About Radiocarbon (14C) in environment
Natural 14C
Naturally occurring carbon has three main isotopes: 12C (98.89%), 13C (1.11%) and 14C (0.0000000001%). Commonly known as radiocarbon, 14C is the only naturally occurring radioisotope of carbon. 14C forms in the stratosphere, by action of secondary thermal neutrons (product of spallation reaction of galactic cosmic rays with atmospheric gases) with atmospheric nitrogen.
14N7 + 1n0 → 14C6 + 1H1 + 0.62MeV
There are other modes of production of 14C in the atmosphere, but the dominant mode is 14N7(n, p)14C6 which has cross section of 1.81barns. Once formed in the atmosphere, hot energetic 14C atoms soon oxidises to CO and then to 14CO2. Indistinguishable from 12CO2, 14CO2 enters the carbon cycle and food chain through photosynthetic fixation of carbon by living plants, and also dissolved inorganic carbon of oceans through air-sea CO2 exchange.
14C decays with a half-life of 5730±40years, emitting a
β-particle with endpoint energy of 0.156MeV and forming stable nitrogen (14N).
14C6 → 14N7 + b - + n (neutrino) + Energy
Steady state atmospheric production rate of 14C at sea level is estimated at ~2.1atoms.cm-2.sec-1, which is balanced by radioactive decay in various exchangeable carbon reservoirs. Most living terrestrial organisms (trees, plants, and most herbivorous animals) which are in equilibrium with the atmosphere have similar 14C/12C ratio as in the atmosphere, but with the death of the organism the exchange of carbon stops, and 14C/12C ratio of the dead organism starts decreasing with radioactive decay of 14C . If A 0 is the initial specific 14C activity in an organism when it died t years ago, its residual specific 14C activity A is given by:
A = A0e-lt where l is 14C decay constant (1.209x10-4 year-1)
Thus, if the initial activity (A0) is known, on measuring the residual specific 14C activity in a dead organism in most cases it is possible to determine the age t (time of death of the organism) from the equation:
t = (1/l) ln[A0/A]
In mid 1940s, Willard Frank Libby of University of Chicago and his colleagues first demonstrated application of 14C as dating tool, for which he was awarded the Nobel Prize in Chemistry in 1960.
Learn more about Prof. Libby and his Nobel Prize here.
Natural 14C/12C ratio in CO2 of pre-industrial tropospheric air was about 1.176x10-12 (corresponding to specific 14C activity of 13.56dpm.g-1 of carbon). The natural level of 14C in the atmosphere had undergone changes in past, through changes in earth's geomagnetic field, solar activity (which modulates galactic cosmic ray activity on earth) and climate changes (which influence atmosphere-ocean CO2 exchange through global changes in oceanic thermo-haline circulation). 'Calibration curves' for 14C dates can be constructed by determining 14C age of known-aged (dated by dendro-chronology) tree-rings. In case of marine samples, calibration curves are constructed from 14C ages of fossil coral samples whose absolute ages are known by U/Th (Uranium/Thorium) dating method. Modern methods of 14C analyses are capable of measuring 14C/12C ratios in order of 10-14 or less in very small samples, thus enabling samples of age up to 10 half-lives of 14C (~55,000 years) to be dated by this method.
Anthropogenic 14C
Anthropogenic 14C were introduced in the environment mainly between early 1950s and late 1960s, due to large number of atmospheric nuclear testing, when natural level of 14C/12C in the atmosphere nearly doubled. Following the Test Ban Treaty of 1963, the level of 14C in global atmosphere is decreasing quasi-exponentially with e-folding time of ~17years. Nuclear power plants also release some amount of 14C to the environment. 14C ages of post-bomb samples can also be dated using the 14C curve for the post-bomb period, a technique known as 'bomb-pulse dating'. This technique had been used for dating of wines and also in forensic science.
Applications of 14C
Due to ubiquitous distribution of carbon and unique half life of 14C (5730±40years), which is in the time scale of archaeological interest, this isotope finds its most common application for archeological dating. As a chronometer, 14C is commonly used in paleoclimate studies to determine ages of natural carbon containing materials (e.g., tree-rings, lake sediments, marine sediments, peat, corals, speleothems, etc.), which archive valuable information for past climate changes. Natural 14C is also used for estimating the time scales of deep ocean circulation. Using bomb produced 14C one can trace the pathways of carbon across various exchangeable reservoirs of carbon (e.g. oceans, atmosphere, biosphere) in decadal time scales. Such studies have important implications to global carbon cycle and its anthropogenic perturbation. 14C is also used as a tracer in biomedicine, for tracing pathways of 14C labeled drugs in human body.
With new and more sensitive instrumentation being available for 14C analysis (mostly by accelerator mass spectrometry), new avenues of research are opening up, thus broadening the scope of applications of 14C in archaeology, environment, and biomedicine.
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