Radiocarbon dating burnt wood
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).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.
But when we stop eating, or when plants stop photosynthesising, our carbon-14 levels no longer get topped up.So calculating the age range of a once-living sample involves measuring the 14C/12C ratio, and using this the known half-life to estimate the length of time since the sample died.That age range is then compared with known 14C/12C ratios from the tree ring/marine record to find the best match, and the result is a calibrated age range you can be 95 per cent sure of.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.
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.