Spike

Heaton and his team looked at 140 tree trunks buried in a bank of the Durance river in Provence. As the bank eroded the trunks were exposed and the team could look for  raised levels of carbon-14, a kind of carbon that has two more neutrons than normal and is produced by energetic particles hitting Earth’s atmosphere.

By comparing the tree rings and constructing a time line of when each tree lived, the researchers dated a huge spike in carbon-14 to 14,300 years ago. They also matched this spike to elevated levels of beryllium from Green land ice cores, which is produced in a similar way to carbon-14.