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.
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.
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.
