How to do carbon dating calculation
By about 58,000 years (ten half-lives) after an organism has died, there's so little radioactive carbon left (less than 1/1000) that calculations of age are no longer accurate.That's why radiocarbon dating is only reliable for samples up to 50,000 years old.It's not that the radioactive carbon in air or food doesn't decay, it does.But something else is going on that keeps producing new carbon-14 — otherwise it would have all turned to nitrogen millions of years ago.
The level of radiocarbon in the atmosphere has varied over time — it was about two per cent higher 3,500 years ago, possibly due to factors affecting cosmic rays (like changes in solar cycles or the Earth's magnetic field).
Chemically, carbon-14 is no different from non-radioactive carbon atoms, so it ends up in all the usual carbon places — one trillionth of the carbon atoms in air, plants, animals and us are radioactive.
All radioactive atoms eventually decay into something more stable, and carbon-14 decays into nitrogen.
It's been painstakingly pieced together from the carbon content in living and long-dead tree rings. The layer (or ring) directly reflects the carbon-14 content of that year, so a ring that was formed 500 years ago tells us the ratio of carbon-14 to regular carbon-12 (14C/12C) of something that died 500 years ago.
As well as the tree ring record, scientists have used the carbon record from corals to calculate C14/C12 levels right back to 50,000 years ago.