Radiocarbon dating

Steffen Allner – Oliver Sala

10. Nov. 2009

Introduction

Nobelprize, W. F. Libby

Basic principles

Methods

Applications

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

Method Samples Time range

40K + e- 40Ar + Rocks, minerals 100 000 years back

238U … 206Pb (Radium series)

Zirkons, meteorites Age of the earth

239U … 207Pb(Actinium series)

Dito Dito

238U … 230ThCorals, marine sediments

1000 – 300 000 years back

87Rb 87Sr + -

Rocks for age determ., teeth and bones (geographic issues)

106 – 109 years

Due to radio activity of 14C

Constant formation rate of this isotop (by cosmic radiation)

Carbon-cycle:

Due to „constant“ ratio of 14C / 12C

Neutrons formed in the upper troposphere by spallation.

Half life of 12 min. Approx. 4 MeV

14 1 17 0 1

14

146

14 07 -1

1 1 0 -0 1 -

6

1

N + n + p Formation

N + e + Decay

n p + e + β -Deca

C

C

y

v

v

Source: W.F.Libby, „Altersbestimmung mit der C14-Methode“, Mannheim, 1969

In living organisms continuous 14C up-takedue 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

14 14

4 114 1412 12

exp 1.21 10Air

C Ct a

C C

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

Total amount of 14C on earth:

81 tons

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

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 Carbon quantities [g cm-2]

Carbonate in oceans

7.25

Org. Subst. dissolved in ocean

0.59

Biosphere 0.33

Atmosphere 0.12

Total 8.3

Source: W.F.Libby, „Altersbestimmung mit der C14-Methode“, Mannheim, 1969

13 14anorg. anorg.

13 14biol. biol.

C C=1.025 =1.05

C C

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

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

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 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 AMSLimit of detection of 14C : 1 part per quadrillion (1 ppq), by AMS

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

Contaminationwith carbon of different radiocarbon content

Reservoir effectRadiocarbon 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

Dendrochronology:(up to 12000 years back)

By means of very old trees (year rings) and age determinationthrough 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)

Method Advantages Disadvantages

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 sensitivityLimit 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 secondIn order to achieve an uncertainty of 40 yr (1σ) 40000 counts needed

Problem: Isobares or molecules having the same mass as 14C

14N, 12CH2- or 13CH-

Solution:

1) 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)

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 12CH2

- , 13CH- are removed.

4) The second magnet selects ions with the momentum expected of 14C ions.

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

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

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

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

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

Measurements taken from the parchment

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

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

W. F. Libby, „Altersbestimmung mit der C14-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 )