Carbon dating, also called radiocarbon dating, was discovered by Willard Frank Libby in the 1940s. It is a method used to determine the age of organic substances. It works on the principle of radioactive decay of carbon-14 in dead living organisms. Libby was awarded the Nobel prize in 1960 for his work on carbon dating.
Radiocarbon dating is different from other dating methods as it is specific to fossils. Besides age, it also tells us the time since the living organisms were dead, which makes it very useful. It cannot be used to date inorganic substances such as rocks, sediments, etc.
Carbon cycle
Radiocarbon (14C) produced in the atmosphere by the decay of nitrogen (14N) enters the living organisms through the biological carbon cycle. This carbon-14 reacts with oxygen to form carbon monoxide and then carbon dioxide. Carbon dioxide is consumed by living organisms (plants) through photosynthesis for food.
1. Production of neutrons by cosmic rays
Cosmic rays strike atmospheric species such as gases and particulate matter in the troposphere to release high-speed neutrons.
2. Production of 14C in the troposphere
High energy neutrons produced by cosmic rays attack the nitrogen atoms (14N) (78% of air). During this nuclear reaction, a positron is emitted from a nitrogen atom thus creating a carbon-14 isotope (14C).
n + 14N → 14C + e+
3. 14C gets involved in the carbon cycle
As with normal carbon atoms, they undergo a carbon cycle to produce various compounds necessary for living processes to go on. The carbonates and carbon dioxide constitute the main products of this cycle.
14C + O2 → CO
CO + OH → 14CO2 + H
4. Uptake of 14CO2 by plants through photosynthesis
Photosynthesis is a photocatalytic reaction that occurs in plants for food (glucose) synthesis. As 14CO2 is now a part of the atmosphere, it is also consumed by plants.
6H2O + 614CO2 + んν → 6O2 + 14C6H12O6
The above reaction shows how plants take up the radioactive isotope of carbon (14C). Animals eat these plants thus, 14C is distributed in the whole biosphere.
Radiocarbon Decay (Half-life)
The amount of carbon-14 in the atmosphere is always in equilibrium with the amount of carbon-14 in the living organism as long as the living organism is alive. After death, carbon-14 starts decreasing due to radioactive decay.
The decay equation is:
Carbon decays to 14N by emitting beta particles and energy.
14C → 14N+ e– + γ
Half-life = 5730 years
The difference between atmospheric carbon-14 and the remaining carbon-14 in that fossils is calculated and is used to find out the age of that species. The following half-life equations (1) and (2) are implied for this function.
N = No e-λt (1)
ln (N/No) = -λt (2)
where,
- N = Number of remaining items after time (t)
- N0 = Number of atoms in an original sample
- λ = Decay constant = 0.693 / t1/2
Measurement of 14C in old remnants
The measurement of radiocarbon in the old incense remains (fossils) is done by following steps:
Sample preparation
The amount of 14C is very small in old samples (fossils) due to the short half-life time of 14C (5730 years), therefore, it is first enriched before analysis. The sample is placed in a thermal diffusion column for a month to remove the impurities. Hence, the concentration of 14C increases.
Liquid scintillation
It measures the number of beta rays emitted per gram per minute by 14C. It is an old technique now replaced by modern techniques.
GIEGER Counter
The Geiger counter is a detector that was used by Libby for the quantification of 14C. It records the emitted β-particles from 14C as a result of a spontaneous conversion of 14C to 14N. This method is not so precise because the ionization potential of emitted rays is very low and it cannot produce accurate results.
Accelerated mass spectrometry
Accelerated MS is one of the most used and modern radiocarbon measurement instruments. In this method, carbon-14 is directly quantified relative to carbon-12 and carbon-13 by mass spectrometry using their percentage compositions.
- 12C ≈ 98.90%
- 13C ≈ 1.07%
- 14C ≈ 0.03%
Example of Carbon Dating
The amount of carbon-14 in a fossil-remain is found to be 20 percent of its fresh live piece. Calculate its age.
Half-life of carbon-14 = 5730 years
Decay constant = λ = 0.693 / t1/2 → (i)
λ = 0.693 / 5730
λ = 1.209 x 10-4
Total time lapsed ‘t’ can be found from first order equation:
2.303 log (14Ct / 14Co) = -λ x t → (ii)
2.303 log (0.2/1) = -(1.209 x10-4) t
-3.91 = -(1.209 x 10-4) x t
t = (3.91/1.204) x 104
t = 3.2475 x 104 years
Hence, the fossil is 32475 years old.
Standards for carbon dating
All advanced techniques require standardization or calibration from ancient techniques. 14C-dating is standardized using:
- Oxalic acid prepared from the remains of 1950s plants is considered a secondary standard in carbon dating.
- An old tree ring is a sample used as a standard from older woods. Such a ring is formed in the stems of the tree according to the life span (phloem cycle). It is taken as a control sample (standard) and the 14C method is calibrated against it.
Factors that affect the carbon dating
Carbon dating may be affected by many factors such as a very small fraction of beta particles. Some of these factors are discussed below.
- Contamination
Old samples can be easily contaminated by impurities from other samples. It must be noted that 1% of the sample with a difference of 5000 years of age may differ the results up to 800 years. Hence, during sampling, it is highly emphasized that distinct and pure samples should be taken for analysis.
- Atmospheric effect
Carbon-14 content in the atmosphere somehow changed over a period of time due to atmospheric effects. The atmospheric changes include volcanic eruptions and the burning of excessive carbon. This may alter the concentration of 14C in the air.
Calibration
The age calculation is based on the assumption that the concentration of 14C is the same as at the time of death of the analyte. Some factors influence the level of 14C in the atmosphere due to which calibration curves are drawn with calendar age as shown below.
Datable Materials with Radiocarbon
Any material composed of carbon can be dated with this method such as:
- Woods, such as trees, plants, etc
- Pollen grains
- Ocean animals
- The soil of different regions
- Textiles
- Blood samples of fossil
- Bones and hairs, etc
Limitations of Radiocarbon dating
It has been found that the age of living organisms that died more than 50,000 years ago cannot be calculated precisely with carbon dating. The reason is that the carbon-14 concentration goes down to ultra-trace and therefore, 14C is no longer detectable.
Some other methods of radioactive dating
1. Potassium-Argon dating method
K-Ar dating is the most common technique to date old ingenious rocks. It is based on the accumulation of argon by the decay of 40K. It takes 13 million years for one-half life of K-40, therefore, it is used to date the oldest materials known to date.
2. Uranium-Thorium dating method
This method (U-Th dating) is used to study the age of substances containing carbonates, silica, alumina, and others that can not be studied with carbon dating. As uranium-234 has a half-life of 80,000 years and it decays into thorium-230.
Applications of Dating Methods
Some applications of dating methods (radioactive dating) are as follows:
- All dating methods help in determining the relative and absolute ages of fossils and rocks (radiometric dating).
- Metrology scientists study the changes in sediments to predict the upcoming climate changes.
- Geological events can be arranged according to the calendar.
- Dating methods are widely used as tracing techniques.
- Hydrology, oceanography, paleoclimatology, biomedicine, and several other fields imply radioactive dating methods for several purposes.
Concepts Berg
What is radiocarbon dating?
Radiocarbon dating is a technique that is used to determine the age of living organisms by radioactive decay.
How does carbon dating work?
Carbon dating works on the principle of the decay constant. In an organic substance, carbon-14 stops changing after death. The difference in the concentration of carbon-14 in the atmosphere and the dead organism is calculated to determine its age.
How accurate is carbon-14 dating?
The accuracy of carbon dating is debatable as the amount of carbon-14 in the atmosphere is continuously changing due to volcanic eruptions and acid rains.
Is carbon-14 harmful to humans?
Carbon-14 is present in a trace amount in living organisms so it is not considered to be of much danger to humans.
Can you carbon date live things?
No, a living organism cannot be dated with C-14 as the decay starts on death and the interchange of carbon with the environment is halted.
How do we know the starting amount of C-14?
The starting amount of C-14 is assumed to be the same as in the environment.
When does carbon-14 dating become impractical?
It become difficult to date fossils of more than 10 thousand years ago.
References
- Chemistry | Fifth edition, by Steven S. Zumdhal and Susan A. Zumdhal (University of Illinois, Urbana Champaign, IL, USA)
- Essential of Physical Chemistry: 2nd edition By B.S Bahl (Gurdaspur, India) and Arun Bahl (RSC, UK) and G.D. Tuli (Delhi University, India)
- What is carbon-dating (radiocarbon.com)
- Willard Libby and radiocarbon dating (acs.org)
- Table of Isotopic Masses and Natural Abundances (University of Alberta, USA)
