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Radiocarbon dating Steffen Allner – Oliver Sala 10. Nov. 2009.

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Presentation on theme: "Radiocarbon dating Steffen Allner – Oliver Sala 10. Nov. 2009."— Presentation transcript:

1 Radiocarbon dating Steffen Allner – Oliver Sala 10. Nov. 2009

2 Introduction Nobelprize, W. F. Libby Basic principles Methods Applications

3 1908 – 1980 Physico-chemist Radioactivity and Isotope research Responsible for enrichement of 235 U development of radiocarbon dating in 1949 Nobel prize in chemistry in 1960

4 MethodSamplesTime range 40 K + e - 40 Ar + Rocks, minerals100 000 years back 238 U … 206 Pb (Radium series) Zirkons, meteoritesAge of the earth 239 U … 207 Pb (Actinium series) Dito 238 U … 230 ThCorals, marine sediments1000 – 300 000 years back 87 Rb 87 Sr + - Rocks for age determ., teeth and bones (geographic issues) 10 6 – 10 9 years

5 Due to radio activity of 14 C Constant formation rate of this isotop (by cosmic radiation) Carbon-cycle: Due to constant ratio of 14 C / 12 C

6 Neutrons formed in the upper troposphere by spallation. Half life of 12 min. Approx. 4 MeV Source: W.F.Libby, Altersbestimmung mit der C14-Methode, Mannheim, 1969

7 In living organisms continuous 14 C up-take due to constant 14 C formation in atmosphere Therefore, 14 C/ 12 C ratio (e.g. in plants) is the same as in atmosphere Through death, 14 C up-take is cut off http://c14.arch.ox.ac.uk/embed.php?File=dating.html Total amount of 14 C on earth: 81 tons

8 J. Res. Natl. Inst. Stand. Technol. 109, 185-217 (2004)

9 Slight deviation of 14 C / 12 C ratio in living organsims with respect to atmospheric ratio due to fractionation. (kinetic effects) 14 C / 12 C ratio in atmosphere influenced by: Natural fluctuations Sun activity -> cosmic radiation Change of the geomagnetic dipole field Terrestral carbon reservoir atmosphere (ice age) Nuclear weapon tests Suess-effect (fossil fuels) SourceCarbon quantities [g cm -2 ] Carbonate in oceans7.25 Org. Subst. dissolved in ocean 0.59 Biosphere0.33 Atmosphere0.12 Total8.3 Source: W.F.Libby, Altersbestimmung mit der C14-Methode, Mannheim, 1969

10 http://de.wikipedia.org/wiki/Radiokohlenstoffdatierung

11 14 C / 12 C ratio is constant over time (not true, as seen before) Original measured half-life value of 5568 years is not correct. 5730 years If BP appears in a graph: BP = before present = before 1950

12 Time range limited to approx. 50 000 years, due to very low 14 C abundance. Generally spoken, the very low abundance of the 14 C isotope represents the main difficulty concerning age determination. 14 C : 1 part per trillion (0.0000000001%) of the carbon in the atmosphere Limit of detection of 14 C : 1 part per quadrillion (1 ppq), by AMS

13 Half-life: Originally not exactly correct measured by Libby (but still used for the sake of consistancy!) Atmospheric variations: Not constant 14 C concentration in the past Contamination with carbon of different radiocarbon content Reservoir effect Radiocarbon composition of the ocean (e.g.) differs from that of the atmosphere dealt with by calibration of the radiocarbon dates against material of known age

14 Dendrochronology: (up to 12000 years back) By means of very old trees (year rings) and age determination through 14 C-method Corals, marine sediments (up to 26000 years back) Probabilistic methods (up to 50000 year back) J. Res. Natl. Inst. Stand. Technol. 109, 185-217 (2004)

15 MethodAdvantagesDisadvantages Liquid scintillation counting (LSC) Sensitive, low cost, shorter measurement time, compared to proportional counting Sample has to be converted into benzene Proportional counting (Libby)Used for calibration (higher precision than AMS, provided large and pure sample is available) Time consuming and at least 1kg of original sample needed AMS (Accelerated MS)Greater sensitivity Limit of detection: 1 ppq (reached after 10 half life cycles approx. 55000 years) Expencive and complex method Remark to proportional counting method: 1 mol modern carbon 3 disintegrations per second In order to achieve an uncertainty of 40 yr (1σ) 40000 counts needed

16 Problem: Isobares or molecules having the same mass as 14 C 14 N, 12 CH 2 - or 13 CH - Solution: 1) Sample (in form of graphit) is bombarded with Cs-ions negative ion-beam 14 N is not stable having a negative charge! (It is hence filtered out, but not 12 CH 2 -, 13 CH - and 14 C) 2) The mass of interest is focused in a mass spectrometer (e.g. sector field) 3) Ions are accelerated in the tandem-accelerator, whereby they pass a stripper (carbon- foil or gas molecules) and lose (valence) electrons cations are formed during this process bonds are broken up molecules like 12 CH 2 -, 13 CH - are removed. 4) The second magnet selects ions with the momentum expected of 14 C ions.

17 http://c14.arch.ox.ac.uk/embed.php?File=ams.html

18 http://www.ams.ethz.ch/about/index

19 I.Hajdas, Radiocarbon, Vol 51, Nr 1, 2009, p 79–90

20 As indicated on www.ams.ethz.ch/services/radiocarbonwww.ams.ethz.ch/services/radiocarbon

21 Shows the Island of Vinland, discovered by Bjarni and Leif First known cartographic representation of North America Associated with the Council of Basel (AD 1431 – 1449) Question of forgery: Ink contains a certain amount of TiO2 as anatase (1970) http://en.wikipedia.org/wiki/File:Vinland_Map_HiRes.jpg

22 Measurements taken from the parchment D.J.Donahue et al., Radiocarbon, 44, 2002, p. 45-52

23 Results : Method A was neglected Method B-E mean value 467 ± 27 BP Converted date via tree ring data: AD 1434 ± 11 1 σ : AD 1423 -1445 2 σ : AD 1411 -1468 -> 95 % confidence Method A : sample contamination from the years 1958 – 1962 Transport from European book trader to Yale

24 W. F. Libby, Altersbestimmung mit der C 14 -Methode, Mannheim, 1969 I. Hajdas, Radiocarbon 2009, 51, 79–90 I. U. Olsson, Radiocarbon 2009, 51, 1-43 D. J. Donahue et al., Radiocarbon 2002, 44, 45-52 Lloyd A. Currie, J. Res. Natl. Inst. Stand. Technol. 2004, 109, 185-217 http://c14.arch.ox.ac.uk/embed.php?File=dating.html www.ams.ethz.ch/services/radiocarbon (http://de.wikipedia.org/wiki/Radiokohlenstoffdatierung )


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