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KCC's Quizzes AQQ245 on Fossil Dating with Carbon14

   

You probably heard the method of dating fossils by measuring Carbon14 (also note C14 concentration. To refresh, the concentration of C14 in the air and on earth surface is constant (thanks to cosmic radiation) since ever (in fact it is assumed so). C14 is not stable and is changed into Nitrogen and an electron: C14 concentration versus the standard carbon (C12) decades with time. That speed is characterized by Its half-life period which is 5568 years for C14 (quantity of C14 still present in an isolated place decays by half after each 5568 years).  

Today, in free air and on the earth surface, the ratio N0 = C14/C12 is 0.8 10-12

A mammoth bone has been just extracted and the concentration of C14/C12 is 0.2 10-12.

 Questions:

1. How old is that bone?

2. Since humanity is constantly putting more carbon in the atmosphere. How it affects the Carbon14 dating? For example, if C12 concentration is doubled. How will be the dating of our previous bone?

Good Luck!

P.S. If you think there are colleagues who should try those quizzes, please forward them the link!



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[edited by: StephenV at 3:24 PM (GMT -4) on 18 Oct 2023]
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  • I agree,

    somehow.


    I find it analogous to discharging a capacitor. Parameters:  Vc0, the initial voltage inside the capacitor, is like the initial ratio C14/(C12+C14); the time of discharge is the time since death of the living part we analyzed, the RC constant is similar to the decay speed of C14, and finally, Cvt, the tested actual voltage at the capacitor is the actual ratio C14/(C12+C14). We have two parameters left: Vc0, the initial voltage or the initial ratio, and t, the time since we started the discharge. Giving one of these allows us to find the other.


    But.


    For a dead part, that component does not "refill" itself with "today" Carbon of any kind. Assuming that C14 was in a steady proportion (what is destroyed by fission equals what was created in the atmosphere) we were able to assume Vc0. But since the Carbon source of the new carbon for C02 comes mostly from a carbon source depleted of C14  (long time dead living parts, as coal or oil) we won't be able to use that technique for "dead" parts not older than about 200 years. It is unfortunate that unlike Uranium contained in a Zircon crystal, we can count the Uranium atoms AND the Lead atoms to get the initial total number of Uranium atoms.

  • I think we are using different assumptions? Under my hypothesis (If, to calculate the age of a "carbon isolated" bone - so N is still 0.2 -, we compare the 14C/12C concentration we find in this bone with current atmosphere concentration, then, if the 12C concentration in the atmosphere NOW is doubled, No=14C/12C is halved) the formula t = Ln(No/N) * (T/Ln2) provides shorter times.