A friend of mine told my wife that carbon dating is unreliable because it has been performed on live humans and indicated that the test subject(s) are 4000 years old.

Is there any truth to this claim? I'm having a hard time finding anything about it.

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    You can't do carbon dating on living tissue... Because of respiration, the C-14 is constantly being replaced thus invalidating any results.
    – JasonR
    Dec 1 '14 at 18:53
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    Maybe you mean "contemporary" and not "live" humans?
    – Sklivvz
    Dec 1 '14 at 19:14
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    @Skliwz I can only guess at what she actually said/meant. Would changing the statement to "comtemporary" make it any more truthful? Dec 1 '14 at 19:55
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    Most dating used to determine age for bones/artifacts/etc that are expected to be over 50k years is done through means other than carbon-dating like Thermoluminescence, Tephrochronology, Magnetostratigraphy, etc. Typically several forms of analysis are used on the objects and surrounding sediment in order to get multiple confirmations and greater accuracy. I know this doesn't directly address carbon-dating which is why I am supplying it as a comment
    – kcar
    Dec 1 '14 at 20:29
  • Performing carbon dating on live humans also sounds painful.
    – Shadur
    Dec 2 '14 at 12:10

The use of Carbon-14 for dating is not completely precise. In general, 500 years is the minimum and 50,000 years is the maximum due to the need to calibrate for background C-14 levels, and to have sufficient breakdown to establish the half-life proportions but not so much that the sample is too small to measure.

That said, they're using Carbon-14 dating on recent human remains in forensic science , although the technique works best on bodies around the 1940s to 1960s due to the increased presence of C-14 due to atomic bomb testing. Currently, the atmospheric levels of C-14 are dropping again, so the method will be less feasible for people deceased after that point.

Given the difficulty of dating samples less than 500 years old, I could readily see your friend, or the person who gave him that information, latching on to the idea of less aged items being hard to date and the actual figure involved getting enlarged with the telling.

  • 6
    Correct. For other time ranges, you'd have to use another isotope. Dec 2 '14 at 3:52
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    To sum it up, carbon dating is fairly reliable when used to determine the age of formerly living organic material that died between 500 and 50000 years ago, and becomes drastically less useful for anything outside those parameters.
    – Shadur
    Dec 2 '14 at 12:12
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    @Shadur Not directly relevant to this question I would add the caveat that the organsism used to be airbreathing, and did not feed on waterbreathers to a large extent.
    – Taemyr
    Dec 2 '14 at 15:18
  • Must one be sure at first that the item is more than 500 years old before using C14 dating ? (that would means that at least 2 dating methods must be used)
    – Emmanuel
    Dec 2 '14 at 16:52
  • @Emmanuel: I'm not an expert, but I think it would just come out as "within 500 years old". But yes, for any form of destructive testing, such as radiocardon dating, they usually use other methods to gauge the age of the artifact or specimen. Dec 2 '14 at 19:21

A "live human" would by definition have an age of 0 on the carbon dating scale, since the carbon in the live human is in active exchange with the environment, and hence the C14:C12 ratio would be identical to the baseline. Carbon dating scales measure the changes in this ratio after death of the organism.

This will be true even if humans somehow are capable of actively removing C14 from their bodies, the baseline would be set to that of a live human. One of the first steps in measurement of radiocarbon age is to find a valid baseline, usually from measuring a living organism. One of the comments above is especially relevant in this aspect:

Not directly relevant to this question I would add the caveat that the organsism used to be airbreathing, and did not feed on waterbreathers to a large extent.

Since waterbreathing organisms contain different ratios of C14:C12, the baseline would have to be adjusted, if you are measuring C14 in a mummified fish, for example, the first step would be normalising the C14 ratios to a recently dead (fresh) fish. This link explains why this has to be done:

Hard water contains high levels of calcium carbonate. Carbonate contains carbon, including carbon-14. However, depending on ocean water circulation, fish and other living creatures can incorporate 'older' carbonate (with less carbon-14) into their bodies. When these organisms die and fossilise, they appear to be much older than they actually are.

This book about radiocarbon dating of the Iceman also used a set of standards (wood samples), it having already been shown that humans and trees have comparable C14 ratios when alive.

Therefore, saying that a human was measured to have an age of 4000 using carbon dating would be equivalent to saying that you placed a thermometer into a bowl of melting ice and measured the temperature to be 50 degrees Celsius: the measurement instrument is incorrectly calibrated, or otherwise nonfunctional.

  • Added additional citations, especially regarding the fish claim.
    – March Ho
    Dec 14 '14 at 11:47

There is another case that might be confusing the issue here:

Living organisms are sometimes carbon-dated and generally return non-current ages when this is done. Multiple times I have seen examples of this used as creationist "evidence" that C-14 dates are unreliable when in reality it's nothing of the kind.

What happens is that the creature being dated lives in a fashion that's isolated from the atmosphere to some degree. You do a C-14 date on that bug you brought out of the cave and it's really telling you about the air exchange between the surface (really, the upper atmosphere where C-14 is formed, but the exchange from the upper atmosphere to the surface is far less than the measurement error) and where the bug was found.

  • 2
    Do you have some evidence, source for that? Also how does that apply to live humans? (which usually don't live in caves)
    – drat
    Dec 3 '14 at 6:20
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    Please add references to support your answer. See meta.skeptics.stackexchange.com/questions/1505/…
    – nico
    Dec 3 '14 at 8:07
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    Not a bad answer, but you do need references. They accept links to other sites, although I get the impression that they prefer more lofty references such as academic papers. Dec 3 '14 at 13:19

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