Radiocarbon Dating -----Natural Clock Yanhua li

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Radiocarbon Dating -----Natural Clock Yanhua li

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Radiocarbon Dating -----Natural Clock Yanhua li . “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”----- - PowerPoint PPT Presentation

Transcript of Radiocarbon Dating -----Natural Clock Yanhua li

Page 1: Radiocarbon Dating  -----Natural Clock  Yanhua li

Radiocarbon Dating -----Natural Clock

Yanhua li

Page 2: Radiocarbon Dating  -----Natural Clock  Yanhua li

• “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 structures5. Well-preserved shell;

Sample should contain the original carbon atoms at thetime 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 Pakistan

Ulrich 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