As Dr Dominik Koll was investigating a new method of radio-dating ancient samples, he stumbled across evidence of a major event in the Earth’s history, around ten million years ago.

“It was completely unexpected. I was just dating an environmental archive with the radio isotope beryllium-10 and I didn’t expect anything exciting to happen,” said Dr Koll, who did the work as part of his PhD in the Department of Nuclear Physics and Accelerator Applications.

“I stumbled on an unexpected accumulation of beryllium-10 around ten million years ago; what caused it is unknown, but it may point to major events such as global shifts in ocean currents, or nearby astrophysical events.”

Dr Koll, who was a student jointly with Helmholtz-Zentrum Dresden-Rossendorf (HZDR), is lead author of a paper detailing the surprise, published in Nature Communications.

The team, which included researchers from ANU, HZDR and Dresden University of Technology was exploring the dating of samples of deep ocean crust from the Pacific Ocean, using beryllium-10, similar to radiocarbon (carbon-14) dating. 

Like carbon-14, beryllium-10 is continuously created in the upper atmosphere by cosmic rays, but its half-life is much longer. At 1.4 million years it offers dating about 300 times further back through history than carbon-14 (half-life 5,700 years), which is limited to about 50,000 years ago. The catch is beryllium-10 is produced only in minuscule amounts in the upper atmosphere. But Dr Koll has at his disposal the incredibly sensitive measurement technique of accelerator mass spectrometry.

Atoms such as beryllium-10 gradually fall from the atmosphere, some landing on the ocean, and sinking to become part of the crust. Previously, the team had found other interesting isotopes in the Pacific Ocean crust. They were looking to develop a solid age model that spanned the millions of years of deposition in the crust sample – beryllium-10 seemed promising so Dr Koll began the dating. 

Detailed scans of the sample made at the ANU MicroCT lab showed age layers, like tree rings, which were carefully machined off by the Research School of Physics workshop, and chemically prepared for analysis at the isotope chemistry labs at the ANU Heavy Ion Accelerator Facility.

The samples were then analysed at HZDR using accelerator mass spectrometry, which can pick out concentrations of beryllium-10 atoms as low as one part in a quadrillion. 

Working back through the layers, Dr Koll found the beryllium-10 concentration dropped as expected in older layers, due to the atoms decaying. Until, at 10 million years ago – about 3.5 cm into the sample – there was an unexpected peak in the otherwise smooth curve.

Dr Koll proposes two possible explanations. One is related to the ocean circulation near Antarctica.

“Ocean currents are thought to have changed drastically 10 to 12 million years ago, which could have caused beryllium-10 to become particularly concentrated in the Pacific Ocean.”

A second explanation could be after-effects of a near-Earth supernova, which could have caused cosmic radiation to become temporarily more intense 10 million years ago. Alternatively, the Earth might have temporarily lost its protective solar shield – the heliosphere – due to a collision with a dense interstellar cloud, making it more vulnerable to cosmic radiation. 

“Only new measurements can indicate whether the beryllium anomaly was caused by changes in ocean currents or has astrophysical reasons,” Dr Koll said.

“We plan to analyse more samples in the future and hope that other research groups will do the same – if the anomaly is detected only in specific regions, altered ocean currents will be more plausible, if not, then astrophysical explanations seem more likely.”