CARBON Energetic neutrons are produced in upper layer of

CARBON DATING

INTRODUCTION

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Carbon dating is a method which determines the age of organic
materials like wood, cloth, paper and bones. This technique is dependent upon
the decay of radiocarbon (C-14) to nitrogen. Energetic neutrons are produced in
upper layer of atmosphere by the collision of cosmic rays coming from sun with
the atoms existing in that layer. These neutron then displaces the protons of
Nitrogen-14 which leads toward the formation of a new isotope, known
radiocarbon C-14. This form of carbon is unstable therefore it decays back to
Nitrogen-14 after passing a particular time period. In a given sample, 5730
years are required for about half of C-14 decay into nitrogen. This period for
half of decay of sample’s radiocarbon is known as “half-life”.

Figure 1: Formation of radiocarbon by the introduction of neutrons
in N-14

 

Atmospheric activities like storms are responsible for the transfer
of unstable radiocarbon to the lower atmosphere.

Carbon-14 is oxidised to (C-14) dioxide by interaction with C-12.
Organic molecules are present in all living organisms existing on earth. Carbon
atoms present in organic molecules have C-14 atoms, as these atoms of carbon
are derived from atmosphere. Ratio of C-14 to C-12 in atmosphere remains
constant with the ratio occurring in living organisms that absorbs C-14
throughout their lives. When an organisms die then this exchange of carbon
become ceased and unstable C-14 in organisms start to decay back to Nitrogen-14
alongwith the emission of beta particle. Hence, the amount of C-14 in an
organisms declines with the passage of time and ratio of C-14 atoms to other
carbon atoms also decreases. This amount is then compared with the
internationally used reference standard to determine the age of an organism.

Figure 2: Transfer of C-14 to environment and its decay back to N-14

APPLICATIONS OF CARBON DATING

Global Carbon Cycle

Energetic neutrons are produced in upper layer of atmosphere by the
collision of cosmic rays coming from sun with the atoms existing in that layer.
These neutron then displaces the protons of Nitrogen-14 which leads toward the
formation of a new isotope, known as radiocarbon C-14. It is immediately
oxidised to C-14 dioxide in air and then enters the global carbon cycle.

Age Determination

Plants and animals take in Carbon-14 from carbon dioxide present in
atmosphere throughout their lives. When an organisms die then exchange of
carbon become ceased and unstable C-14 in organisms start to decay into C-12.
As there is no chance of replacement of carbon after the decay of C-14
therefore, amount of C-14 declines. This amount in a sample of living organism
is then compared with reference standard to determine the age of an organisms.
Hence, radiocarbon dating is helpful to know about the time of survival of an
organism.

 

 

Example

When exchange of gas become ceased in a specific part of body such
as deposits on bones and teeth or at the death of an organism, ratio of C-14 to
C-12 decreases. Decay of unstable C-14 to stable C-12 occur at rapid rate. In
order to know about the time of survival of biological sample, researchers
measure the ratio of carbon isotopes.

Figure 3: Radiocarbon dating on bones

Food Chain                                                    

In the upper layer of atmosphere, radiocarbon is produced by the
interaction of Nitrogen-14 with cosmic radiations. The process of
photosynthesis then enables plants to consume radiocarbon from atmosphere. As a
result, all of the living organisms that are involved in food chain take in
radiocarbon from plants. Hence, it travels to all living organisms that are
existing on earth.

Figure 4: Trravelling of C-14 through atmosphere to entire food
chain

 

Example

Green plants and algae that form the base of food chain use carbon
of earth’s atmosphere to photosynthesize. These organisms have same ratio of
C-14 to C-12 as in atmosphere. Similar ratio passes all the way of entire food
chain to apex predators such as sharks.

Biological Carbon cycle

Atmospheric molecules of carbon-dioxide contains radiocarbon that
enters the biological carbon cycle through its absorption by plants having
green colour. It then travels through food chain to animals. It decays
gradually in living organisms and replenishing process occurs to compensate the
lost amount of radiocarbon by taking in air or food. When an organism die then
absorption of C-14 decreases as a result amount of radiocarbon in tissues of
organism also decline. This amount (residual radiocarbon) then gives an
estimate of the date at which organism die.

Planktons

Likewise terrestrial plants absorb C-14 from air, planktons are
also capable of absorbing radiocarbon from the ocean. As plankton is the base
of marine food chain therefore radiocarbon travels to whole of the aquatic
organisms.

Figure 5: Plankton

Archaeology

Different methods are adapted by archaeologist to date objects. But
when object is organic like bone or wood then researchers uses a method known
as radiocarbon dating. In this technique, decay of Carbon-14 is used to give an
estimate about the date of organic materials. Many renowned substances like
Dead Sea Scrolls, Otzi the iceman and Egyptian articrafts (to supply a
chronology of Dynastic Egypt) have been assigned dates through the use of
“Radiocarbon Dating”. From about 58,000 to 60,000 years, this technique
appeared to be most efficient in the field of archaeology.

C-14 and Climate

During the span of time that can be radiocarbon dated the level of
C-14 has not been constant due to which raw date cannot be used directly as a
calendar date. Variations in Earth’s magnetosphere are responsible for the
variations in cosmic ray intensity and hence level of C-14 is affected. In
addition to that there are significant reservoirs of carbon in organic matter
like sedimentary rocks, ocean and ocean sediments. Flow of carbon between
reservoirs and atmosphere is affected by change in climate of earth. As a
result, amount of C-14 in atmosphere varies.

Marine Effect

Surface water of oceans absorb carbon-dioxide from atmosphere and
forms carbonate or bicarbonate ions. At the same time, returning of carbonate
ions in air also occurs in the form of CO. During this exchange process, C-14
enters in to surface water of ocean from atmosphere. However, percolation of
C-14 in to entire water of ocean consume a lot of time.  Upwelling is the process that brings deep
water to the surface. Mixing of surface and deep water takes a long time than
that of mixing of carbon dioxide with surface water. As a result, radiocarbon
age of several thousand years is shown by deep ocean water. Mixing of surface
water with old water occurs by upwelling which means surface water has an age
of several hundred years. This effect is not constant and average effect is
considered to be 440 years. Geographically closer areas have local deviations
of several thousand years. This effect is also applicable upon marine organisms
like marine mammals (whales and seals) and shells that have radiocarbon age of
about hundreds of years old.

Bomb
Radiocarbon Dating

This term is for radiocarbon dating dependent upon timestamps
leftward by nuclear explosion that were carried out above of the ground. It is
very helpful for giving an absolute estimate of organism’s age that lived
through those events.

 

Hemisphere Effect    

Atmospheric circulation between northern and southern hemisphere
are independent of each other due to which there is perceptible time lag in
mixing of these two hemisphere. Atmospheric ratio of 14-C to 12-C is less in
southern hemisphere, with an apparent additional age of 30 years for
radiocarbon results from south than that of north. This is due to the larger
surface area of ocean in southern hemisphere which in turn causes greater
exchange of carbon between atmosphere and ocean as compared to that of north.
Since, marine effect causes depletion of 14-C in surface ocean therefore,
removal of 14-C from southern atmosphere occurs rapidly as compared to that of
north.

Figure
6:
Northern and southern hemisphere