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Dating

Radiocarbon dating Steffen Allner – Oliver Sala. Dating. 10. Nov. 2009. Contents. Introduction Nobelprize, W. F. Libby Basic principles Methods Applications. Willard Frank Libby. 1908 – 1980 Physico-chemist Radioactivity and Isotope research Responsible for enrichement of 235 U

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Dating

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

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

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

  4. Alternative methods

  5. Why does it work? • Due to radio activity of 14C • Constant formation rate of this isotop (by cosmic radiation) • Carbon-cycle: • Due to „constant“ ratio of 14C / 12C

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

  8. Geo-chemical 14C cycle J. Res. Natl. Inst. Stand. Technol. 109, 185-217 (2004)

  9. Uncertainties • Slight deviation of 14C / 12C ratio in living organsims with respect to atmospheric ratio due to fractionation. (kinetic effects) • 14C / 12C 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) Source: W.F.Libby, „Altersbestimmung mit der C14-Methode“, Mannheim, 1969

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

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

  13. Summary of complications • Half-life: Originally not exactly correct measured by Libby (but still used for the sake of consistancy!) • Atmospheric variations: Not constant 14C 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. Calibration • Dendrochronology: (up to 12000 years back) By means of very old trees (year rings) and age determination through 14C-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. Methods of 14C determination 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. AMS • Problem: Isobares or molecules having the same mass as 14C 14N, 12CH2- or 13CH- • Solution: • Sample (in form of graphit) is bombarded with Cs-ions  negative ion-beam 14N is not stable having a negative charge! (It is hence filtered out, but not 12CH2- , 13CH- and 14C) • The mass of interest is focused in a mass spectrometer (e.g. sector field) • 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 12CH2- , 13CH- are removed. • The second magnet selects ions with the momentum expected of 14C ions.

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

  18. Tandem-accelerator ETH http://www.ams.ethz.ch/about/index

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

  20. Sample sizes • As indicated on www.ams.ethz.ch/services/radiocarbon

  21. Examination of the Vinland Map • 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. Examination of the Vinland Map • Measurements taken from the parchment D.J.Donahue et al., Radiocarbon, 44, 2002, p. 45-52

  23. Examination of the Vinland Map • 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. Literature • W. F. Libby, „Altersbestimmung mit der C14-Methode“, Mannheim, 1969 • I. Hajdas, Radiocarbon 2009, 51, 79–90 • I. U. Olsson, Radiocarbon2009, 51, 1-43 • D. J. Donahue et al., Radiocarbon2002, 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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