquid scintillation counting (Benzene)
Radionuclides interact with the solvent molecules (called scintillators), causes those molecules to emit photons. These photons are detected quantitatively by determining the anode current of a photomultiplier tube (PMT). The number of photons ,is proportional to the energy of the radioactivity.
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2) Accelerator Mass Spectrometry (AMS) revolution;
http://www.ansto.gov.au/ansto/environment1/ams/images/AMS_Sketch.jpg
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• The ion source produces a beam of negative ions from a few milligrams of solid material by the cesium sputter source ;
• Low energy mass analysis select the mass of interest, a radioisotope of the element inserted in the sample holder, and reject the much-more-intense neighboring stable isotopes.
• Accelerators: accelerate ions, remove several electrons, turning the negative carbon ions into positive ones. The accelerator functions as a molecular dissociator. The final velocity is a few percent of the speed of light or about 50
million miles per hour. • High energy mass analysis: counts C14 ions with a gas
ionization detector. measures Isotope ratios• Measuring time just a few hours
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• Application
Archaeology
Earth sciences
Environmental sciences
Biomedical applications
Hydrology: groundwater dating
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• The Shroud of Turin
• “ The results of radiocarbon measurements at Arizona, Oxford and Zurich yield a calibrated calendar age range with at least 95% confidence for the linen of the Shroud of Turin of AD 1260 - 1390 (rounded down/up to nearest 10 yr). These results therefore provide conclusive evidence that the linen of the Shroud of Turin is mediaeval ”
• (P. E. Damon, etal., Radiocarbon Dating of the Shroud of Turin, Nature 337:6208, 16 February 1989, pp 611-615)
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• Basic two assumptions in radiocarbon dating :
• 1.the planetary distribution of C14 in the biosphere is uniform in time and space;
• 2.the sample, as measured, contains carbon that come only from a living organism, and that living organism took its carbon only from the biosphere;
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• Things That Can't Be Carbon-Dated
1 things which didn't get their carbon from the air. aquatic creatures, their carbon might (for example) come from dissolved carbonate rock. any animal that eats seafood.
2 things that are too old. After about ten half-lives, there's very little C14 left. So, anything more than about 50,000 years old probably can't be dated at all.
3 oil paints, because their oil is "old" carbon from petroleum.
4 fossils, for three reasons. First, they are almost always too old. Second, they rarely contain any of the original carbon. And third, it is common to soak new-found fossils in a preservative, such as shellac.
5 things that are too young. The nuclear tests of the 1950's created a lot of C14. Also, humans are now burning large amounts of "fossil fuel". As the name suggests, fossil fuel is old, and no longer contains C14. Both of these man-made changes are a nuisance to carbon dating.
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• Calibration and tree-ring correction
• Measure the C14 activity of wood samples that have known ages ( dendrochronologically determined).
• Create the calibration curves by plotting the wood radiocarbon ages versus the calibrated (cal) ages. (dated by another radioactive material, Thorium-230 or by ice layer
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Jim Parks prepares to analyze one of hundreds of core samples
collected at Mesa Verde National Park, Colorado. (Photo by John Florence.)
http://www.ltrr.arizona.edu/archaeology/progandhist.htm
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AMS Radiocarbon Dating of Bones At LSCEN Tisnérat-Laborde; H Valladas; E Kaltnecker; M Arnold
Development of an Automated System for Preparation of Organic SamplesChristine Hatté; Jean-Jacques Poupeau; Jean-François Tannau; Martine Paterne
Environmental Studies
Bomb Radiocarbon in Tree Rings from Northern New South Wales, Australia: Implications for Dendochronology, Atmospheric Transport, and Air-Sea Exchange of CO2Quan Hua; Mike Barbetti; Ugo Zoppi; David M Chapman; Bruce Thomson Freshwater Reservoir Effect in 14C Dates of Food Residue on PotteryAnders Fischer; Jan Heinemeier
Soils and Sediments
14C Ages of a Varved Last Glacial Maximum Section Off PakistanUlrich von Rad; Michael Sarnthein; Pieter M Grootes; Heidi Doose-Rolinski; Jochen Erbacher
AMS Dating of Pollen Concentrates - A Methodological Study of Late Quaternary Sediments from South Westland, New Zealand Marcus J Vandergoes; Christine A Prior
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• Thank you