Editing Carbon-14 (section)

==Occurrence==

=== Dispersion in the environment ===
After production in the upper atmosphere, the carbon-14 reacts rapidly to form mostly (about 93%) {{sup|14}}CO ([[carbon monoxide]]), which subsequently oxidizes at a slower rate to form {{chem|14|CO|2}}, radioactive [[carbon dioxide]]. The gas mixes rapidly and becomes evenly distributed throughout the atmosphere (the mixing timescale on the order of weeks). Carbon dioxide also dissolves in water and thus permeates the [[ocean]]s, but at a slower rate.<ref name="Ramsey-2008"/> The atmospheric half-life for removal of {{chem|14|CO|2}} has been estimated at roughly 12 to 16 years in the Northern Hemisphere. The transfer between the ocean shallow layer and the large reservoir of [[bicarbonate]]s in the ocean depths occurs at a limited rate.<ref name="Yim-2006">{{cite journal|doi=10.1016/j.pnucene.2005.04.002 |title=Life cycle and management of carbon-14 from nuclear power generation |year=2006 | vauthors = Yim MS, Caron F |journal=Progress in Nuclear Energy |volume=48 |pages=2–36 }}</ref>
In 2009 the activity of {{chem|14|C}} was 238&nbsp;Bq per kg carbon of fresh terrestrial biomatter, close to the values before atmospheric nuclear testing (226 Bq/kg C; 1950).<ref>{{cite web | url= http://www.irsn.fr/EN/Research/publications-documentation/radionuclides-sheets/environment/Pages/carbon14-environment.aspx#3 | publisher= Institute for Radiological Protection and Nuclear Safety | title= Carbon-14 and the environment | url-status= live | archive-url= https://web.archive.org/web/20150418012710/http://www.irsn.fr/EN/Research/publications-documentation/radionuclides-sheets/environment/Pages/carbon14-environment.aspx#3 | archive-date= 2015-04-18 }}</ref>

===Total inventory===
The inventory of carbon-14 in Earth's biosphere is about 300 [[Curie (unit)|megacuries]] (11&nbsp;[[Exa-|E]][[Becquerel|Bq]]), of which most is in the oceans.<ref>{{cite web|title=Human Health Fact Sheet – Carbon 14 |publisher=Argonne National Laboratory, EVS |date=August 2005 |url=http://www.ead.anl.gov/pub/doc/carbon14.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110716164724/http://www.ead.anl.gov/pub/doc/carbon14.pdf |archive-date=2011-07-16 }}</ref>
The following inventory of carbon-14 has been given:<ref name="Choppin-2002">{{cite book | vauthors = Choppin GR, [[Jan-Olov Liljenzin|Liljenzin JO]], Rydberg J | date = 2002 | title = Radiochemistry and Nuclear Chemistry | edition = 3rd | publisher = Butterworth-Heinemann | isbn = 978-0-7506-7463-8}}</ref>
* Global inventory: ~8500&nbsp;PBq (about 50&nbsp;[[tonne|t]])
** Atmosphere: 140&nbsp;PBq (840&nbsp;kg)
** Terrestrial materials: the balance
* From nuclear testing (until 1990): 220&nbsp;PBq (1.3&nbsp;t)

===In fossil fuels===

{{main|Suess effect}}

Many human-made chemicals are derived from [[fossil fuel]]s (such as [[petroleum]] or [[coal]]) in which {{sup|14}}C is greatly depleted because the age of fossils far exceeds the half-life of {{sup|14}}C. The relative absence of {{chem|14|CO|2}} is therefore used to determine the relative contribution (or [[mixing ratio]]) of fossil fuel oxidation to the total [[carbon dioxide]] in a given region of Earth's [[atmosphere]].<ref name="NOAA-2015">{{cite web
 | url = http://www.esrl.noaa.gov/gmd/outreach/isotopes/c14tracer.html
 | title = The Basics: 14C and Fossil Fuels
 | website = NOAA ESRL GMD Education and Outreach
 | archive-url = https://web.archive.org/web/20150925082306/http://esrl.noaa.gov/gmd/outreach/isotopes/c14tracer.html
 | archive-date = 25 September 2015
 | access-date = 9 Dec 2015
 | quote = All other atmospheric carbon dioxide comes from young sources–namely land-use changes (for example, cutting down a forest in order to create a farm) and exchange with the ocean and terrestrial biosphere. This makes 14C an ideal tracer of carbon dioxide coming from the combustion of fossil fuels. Scientists can use 14C measurements to determine the age of carbon dioxide collected in air samples, and from this can calculate what proportion of the carbon dioxide in the sample comes from fossil fuels.
}}</ref>

Dating a specific sample of fossilized carbonaceous material is more complicated. Such deposits often contain trace amounts of {{sup|14}}C. These amounts can vary significantly between samples, ranging up to 1% of the ratio found in living organisms (an apparent age of about 40,000 years).<ref>{{cite journal|title=Problems associated with the use of coal as a source of C14-free background material| vauthors = Lowe D |journal=Radiocarbon|year=1989|volume=31|issue=2|pages=117–120|url=https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/1127/1132|url-status=live|archive-url=https://web.archive.org/web/20130724153305/https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/1127/1132|archive-date=2013-07-24|doi=10.1017/S0033822200044775|doi-access=free| bibcode = 1989Radcb..31..117L }}</ref> This may indicate contamination by small amounts of bacteria, underground sources of radiation causing a {{sup|14}}N(n,p){{sup|14}}C reaction, direct [[uranium]] decay (though reported measured ratios of {{sup|14}}C/U in uranium-bearing ores<ref>{{cite journal |title=Carbon-14 Abundances in Uranium Ores and Possible Spontaneous Exotic Emission from U-Series Nuclides | vauthors = Jull AJ, Barker D, Donahue DJ |journal=Meteoritics |volume=20 |year=1985 |page=676 |bibcode=1985Metic..20..676J}}</ref> would imply roughly 1 uranium atom for every two carbon atoms in order to cause the {{sup|14}}C/{{sup|12}}C ratio, measured to be on the order of 10{{sup|−15}}), or other unknown secondary sources of {{sup|14}}C production. The presence of {{sup|14}}C in the [[isotopic signature]] of a sample of carbonaceous material possibly indicates its contamination by biogenic sources or the decay of radioactive material in surrounding geologic strata. In connection with building the [[Borexino]] solar neutrino observatory, petroleum feedstock (for synthesizing the primary scintillant) was obtained with low {{sup|14}}C content. In the Borexino Counting Test Facility, a {{sup|14}}C/{{sup|12}}C ratio of 1.94×10{{sup|−18}} was determined;<ref>{{cite journal | vauthors = Alimonti G, Angloher G, Arpesella C, Balata M, Bellini G, Benziger J, Bonetti S, Cadonati L, Calaprice FP, Cecchet G, Chen M | display-authors = 6 |doi = 10.1016/S0370-2693(97)01565-7|title = Measurement of the <sup>14</sup>C abundance in a low-background liquid scintillator |journal = Physics Letters B|volume = 422 |issue=1–4 |year=1998|pages=349–358|bibcode = 1998PhLB..422..349B }}</ref> probable reactions responsible for varied levels of {{sup|14}}C in different [[petroleum reservoir]]s, and the lower {{sup|14}}C levels in methane, have been discussed by Bonvicini et al.<ref>{{cite arXiv|eprint=hep-ex/0308025| vauthors = Bonvicini G, Harris N, Paolone V |title=The chemical history of <sup>14</sup>C in deep oilfields|year=2003}}</ref>

===In the human body===
Since many sources of human food are ultimately derived from terrestrial plants, the relative concentration of {{sup|14}}C in human bodies is nearly identical to the relative concentration in the atmosphere. The rates of disintegration of [[potassium-40]] ({{sup|40}}K) and {{sup|14}}C in the normal adult body are comparable (a few thousand decays per second).<ref>{{cite web | vauthors = Rowland RE | url = http://www.rerowland.com/BodyActivity.htm | title = The Radioactivity of the Normal Adult Body | archive-url = https://web.archive.org/web/20110205025628/http://www.rerowland.com/BodyActivity.htm | archive-date=2011-02-05 | work = rerowland.com }}</ref> The beta decays from external (environmental) radiocarbon contribute about 0.01&nbsp;[[mSv]]/year (1&nbsp;mrem/year) to each person's [[Equivalent dose|dose]] of [[ionizing radiation]].<ref>{{cite book| title=Ionizing Radiation Exposure of the Population of the United States {{!}} NCRP Report No. 93 | publisher=National Council on Radiation Protection and Measurements | year=1987 | url = http://lbl.gov/abc/wallchart/chapters/15/3.html | archive-url = https://web.archive.org/web/20070711052408/http://www.lbl.gov/abc/wallchart/chapters/15/3.html | archive-date=2007-07-11 }})</ref> This is small compared to the doses from potassium-40, {{sup|40}}K (0.39&nbsp;mSv/year) and [[radon]] (variable depending on where you live).

{{sup|14}}C can be used as a [[radioactive tracer]] in medicine. In the initial variant of the [[urea breath test]], a diagnostic test for ''[[Helicobacter pylori]]'', urea labeled with about {{convert|37|kBq|uCi|abbr=on|lk=on}} {{sup|14}}C is fed to a patient (i.e. 37,000 decays per second). In the event of a ''H. pylori'' infection, the bacterial [[urease]] enzyme breaks down the [[urea]] into [[ammonia]] and radioactively-labeled [[carbon dioxide]], which can be detected by low-level counting of the patient's breath.<ref>{{cite web|title=Society of Nuclear Medicine Procedure Guideline for C-14 Urea Breath Test |date=2001-06-23 |url=http://interactive.snm.org/docs/pg_ch07_0403.pdf |access-date=2007-07-04 |work=snm.org |url-status=dead |archive-url=https://web.archive.org/web/20070926152956/http://interactive.snm.org/docs/pg_ch07_0403.pdf |archive-date=2007-09-26 }}</ref>