4
$\begingroup$

The life of carbon-14 isotope is about 5 thousand years. But we still are able to detect traces of it in fossils which are older, more than 10 thousand years. Why?

$\endgroup$
2

1 Answer 1

9
$\begingroup$

The half-life of $^{14}$C is $5730\pm40$ years. That means that after 5730 years, we would have 0.5 the original amount of it. After 11460 years we should have 0.25, and after 17190 years we should have 0.125, and after 22920 years we should have 0.0625. Since 6% of original should still be pretty observable, I am not sure what you are having problems with.

Nobody is saying that exactly after 5730 years, there will be no more $^{14}$C. Instead, we should have quite a lot left.

$\endgroup$
7
  • 3
    $\begingroup$ Another related fact: in radiometric dating, it's usually preferable (and accurate) to date objects whose age is presumably within ten half-lives of the target element. For instance, in this case it would be approximately 57300 years, and any older than that researchers usually look for other elements for radio dating. $\endgroup$
    – Jono94
    May 29, 2023 at 10:20
  • 1
    $\begingroup$ I personally haven't read about that, but a search on Google led me to nps.gov/subjects/geology/…., which says that it's about 100 years. I don't know how it's estimated though. $\endgroup$
    – Jono94
    May 29, 2023 at 10:39
  • 1
    $\begingroup$ Radiometric dating objects from the last century or so is a bit messy, for several reasons. The industrial revolution has dumped a lot of old carbon (which has virtually no C-14) into the atmosphere. OTOH, coal has non-trivial amounts of radioactive isotopes, including uranium. Also, nuclear testing in the mid 20th century messed up various other radioisotope profiles. $\endgroup$
    – PM 2Ring
    May 29, 2023 at 11:45
  • 2
    $\begingroup$ @PM2Ring the "messing up" caused by Nuclear testing actually means that radiocarbon dating still works (to some extent) right up to the present day. radiocarbon.com/ams-dating-forensics.htm and see physics.stackexchange.com/questions/346525/… $\endgroup$
    – ProfRob
    May 29, 2023 at 12:50
  • 1
    $\begingroup$ @naturallyInconsistent Another isotope of interest is chlorine-36, which has a half-life of ~300,000 years. It was produced in bomb tests conducted above & under seawater, although at the time the short-lived radioactive sodium was more of a concern due to its high activity. $\endgroup$
    – PM 2Ring
    May 29, 2023 at 13:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.