Editing Radiocarbon dating (section)

===Carbon exchange reservoir===
[[File:Carbon exchange reservoir 2.svg|right|thumb|upright=1.8|Simplified version of the carbon exchange reservoir, showing proportions of carbon and relative activity of the {{chem|14|C}} in each reservoir<ref name=Bowman_9/>{{#tag:ref|The data on carbon percentages in each part of the reservoir is drawn from an estimate of reservoir carbon for the mid-1990s; estimates of carbon distribution during pre-industrial times are significantly different.<ref name=GC_128>Post (2001) pp.&nbsp;128–129.</ref>|group=note}}]]

Carbon is distributed throughout the atmosphere, the biosphere, and the oceans; these are referred to collectively as the carbon exchange reservoir,<ref>Aitken (2003), p. 506.</ref> and each component is also referred to individually as a carbon exchange reservoir. The different elements of the carbon exchange reservoir vary in how much carbon they store, and in how long it takes for the {{chem|14|C}} generated by cosmic rays to fully mix with them. This affects the ratio of {{chem|14|C}} to {{chem|12|C}} in the different reservoirs, and hence the radiocarbon ages of samples that originated in each reservoir.<ref name=Bowman_9/> The atmosphere, which is where {{chem|14|C}} is generated, contains about 1.9% of the total carbon in the reservoirs, and the {{chem|14|C}} it contains mixes in less than seven years.<ref name=Warneck_690>Warneck (2000), p.&nbsp;690.</ref> The ratio of {{chem|14|C}} to {{chem|12|C}} in the atmosphere is taken as the baseline for the other reservoirs: if another reservoir has a lower ratio of {{chem|14|C}} to {{chem|12|C}}, it indicates that the carbon is older and hence that either some of the {{chem|14|C}} has decayed, or the reservoir is receiving carbon that is not at the atmospheric baseline.<ref name=Aitken1990/> The ocean surface is an example: it contains 2.4% of the carbon in the exchange reservoir, but there is only about 95% as much {{chem|14|C}} as would be expected if the ratio were the same as in the atmosphere.<ref name=Bowman_9/> The time it takes for carbon from the atmosphere to mix with the surface ocean is only a few years,<ref>Ferronsky & Polyakov (2012), p. 372.</ref> but the surface waters also receive water from the deep ocean, which has more than 90% of the carbon in the reservoir.<ref name=Aitken1990/> Water in the deep ocean takes about 1,000 years to circulate back through surface waters, and so the surface waters contain a combination of older water, with depleted {{chem|14|C}}, and water recently at the surface, with {{chem|14|C}} in equilibrium with the atmosphere.<ref name=Aitken1990/>

Creatures living at the ocean surface have the same {{chem|14|C}} ratios as the water they live in, and as a result of the reduced {{chem|14|C}}/{{chem|12|C}} ratio, the radiocarbon age of marine life is typically about 400 years.<ref name=Bowman1995>Bowman (1995), pp. 24–27.</ref><ref name=Cronin2010>Cronin (2010), p. 35.</ref> Organisms on land are in closer equilibrium with the atmosphere and have the same {{chem|14|C}}/{{chem|12|C}} ratio as the atmosphere.<ref name=Bowman_9/>{{#tag:ref|For marine life, the age only appears to be 400 years once a correction for [[Radiocarbon dating#Fractionation|fractionation]] is made. This effect is accounted for during calibration by using a different marine calibration curve; without this curve, modern marine life would appear to be 400 years old when radiocarbon dated. Similarly, the statement about land organisms is only true once fractionation is taken into account.|group=note}} These organisms contain about 1.3% of the carbon in the reservoir; sea organisms have a mass of less than 1% of those on land and are not shown in the diagram. Accumulated dead organic matter, of both plants and animals, exceeds the mass of the biosphere by a factor of nearly 3, and since this matter is no longer exchanging carbon with its environment, it has a {{chem|14|C}}/{{chem|12|C}} ratio lower than that of the biosphere.<ref name=Bowman_9/>