CARBON DATINGINTRODUCTIONCarbon dating is a method which determines the age of organicmaterials like wood, cloth, paper and bones. This technique is dependent uponthe decay of radiocarbon (C-14) to nitrogen.
Energetic neutrons are produced inupper layer of atmosphere by the collision of cosmic rays coming from sun withthe atoms existing in that layer. These neutron then displaces the protons ofNitrogen-14 which leads toward the formation of a new isotope, knownradiocarbon C-14. This form of carbon is unstable therefore it decays back toNitrogen-14 after passing a particular time period. In a given sample, 5730years are required for about half of C-14 decay into nitrogen.
This period forhalf of decay of sample’s radiocarbon is known as “half-life”.Figure 1: Formation of radiocarbon by the introduction of neutronsin N-14 Atmospheric activities like storms are responsible for the transferof 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. Carbonatoms present in organic molecules have C-14 atoms, as these atoms of carbonare derived from atmosphere.
Ratio of C-14 to C-12 in atmosphere remainsconstant with the ratio occurring in living organisms that absorbs C-14throughout their lives. When an organisms die then this exchange of carbonbecome ceased and unstable C-14 in organisms start to decay back to Nitrogen-14alongwith the emission of beta particle. Hence, the amount of C-14 in anorganisms declines with the passage of time and ratio of C-14 atoms to othercarbon atoms also decreases. This amount is then compared with theinternationally used reference standard to determine the age of an organism.Figure 2: Transfer of C-14 to environment and its decay back to N-14APPLICATIONS OF CARBON DATINGGlobal Carbon CycleEnergetic neutrons are produced in upper layer of atmosphere by thecollision 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 theformation of a new isotope, known as radiocarbon C-14. It is immediatelyoxidised 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 inatmosphere throughout their lives. When an organisms die then exchange ofcarbon 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-14therefore, amount of C-14 declines. This amount in a sample of living organismis 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 anorganism. ExampleWhen exchange of gas become ceased in a specific part of body suchas deposits on bones and teeth or at the death of an organism, ratio of C-14 toC-12 decreases. Decay of unstable C-14 to stable C-12 occur at rapid rate. Inorder to know about the time of survival of biological sample, researchersmeasure the ratio of carbon isotopes.Figure 3: Radiocarbon dating on bonesFood Chain In the upper layer of atmosphere, radiocarbon is produced by theinteraction of Nitrogen-14 with cosmic radiations.
The process ofphotosynthesis then enables plants to consume radiocarbon from atmosphere. As aresult, all of the living organisms that are involved in food chain take inradiocarbon from plants. Hence, it travels to all living organisms that areexisting on earth.Figure 4: Trravelling of C-14 through atmosphere to entire foodchain ExampleGreen plants and algae that form the base of food chain use carbonof earth’s atmosphere to photosynthesize. These organisms have same ratio ofC-14 to C-12 as in atmosphere. Similar ratio passes all the way of entire foodchain to apex predators such as sharks.Biological Carbon cycleAtmospheric molecules of carbon-dioxide contains radiocarbon thatenters the biological carbon cycle through its absorption by plants havinggreen colour.
It then travels through food chain to animals. It decaysgradually in living organisms and replenishing process occurs to compensate thelost amount of radiocarbon by taking in air or food. When an organism die thenabsorption of C-14 decreases as a result amount of radiocarbon in tissues oforganism also decline. This amount (residual radiocarbon) then gives anestimate of the date at which organism die.PlanktonsLikewise terrestrial plants absorb C-14 from air, planktons arealso capable of absorbing radiocarbon from the ocean. As plankton is the baseof marine food chain therefore radiocarbon travels to whole of the aquaticorganisms.
Figure 5: PlanktonArchaeologyDifferent methods are adapted by archaeologist to date objects. Butwhen object is organic like bone or wood then researchers uses a method knownas radiocarbon dating. In this technique, decay of Carbon-14 is used to give anestimate about the date of organic materials.
Many renowned substances likeDead Sea Scrolls, Otzi the iceman and Egyptian articrafts (to supply achronology of Dynastic Egypt) have been assigned dates through the use of”Radiocarbon Dating”. From about 58,000 to 60,000 years, this techniqueappeared to be most efficient in the field of archaeology.C-14 and ClimateDuring the span of time that can be radiocarbon dated the level ofC-14 has not been constant due to which raw date cannot be used directly as acalendar date. Variations in Earth’s magnetosphere are responsible for thevariations in cosmic ray intensity and hence level of C-14 is affected. Inaddition to that there are significant reservoirs of carbon in organic matterlike sedimentary rocks, ocean and ocean sediments. Flow of carbon betweenreservoirs and atmosphere is affected by change in climate of earth.
As aresult, amount of C-14 in atmosphere varies.Marine EffectSurface water of oceans absorb carbon-dioxide from atmosphere andforms carbonate or bicarbonate ions. At the same time, returning of carbonateions in air also occurs in the form of CO. During this exchange process, C-14enters in to surface water of ocean from atmosphere.
However, percolation ofC-14 in to entire water of ocean consume a lot of time. Upwelling is the process that brings deepwater to the surface. Mixing of surface and deep water takes a long time thanthat of mixing of carbon dioxide with surface water.
As a result, radiocarbonage of several thousand years is shown by deep ocean water. Mixing of surfacewater with old water occurs by upwelling which means surface water has an ageof several hundred years. This effect is not constant and average effect isconsidered to be 440 years.
Geographically closer areas have local deviationsof several thousand years. This effect is also applicable upon marine organismslike marine mammals (whales and seals) and shells that have radiocarbon age ofabout hundreds of years old.BombRadiocarbon DatingThis term is for radiocarbon dating dependent upon timestampsleftward by nuclear explosion that were carried out above of the ground. It isvery helpful for giving an absolute estimate of organism’s age that livedthrough those events. Hemisphere Effect Atmospheric circulation between northern and southern hemisphereare independent of each other due to which there is perceptible time lag inmixing of these two hemisphere.
Atmospheric ratio of 14-C to 12-C is less insouthern hemisphere, with an apparent additional age of 30 years forradiocarbon results from south than that of north. This is due to the largersurface area of ocean in southern hemisphere which in turn causes greaterexchange 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 ofnorth.Figure6:Northern and southern hemisphere