Lauren Fuge
A European-Australian research team has stumbled upon an unexpected accumulation of the rare radioactive isotope Beryllium-10 at the bottom of the Pacific Ocean. Dating to around 10 million years ago, this anomaly may serve as a global ‘time marker’ that will improve geological dating methods into the deep past.
But the mystery remains as to why so much of this isotope is present from 10 million years ago – and the answer may be out-of-this-world.
Deep dive on dating
Beryllium-10 is an example of a radionuclide: atomic nuclei (known as isotopes) that decay into other elements over time by releasing protons or neutrons.
If an isotope decays at a stable rate and its half-life (the time it takes for half the ‘parent’ atoms to decay into ‘daughter’ ones) is known, scientists can measure the relative concentrations of elements in a given sample and calculate its age.
This is called radiometric dating, and it can be used to determine the age of ancient things from a rock to a fossil to a wooden artefact.
The most common method is carbon dating, using the radioactive isotope of carbon, 14C. But 14C only has a short half life of approximately 5,700 years.
Credit: HZDR / blrck.de
“The radiocarbon method is limited to dating samples no more than 50,000 years old,” says Dominik Koll, a physicist from Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and lead author on the recent research. “To date older samples, we need to use other isotopes, such as cosmogenic beryllium-10 (10Be).”
This isotope has a half-life of 1.4 million years – allowing researchers to date objects back to 10 million years ago.
For example, 10Be has previously been used to push back the dating of Australopithecus fossils by a million years, and to confirm a massive solar storm indicated by ancient tree rings.
Koll’s research group discovered an unexpected amount of 10Be in samples of ferromanganese crust, taken from the sea bed of the Pacific Ocean.
Formed slowly and steadily out of iron and manganese, this crust is “one of the most pristine geological archives,” as Koll and colleagues write in their new paper.
When the team measured its 10Be content, the results were surprising.
“At around 10 million years, we found almost twice as much 10Be as we had anticipated,” reports Koll. “We had stumbled upon a previously undiscovered anomaly.”
This deviation could improve geological dating methods into deep time, by providing an independent ‘time marker’ to help synchronise different datasets.
“For periods spanning millions of years, such cosmogenic time markers do not yet exist,” Koll explains. “This beryllium anomaly has the potential to serve as such a marker.”
The unsolved anomaly
But the question remains: why does this striking accumulation exist?
The key may lie in how 10Be is formed. The vast majority of it is created when cosmic rays slam into the Earth’s atmosphere and interact with oxygen and nitrogen atoms. The 10Be is then deposited across the planet through precipitation. On land, it becomes fixed in soils, locked in ice, or transported through river systems. Some isotopes make it to the seafloor, where slow-growing manganese nodules can absorb it.
The research team suggest two possible explanations for the massive accumulation of 10Be 10 million years ago.
Firstly, evidence suggests that around 10 to 12 million years ago, the oceanic circulation around Antarctica changed dramatically. This may have caused 10Be to be unevenly distributed around the planet, with a concentration in the Pacific.
The alternative options are astrophysical. The researchers propose that a supernova may have exploded near Earth at this time, temporarily increasing the cosmic rays bombarding our atmosphere and thus increasing the overall production of 10Be. Or, perhaps the Earth’s protective heliosphere was damaged, leaving it more vulnerable to cosmic rays.
“Only new measurements can indicate whether the beryllium anomaly was caused by changes in ocean currents or has astrophysical reasons,” says Koll.
The research appears in the journal Nature Communications.
