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The radiocarbon dating method is based on certain assumptions on the global concentration of carbon 14 at any given time. One assumption is that the global levels of carbon 14 (also called radiocarbon) in the atmosphere has not changed over time. The other assumption is the corollary of the first; the biosphere has the same overall concentration of radiocarbon as the atmosphere due to equilibrium.
Radiocarbon’s entry into the global carbon cycle starts in the atmosphere where it is formed by the interaction of neutrons produced by cosmic rays with nitrogen atoms. The carbon 14 produced reacts with oxygen atoms in the atmosphere to form carbon dioxide. This carbon dioxide is no different from those produced by carbon 12 and carbon 13; hence, carbon dioxide with carbon 14 has the same fate as those produced with the other carbon isotopes.
Mixing and exchanges happen between the atmosphere and the biosphere until such time that equilibrium is established. Radiocarbon dating rests heavily on this assumption such that other sources of carbon 14 had, at first, not been considered nor accounted for.
Nowadays, radiocarbon scientists had to perform calibration not only to convert their radiocarbon year results into calendar year but also to take into account the various factors that have major effects on the global levels of carbon 14, one of which is nuclear weapons testing.
There are two human activities recognized to have irreparably changed the global radiocarbon levels—the burning of fossil fuel and nuclear weapons testing.
Burning of large quantities of fossil fuels like coal, referred as the Suess effect, had significantly lowered the radiocarbon concentration of the atmospheric carbon reservoir. In contrast, nuclear weapons testing in the 1950s and 1960s dramatically increased the level of carbon 14 in the atmosphere. The phenomenon is often referred to as the bomb effect.
The bomb effect refers to the phenomenon that produced “artificial” radiocarbon in the atmosphere due to nuclear bombs.
Nuclear weapons testing brought about a reaction that simulated atmospheric production of carbon 14 in unnatural quantities. The huge thermal neutron flux produced by nuclear bombs reacted with nitrogen atoms present in the atmosphere to form carbon 14. The carbon 14 produced is what is known as bomb carbon or artificial radiocarbon.
According to literature, nuclear weapons testing in the 1950s and 1960s have nearly doubled the atmospheric carbon 14 content as measured in around 1965. The level of bomb carbon was about 100% above normal levels between 1963 and 1965. The level of bomb carbon in the northern hemisphere reached a peak in 1963, and in the southern hemisphere around 1965.
The change in global radiocarbon levels brought about by human activities necessitated the use of a reference standard for carbon 14 dating. Radiocarbon dating needed an organic material that was not contaminated with carbon 14 from fossil fuel burning or nuclear weapons testing.
Oxalic acid stocked by the U.S. National Bureau of Standards had been adopted as standard for radiocarbon dating. Its radiocarbon content was theoretically the same as a wood sample grown in AD 1950, the zero point of the radiocarbon timescale used in quoting carbon dating results.
Even after nuclear weapon testing was banned, the bomb effect still remains. According to literature, the excess carbon 14 produced during nuclear weapons testing has already decreased due in part to the global carbon exchange cycle. By the 1990s, the carbon 14 level was only about 20% higher than the theoretical 1950 level as measured by the activity of the oxalic acid reference standard.
Bomb carbon is essentially an artificial injection of carbon 14. Radiocarbon scientists used this knowledge to test their theories regarding the mixing rates of carbon 14 through various carbon reservoirs. They found out that tree rings do not exchange radiocarbon with other tree rings. This fact has supported the use of dendrochronology in radiocarbon dating, particularly in constructing radiocarbon calibration curves.
There are also other studies that monitored the presence of bomb carbon or radiocarbon in general.
The Geochemical Ocean Section Study analyzed ocean water samples from the Atlantic, Pacific, Indian, and Mediterranean Oceans and mapped the presence of bomb carbon. Results of the study have enabled modelers to analyze the pathway of radiocarbon and its exchange and residence times.
The World Ocean Circulation Experiment from 1990 to 2002 obtained radiocarbon measurements from dissolved inorganic carbon.
Reidar Nydal and Knut Lovseth have made radiocarbon measurements in atmospheric carbon dioxide from the northern and southern hemispheres from 1962 to 1993.