Radiocarbon carbon testing

Nuclear testing has increased the amount of carbon-14 in the air, and sensitive radiocarbon dating techniques take this increase into account. 

Fossil fuel emissions alter the isotopic composition of atmospheric carbon, since they contain no C and are depleted in C. Releasing radiocarbon-free CO2 to the atmosphere dilutes the atmospheric C content, 3delding lower C/C ratios ("the Suess effect"). From 1850 to 1954 the C/C ratio in the atmosphere decreased by 2.0 to 2.5% (Fig. 11-23) (Suess, 1965 Stuiver and Quay, 1981). Then, this downward trend in C was disrupted by a series of atmospheric nuclear tests. Many large fission explosions set off by the United States with high emission of neutrons took place in 1958 in the atmosphere and the Soviet Union held extensive tests during... 

A mass of evidence seems to confirm that the mixing rate of radiocarbon in the atmosphere is rapid, and that with respect to its radiocarbon content the atmosphere can be considered as a homogeneous entirety. The contamination of samples with matter from an extraneous source can nevertheless invalidate this assumption. Two types of contamination can be differentiated physicochemical contamination and mechanical intrusion. There are two forms of physicochemical contamination. One is due to the dilution of the concentration of radiocarbon in the atmosphere by very old carbon, practically depleted of radiocarbon, released by the combustion of fossil fuel, such as coal and oil. The other is by the contamination with radiocarbon produced by nuclear bomb tests during the 1950s and later in the twentieth century. The uncertainties introduced by these forms of contamination complicate the interpretation of data obtained by the radiocarbon dating method and restrict its accuracy and the effective time range of dating. 

Table 2 gives details of some conventional regimes (see British National Formulary, 2008). The efficacy of therapy can be checked by Radiocarbon-labelled urea breath testing (which depends upon release of labelled carbon dioxide by bacterial urease) or by testing gastric biopsy material for persistence of gastric urease, but should only be done after eradication therapy has been discontinued for at least a month, and whilst any anti-secretory treatment has been discontinued (because it tends to suppress but does not eradicate the organism). 

Variability in the Amount of Carbon in Reservoirs. In addition to variations in the production and distribution of radiocarbon over time and within portions of various carbon reservoirs, variations may result in situations where carbon not in equilibrium with the contemporary standard values is added or removed from any reservoir. Two instances of this are well documented since they occurred within the last century as a result of human intervention. The first is known as the industrial or Suess effect and is caused by the combustion of fossil fuels beginning about 1890, resulting in a depletion of atmospheric activities by about 3% (76). A more recent occurrence has been called the atomic bomb or Libby effect. The detonation of nuclear devices in the atmosphere beginning in 1945 produced large amounts of artificial increasing the radiocarbon concentrations in the atmosphere by more than 100% in the Northern Hemisphere (77). Because of equilibration with the oceans, the levels have been diminishing steadily since the atmospheric testing was terminated by the major nuclear powers except France and the People s Repub-... 

The activity of radiocarbon (A 4c%o) in the atmosphere and surface ocean in response to atmospheric nuciear weapons testing. Atmospheric values are from the Carbon Dioxide Information and Analysis Center (CDIAC), Oakridge, TN. Surface ocean data are from measurement of the C activity of growth bands of corals (Druffel and Linick, 1978). 

It is unusual to think of any type of atmospheric contamination - especially by a radioactive species -as beneficial however, bomb-produced radiocarbon (and tritium) has proven to be extremely valuable to oceanographers. The majority of the atmospheric testing, in terms of number of tests and production, occurred over a short time interval, between 1958 and 1963, relative to many ocean circulation processes. This time history, coupled with the level of contamination and the fact that becomes intimately involved in the oceanic carbon cycle, allows bomb-produced radiocarbon to be valuable as a tracer for several ocean processes including biological activity, air-sea gas exchange, thermocline ventilation, upper ocean circulation, and upwelling. 

In his 1956 paper on radioactive fallout (7) Libby pointed out that neutrons released in the explosions of nuclear weapons react with nitrogen nuclei in the air to make carbon-14, which has a half-life of about 5600 years. In his discussion of bomb-test carbon-14 he said that Fortunately, this radioactivity is essentially safe because of its long lifetime and the enormous amount of diluting carbon dioxide in the atmosphere. He pointed out that 5.2 tons of neutrons would be needed to double the feeble natural radioactivity of living matter due to radiocarbon. Such an increase would have no significance from the standpoint of health. He mentioned that, for a given energy release, thermonuclear weapons produce more neutrons than fission weapons, and concluded that the essential point is that the atmosphere is difficult to activate and the activities produced are safe, ... 

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