Radiocarbon Dating -----Natural Clock Yanhua li

1. Radiocarbon Dating -----Natural Clock Yanhua li

2. “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

3. Outline • Principles • Sample and measurement • Application • Advantages and limitations • Future developments

4. 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

5. Url: http://id-archserve.ucsb.edu/Anth3/Courseware/Chronology/Movies/Reaction.html

6. Url: http://id-archserve.ucsb.edu/Anth3/Courseware/Chronology/Movies/Reaction.html

7. http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/carbondating.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/carbondating.htm

8. 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.

9. 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.

10. Radiocarbon dating by W.F.Libby,1955

11. 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.

12. 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.

13. 2) Accelerator Mass Spectrometry (AMS) revolution; http://www.ansto.gov.au/ansto/environment1/ams/images/AMS_Sketch.jpg

14. 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

15. Application Archaeology Earth sciences Environmental sciences Biomedical applications Hydrology: groundwater dating

16. 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)

17. 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;

18. Things That Can't Be Carbon-Dated • 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. • 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. • oil paints, because their oil is "old" carbon from petroleum. • 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.

19. 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

20. Jim Parks prepares to analyzeone of hundreds of core samples collected at Mesa Verde National Park, Colorado. (Photo by John Florence.) http://www.ltrr.arizona.edu/archaeology/progandhist.htm

21. 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 ThomsonFreshwater 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 ZealandMarcus J Vandergoes; Christine A Prior

22. Thank you