Carbon 14 dating otzi
With an extra neutron and one less proton, that's no longer a nitrogen atom — six protons plus eight neutrons spells carbon-14.The newly formed carbon-14 atoms end up in carbon dioxide, which ends up in plants, which end up on our dinner plates as fruit, veg or a highly processed version of plants known as meat.And nuclear reactions have seen a leap in carbon-14 activity since 1945.Luckily for us we have a record of atmospheric carbon-14 levels for every one of the last 12,000 years.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.
Everything from the fibres in the Shroud of Turin to Otzi the Iceman has had their birthday determined the carbon-14 way. There's plenty of hydrogen, oxygen and nitrogen in living things too, but carbon's got something none of them do — a radioactive isotope that can take thousands of years to decay.
And that something else starts where Earth meets space.
Earth's upper atmosphere is constantly being bombarded by cosmic rays (usually protons travelling at nearly the speed of light).
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.