Editing Radiocarbon dating (section)

===Atmospheric variation===
[[Image:Hemispheric 14C graphs 1950s to 2010.png|thumb|Atmospheric {{chem|14|C}} for the northern and southern hemispheres, showing percentage excess above pre-bomb levels. The [[Partial Test Ban Treaty]] went into effect on 10 October 1963.<ref name=Hua_etal/>]]

In the early years of using the technique, it was understood that it depended on the atmospheric {{chem|14|C}}/{{chem|12|C}} ratio having remained the same over the preceding few thousand years. To verify the accuracy of the method, several artefacts<!-- This article uses British English, so please do not change this to "artifact". --> that were datable by other techniques were tested; the results of the testing were in reasonable agreement with the true ages of the objects. Over time, however, discrepancies began to appear between the known chronology for the oldest Egyptian dynasties and the radiocarbon dates of Egyptian artefacts. Neither the pre-existing Egyptian chronology nor the new radiocarbon dating method could be assumed to be accurate, but a third possibility was that the {{chem|14|C}}/{{chem|12|C}} ratio had changed over time. The question was resolved by the [[Dendrochronology|study of tree rings]]:<ref name=Bowman_16>Bowman (1995), pp. 16–20.</ref><ref name=Suess_1970>Suess (1970), p.&nbsp;303.</ref><ref name=Taylor2014>Taylor & Bar-Yosef (2014), pp. 50–52.</ref> comparison of overlapping series of tree rings allowed the construction of a continuous sequence of tree-ring data that spanned 8,000 years.<ref name=Bowman_16/> (Since that time the tree-ring data series has been extended to 13,900 years.)<ref name=INTCAL13>{{Cite journal|last1=Reimer|first1=Paula J.|last2=Bard|first2=Edouard|last3=Bayliss|first3=Alex|last4=Beck|first4=J. Warren|last5=Blackwell|first5=Paul G.|last6=Ramsey|first6=Christopher Bronk|last7=Buck|first7=Caitlin E.|last8=Cheng|first8=Hai|last9=Edwards|first9=R. Lawrence|date=2013|title=IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0–50,000 Years cal BP|journal=Radiocarbon|volume=55|issue=4|pages=1869–1887|doi=10.2458/azu_js_rc.55.16947|issn=0033-8222|doi-access=free|bibcode=2013Radcb..55.1869R |hdl=10289/8955|hdl-access=free}}</ref> In the 1960s, [[Hans Suess]] was able to use the tree-ring sequence to show that the dates derived from radiocarbon were consistent with the dates assigned by Egyptologists. This was possible because although annual plants, such as corn, have a {{chem|14|C}}/{{chem|12|C}} ratio that reflects the atmospheric ratio at the time they were growing, trees only add material to their outermost tree ring in any given year, while the inner tree rings don't get their {{chem|14|C}} replenished and instead start losing {{chem|14|C}} through decay. Hence each ring preserves a record of the atmospheric {{chem|14|C}}/{{chem|12|C}} ratio of the year it grew in. Carbon-dating the wood from the tree rings themselves provides the check needed on the atmospheric {{chem|14|C}}/{{chem|12|C}} ratio: with a sample of known date, and a measurement of the value of ''N'' (the number of atoms of {{chem|14|C}} remaining in the sample), the carbon-dating equation allows the calculation of ''N''<sub>0</sub> – the number of atoms of {{chem|14|C}} in the sample at the time the tree ring was formed – and hence the {{chem|14|C}}/{{chem|12|C}} ratio in the atmosphere at that time.<ref name=Bowman_16/><ref name=Taylor2014/> Equipped with the results of carbon-dating the tree rings, it became possible to construct calibration curves designed to correct the errors caused by the variation over time in the {{chem|14|C}}/{{chem|12|C}} ratio.<ref name=renamed_from_18_on_20200701175743>Bowman (1995), pp. 43–49.</ref> These curves are described in more detail [[Radiocarbon dating#Calibration|below]].

Coal and oil began to be burned in large quantities during the 19th century. Both are sufficiently old that they contain little or no detectable {{chem|14|C}} and, as a result, the {{chem|CO|2}} released substantially diluted the atmospheric {{chem|14|C}}/{{chem|12|C}} ratio. Dating an object from the early 20th century hence gives an apparent date older than the true date. For the same reason, {{chem|14|C}} concentrations in the neighbourhood of large cities are lower than the atmospheric average. This fossil fuel effect (also known as the Suess effect, after Hans Suess, who first reported it in 1955) would only amount to a reduction of 0.2% in {{chem|14|C}} activity if the additional carbon from fossil fuels were distributed throughout the carbon exchange reservoir, but because of the long delay in mixing with the deep ocean, the actual effect is a 3% reduction.<ref name=Bowman_16/><ref name=Aitken_71>Aitken (1990), pp.&nbsp;71–72.</ref>

A much larger effect comes from above-ground nuclear testing, which released large numbers of neutrons into the atmosphere, resulting in the creation of {{chem|14|C}}. From about 1950 until 1963, when atmospheric nuclear testing was [[Partial Nuclear Test Ban Treaty|banned]], it is estimated that several tonnes of {{chem|14|C}} were created. If all this extra {{chem|14|C}} had immediately been spread across the entire carbon exchange reservoir, it would have led to an increase in the {{chem|14|C}}/{{chem|12|C}} ratio of only a few per cent, but the immediate effect was to almost double the amount of {{chem|14|C}} in the atmosphere, with the peak level occurring in 1964 for the northern hemisphere, and in 1966 for the southern hemisphere. The level has since dropped, as this [[bomb pulse]] or "bomb carbon" (as it is sometimes called) percolates into the rest of the reservoir.<ref name=Bowman_16/><ref name=Aitken_71/><ref name=PTBT>{{cite web|url=https://2009-2017.state.gov/t/isn/4797.htm|title=Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water|publisher=US Department of State|access-date=2 February 2015}}</ref><ref name=Hua_etal>{{Cite journal|last1=Hua|first1=Quan|last2=Barbetti|first2=Mike|last3=Rakowski|first3=Andrzej Z.|date=2013|title=Atmospheric Radiocarbon for the Period 1950–2010|journal=Radiocarbon|volume=55|issue=4|pages=2059–2072|doi=10.2458/azu_js_rc.v55i2.16177|issn=0033-8222|doi-access=free|bibcode=2013Radcb..55.2059H }}</ref>