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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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radiocarbon dating steffen allner oliver sala
Radiocarbon dating

Steffen Allner – Oliver Sala

Dating

10. Nov. 2009

contents
Contents
  • Introduction
  • Nobelprize, W. F. Libby
  • Basic principles
  • Methods
  • Applications
willard frank libby
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
why does it work
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
formation and decay
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

geo chemical 14 c cycle
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

geo chemical 14 c cycle1
Geo-chemical 14C cycle

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

uncertainties
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

uncertainties1
Uncertainties

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

assumptions made
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

limitations
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

summary of complications
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

calibration
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)

methods of 14 c determination
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

slide16
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.
slide17
AMS

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

tandem accelerator eth
Tandem-accelerator ETH

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

applications
Applications

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

sample sizes
Sample sizes
  • As indicated on www.ams.ethz.ch/services/radiocarbon
examination of the vinland map
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

examination of the vinland map1
Examination of the Vinland Map
  • Measurements taken from the parchment

D.J.Donahue et al., Radiocarbon, 44, 2002, p. 45-52

examination of the vinland map2
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

literature
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 )