Most carbon on Earth exists as the very stable isotope carbon-12, with a very small amount as carbon-13.Carbon-14 is an unstable isotope of carbon that will eventually decay at a known rate to become carbon-12.
Scratching around in a cave in the middle of nowhere, you find a bone.
How do you find out if it’s the remains of an ancient animal that stomped the land tens of thousands of years ago or a discarded scrap from a cooking fire only a few hundred years back?
But when gas exchange is stopped, be it in a particular part of the body like in deposits on bones and teeth, or when the entire organism dies, the ratio of carbon-14 to carbon-12 begins to decrease.
The unstable carbon-14 gradually decays to carbon-12 at a steady rate. Scientists measure the ratio of carbon isotopes to be able to estimate how far back in time a biological sample was active or alive.
Then, only exceptionally well-preserved, pristine samples can provide reliable dates.
At Warratyi rock shelter in the Flinders Ranges, South Australia, which shows signs of the oldest human occupation of the country’s arid interior, the oldest sample – a fragment of emu eggshell – has been radiocarbon dated to 49,000 years with reasonable confidence.
This plot shows the level of carbon-14 in the atmosphere as measured in New Zealand (red) and Austria (green), representing the Northern and Southern Hemispheres, respectively.
Aboveground nuclear testing almost doubled the amount of carbon-14 in the atmosphere. The black arrow shows when the Partial Test Ban Treaty was enacted that banned aboveground nuclear tests. A special kind of radiocarbon dating: Bomb radiocarbon dating.
“Unlike bone or charcoal, carbon preserved in eggshell is very stably locked in and unlikely to have been contaminated,” says Nigel Spooner, a physicist at the University of Adelaide in Australia who specialises in dating techniques.