3. • Results of 14C dating are reported in radiocarbon years, and
calibration is needed to convert radiocarbon years into
calendar years
• Un-calibrated radiocarbon measurements are usually
reported in years BP where 0 (zero) BP is defined as AD 1950
• The most popular and often used method for calibration is by
dendrochronology.
Calibration
the age of a certain carbonaceous
sample can be easily determined by
comparing its radiocarbon content to
that of a tree ring with a known
calendar age.
If a sample has the same proportion
of radiocarbon as that of the tree
ring, it is safe to conclude that they
are of the same age.
9. • Once an organism dies, it ceases to obtain more 14C
• 14C decays reducing the concentration within
organism after death
• 14C decays by beta emission, emission of an electron
and a neutron changing into a proton, thus reverting
back into nitrogen
The emitted beta particles (ß) are what is counted in Libby's "gas
proportional“ method of 14C dating
14C ---> 14N + ß + neutrino
10. Determining the Starting Amount
Two types of carbon used in the dating process: 12C and 14C
• 12C is a stable isotope (it does not decay)
• When an organism is alive it has the same ratio (12C to
14C) that is found in the atmosphere (1-trillion to 1)
Same ratio Different
ratio
11. 1. How fast it decays (measured in half-lives). This
is known (5,730 years --> Cambridge half life).
2. The starting amount of C-14 in the fossil.
A Critical Detail
Two Things Need to Know to determine
how many half-lives have expired
The C-14 dating method relies on measuring the
amount of 14C in the material
12. Applicable range
Within the last 50,000 to 60,000 years
A: Present amount of 14C
A0: Original amount of 14C
t: Time it takes to reduce the original amount to the present amount
k: Half-life of 14C (5,370 years)
(*Libby half-life is 5,568 years)
14. Two ways to measure 14C
(1) Beta-decay counting (14C → 14N + b-): Measure
radioactivity (decay constant x no. of 14C atoms).
(2)Accelerator mass spectrometry (AMS)
Count individual 14C atoms to get 14C/12C ratio
One gram of "modern" carbon produces about 14 beta-
decay events per minute. To measure the age of a 1g
sample to a precision of +/- 20 years one needs 160,000
counts, or about 8 days of beta-counting.
AMS allows you to do the same measurement on a
1 milligram sample in a few minutes.
15. Example of Material
• Charcoal, wood, twigs and seeds
• Bone.
• Marine, estuarine and riverine
shell.
• Peat
• Lake muds and sediments.
• Soil.
• Pollen.
• Corals and
foraminifera.
• Textiles and fabrics.
• Water.
• etc
requirements:
• Carbon originally fixed from
atmospheric CO2
• Not contaminated
• Found in situ
• Well-preserved
17. Sampling error precaution
• samples should be packaged in chemically
neutral materials to avoid picking up new 14C
from the packaging, the packaging should also
be airtight to avoid contact with atmospheric
14C
• the stratigraphy should be carefully examined
to determine that a carbon sample location was
not contaminated by carbon from a later or an
earlier period
18. 3: Pre-treatments 4: CO2 production
5: Graphitization 6: Pressing 7: Sample analysis
b. ABA wash c. Combustion
Sample preparation and analysis
a. Physical separation a. Into quartz tube b. Sealing
a. Adding CO2 b. Adding H2 c. Graphitization AMS
19.
20. Factors affecting the amount of carbon in
the atmosphere
• Atomic bomb testing in the 1950s elevated
atmospheric 14C
• Industrial revolution to present, increase in
values of CO2 in atmosphere which decreases the
ratio of 14C to 12C
• Cosmic ray flux rate changes, e.g. supernova
• Magnetic field changes can modify intensity of
cosmic ray flux
• Short term- sun activity (solar flares) major
factor, paired with low 14C production rates
21. Limitation
• 14C can only be used to date organic material
• Samples can’t be too old or too young, from ~300 -
~50,000 years, limited due to half life
(approximately 9 half lives)
• 14C dating accuracy is dependent upon a consistent
ratio between 12C and 14C (equilibrium)
– The assumption of equilibrium is FALSE
– There are factors that can affect 14C in the atmosphere
• Ancient fossils as well as coal contain 14C residue
22. Solutions
• measure all three C isotopes (12C, 13C, 14C)
• concentrate 14C and extend counting time
• measure individual atoms (AMS)
• precisely identify exchange reservoirs
• calibrate conventional dates to calendar years
(tree rings, corals)
• combine 14C with other dating methods
• understand stratigraphic context
