Dating the undateable

Dr Robyn Pickering, isotope geochemist at the University of Cape Town

Dr Robyn Pickering, isotope geochemist at the University of Cape Town

When a fossil is too old for one technique, and the rock around it too young for another, dating a find can be almost impossible. This conundrum has plagued paleoanthropologists in South Africa for a very long time, but thanks to the work of Dr Robyn Pickering things have changed.

Over the past 10 years, the University of Cape Town (UCT)-based isotope geochemist has successfully adapted uranium-lead dating techniques to provide the first set of direct ages for the caves in which early human fossils were found in South Africa. Her work has changed the way this record is viewed around the world.

“As an undergraduate student I was taught that it was impossible to date our fossil sites,” Pickering remembers. This fallacy was based on the geological youthfulness of South African caves. At only between two and three million years old, local cave rock could not be dated using the uranium-lead technique.

Developed in the 1950s, uranium-lead dating is geology’s cornerstone technique — it was used to determine the age of our planet. Until the early 2000s, its use was confined to truly ancient materials due to uranium’s long half-life. For substances younger than 50 000 years, radiocarbon dating is used.

Naturally-occurring radioactive uranium is locked into rocks as they form, and starts decaying into lead almost immediately. Because scientists know the rate at which uranium decays, they only have to measure the amount of uranium and lead present in a rock to determine its age.

In the early 2000s, technology emerged that finally made it possible to use uranium-lead dating on younger rocks. South African cave rock remained a problem, however, as it contains only miniscule amounts of uranium.

Pickering earned her PhD by adapting uranium-lead dating to South African circumstances. “Uranium is not visible to the naked eye, hence we needed to come up with ways to determine where in a rock it could be, so that we could do better fieldwork to bring viable samples back to the lab.”

The combination of field and laboratory work is a hallmark of Pickering’s approach. “The person who does the dating has to understand where the samples are coming from and where the fossils are in relation to that. The big picture is very important, as is the collaboration between everybody working on a fossil site.”

Having spent 10 years in Switzerland and Australia, Pickering returned to South Africa in 2015 to join UCT’s department of geological sciences. “My dream since undergraduate days was to date local cave sites, and I had to go away to learn how to do that,” she says. “But I’m excited to be back to help train and inspire a new generation of scientists. Married and with two small children, I am aware that I can be a positive role model for young women. I also want to help ensure that expertise and funding come to South Africa and stay here.”

As far as the next scientific challenge is concerned, Pickering has her sights set on calcrete, the carbonate that forms in soil and is present at many open-air fossil sites. “Finding out how to date materials that people believe cannot be dated keeps me going. You are never completely done as a researcher — there are always more questions to answer.”