Next time you discover a mummified corpse, you’ll know who to call…

Ever wonder how carbon dating works? Or how scientists can tell how old something is, just by testing the dirt they found it in?

Well, wonder no more. Below is my article on carbon detective Dr Stewart Fallon, an ANU expert on carbon dating. It first appeared in the magnificent Crinkling News.

Carbon sleuths solve 5,000-year-old crimes

Dr Stewart Fallon is a carbon detective. By measuring how much carbon is in a once-living thing—whether it’s charcoal, shellfish, coral or bone—Dr Fallon can work out when that thing was last alive.

“We can measure back to about 50,000 years,” says Dr Fallon, who works with a team at the Australian National University.

The method is called carbon dating, and Dr Fallon uses it to help solve mysteries, fight crime, and better understand our environment.

Mysterious bones

In 2016, a human jaw bone, leg bone and arm bone were found in a Brisbane park. The police asked Dr Fallon to carbon-date the jaw bone.

Carbon levels in the bone showed it wasn’t a recent murder: the person probably died between 1800 and 1899.

Corals and ivory

Dr Fallon also uses carbon dating on coral reefs. “Corals can live for several hundred years, and have growth rings, just like trees do,” he says. “We’ve dated some deep sea black corals that have been living for 4500 years.”

Horns and tusks can also be dated, helping police work out when the animal died. “We do a lot of work with police on wildlife forensics, trying to help prevent the illegal trade in rhino horn and ivory,” says Dr Fallon.

Ötzi the Iceman

In 1991, after a warm summer melted mountain ice, hikers discovered the mummified remains of a human body, now nicknamed Ötzi the Iceman.

Carbon dating showed Ötzi died more than 5000 years ago. “He was almost perfectly preserved, he’d been covered with ice for a long time,” says Dr Fallon. “They were able to date food in his stomach, and the grass his shoes were made from.”

What are carbon atoms?

Everything is built of tiny packages, called atoms. And every living (or once-living) thing—whether it’s your lunch, your goldfish, or your bones—contains atoms of carbon. Even the air we breathe contains carbon atoms, bound up in carbon dioxide gas. But not all carbon atoms are the same:

¹²C: More than 98% of carbon atoms contain 12 particles in their central nucleus. We call this carbon-12.

¹³C: Nearly 1.1% of carbon atoms are slightly heavier than carbon-12. They’re called carbon-13, because their nucleus contains 13 particles.

¹⁴C: A teensy-tiny percentage of carbon atoms are heavier still, with 14 particles in their nucleus. These carbon-14 atoms are so heavy, they sometimes fall apart. When this happens, they ‘disappear’, decaying into nitrogen atoms instead.

How does carbon dating work?

The number of ¹⁴C atoms compared to ¹²C atoms is called the ¹⁴C to ¹²C ratio.

• While you’re alive: The ¹⁴C to ¹²C ratio in your body is the same as in the air. You add some ¹⁴C when you eat and breathe, and you lose some ¹⁴C when it decays, so the overall level stays the same.
• Once you’re dead: You can’t add more ¹⁴C (because you’re dead, so you can’t eat or breathe). But the ¹⁴C in your body can still decay. We know it takes 5730 years for half of your ¹⁴C to disappear. This means we can use ¹⁴C levels as a kind of clock
• The deadly difference: Because your ¹⁴C has been disappearing, the ¹⁴C to ¹²C ratio in your dead body will be different to the ratio in the air. This difference allows Dr Fallon to work out how long you’ve been dead.

History in the air

The ratio of ¹⁴C to ¹²C in the air is always changing. For example, the nuclear weapons tests that happened from 1955–1963 temporarily doubled ¹⁴C levels.

On the other hand, burning fossil fuels releases loads of carbon, but hardly any of it is ¹⁴C. Fossil fuels are so old that all of their ¹⁴C has decayed, Dr Fallon says.