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The logical conclusion from this was that in order to obtain a modern radiocarbon reference standard, representing the radiocarbon activity of the 'present day', one could not very well use wood which grew in the 1900's since it was affected by this industrial effect.Thus it was that 1890 wood was used as the modern radiocarbon standard, extrapolated for decay to 1950 AD.In the northern hemisphere the amount of artificial carbon in the atmosphere reached a peak in 1963 (in the southern hemisphere around 1965) at about 100% above normal levels.Since that time the amount has declined owing to exchange and dispersal of C14 into the Earth's carbon cycle system.The volcanic effect has a limited distance however. (1980) found that at 200 m away from the source, plants yielded an age in agreement with that expected.They suggested that the influence of depleted CO2 declined rapidly with increasing distance from the source.Since about 1955, thermonuclear tests have added considerably to the C14 atmospheric reservoir.This C14 is 'artificial' or 'bomb' C14, produced because nuclear bombs produce a huge thermal neutron flux.

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Ellen Druffel has called this the silver lining in thermonuclear bomb testing.Plants which grow in the vicinity of active volcanic fumeroles will yield a radiocarbon age which is too old. (1980) measured the radioactivity of modern plants growing near hot springs heated by volcanic rocks in western Germany and demonstrated a deficiency in radiocarbon of up to 1500 years through comparison with modern atmospheric radiocarbon levels.Similarly, this effect has been noted for plants in the bay of Palaea Kameni near the prehistoric site of Akrotiri, which was buried by the eruption of the Thera volcano over 3500 years ago (see Weninger, 1989).Radiocarbon discrepancies due to volcanic CO2 emissions are a popular source of ammunition for fundamentalist viewpoints keen to present evidence to show that the radiocarbon method is somehow fundamentally flawed.Since about 1890, the use of industrial and fossil fuels has resulted in large amounts of CO2 being emitted into the atmosphere.One of the most commonly referenced reservoir effects concerns the ocean.The average difference between a radiocarbon date of a terrestrial sample such as a tree, and a shell from the marine environment is about 400 radiocarbon years (see Stuiver and Braziunas, 1993).Because the source of the industrial fuels has been predominantly material of infinite geological age ( e.g coal, petroleum), whose radiocarbon content is nil, the radiocarbon activity of the atmosphere has been lowered in the early part of the 20th century up until the 1950's.The atmospheric radiocarbon signal has, in effect, been diluted by about 2%.In order to ascertain the ages of samples which were formed in equilibrium with different reservoirs to these materials, it is necessary to provide an age correction.Implicit in the Conventional Radiocarbon Age BP is the fact that it is not adjusted for this correction.