“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.
For archaeologists such as Spooner trying to date the first occupation of Australia, older age limitations of radiocarbon dating are frustrating, as it is exactly this period in which they are most interested.
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But once it dies, no more fresh radiocarbon is absorbed, and what’s left starts to decay.
Once samples are older than around 40,000 years, though, amounts of radiocarbon remaining are very small and difficult to measure.
This fits with the 49,000-year-old radiocarbon date, given that it takes a few hundred years before amassed sand is firmly trampled into the floor and no longer exposed to sunlight.
Previous multi-grain OSL dating at a number of ancient sites have suggested humans arrived in Australia well over 50,000 years ago, but Spooner is sceptical of many of these dates.
Radiocarbon is an isotope with two extra neutrons, created by cosmic rays interacting with nitrogen in Earth's atmosphere.
When a plant or animal is alive, it constantly replenishes trace amounts of radiocarbon in its tissues.
So along with radiocarbon dating, they use a technique known as optically stimulated luminescence (OSL) dating.
It finds the age of the sediment surrounding artefacts – sediment which may have once been outside sand trampled into caves tens of thousands of years ago – by measuring when it was last exposed to the sun.
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.
An archaeologist’s staple is radiocarbon dating: judging the age of an organic sample from its carbon-14 – also known as radiocarbon – content.