Something extraordinary happened on Earth around 10 million years ago, and whatever it was, it left behind a “signature” of radioactive beryllium-10. This finding, which is based on studies of rocks located deep beneath the ocean, could be evidence for a previously-unknown cosmic event or major changes in ocean circulation. With further study, the newly-discovered beryllium anomaly could also become an independent time marker for the geological record.
Most of the beryllium-10 found on Earth originates in the upper atmosphere, where it forms when cosmic rays interact with oxygen and nitrogen molecules. Afterwards, it attaches to aerosols, falls to the ground and is transported into the oceans. Eventually, it reaches the seabed and accumulates, becoming part of what scientists call one of the most pristine geological archives on Earth.
Because beryllium-10 has a half-life of 1.4 million years, it is possible to use its abundance to pin down the dates of geological samples that are more than 10 million years old. This is far beyond the limits of radiocarbon dating, which relies on an isotope (carbon-14) with a half-life of just 5730 years, and can only date samples less than 50 000 years old.
Almost twice as much 10Be than expected
In the new work, which is detailed in Nature Communications, physicists in Germany and Australia measured the amount of beryllium-10 in geological samples taken from the Pacific Ocean. The samples are primarily made up of iron and manganese and formed slowly over millions of years. To date them, the team used a technique called accelerator mass spectrometry (AMS) at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). This method can distinguish beryllium-10 from its decay product, boron-10, which has the same mass, and from other beryllium isotopes.
The researchers found that samples dated to around 10 million years ago, a period known as the late Miocene, contained almost twice as much beryllium-10 as they expected to see. The source of this overabundance is a mystery, says team member Dominik Koll, but he offers three possible explanations. The first is that changes to the ocean circulation near the Antarctic, which scientists recently identified as occurring between 10 and 12 million years ago, could have distributed beryllium-10 unevenly across the Earth. “Beryllium-10 might thus have become particularly concentrated in the Pacific Ocean,” says Koll, a postdoctoral researcher at TU Dresden and an honorary lecturer at the Australian National University.
Another possibility is that a supernova exploded in our galactic neighbourhood 10 million years ago, producing a temporary increase in cosmic radiation. The third option is that the Sun’s magnetic shield, which deflects cosmic rays away from the Earth, became weaker through a collision with an interstellar cloud, making our planet more vulnerable to cosmic rays. Both scenarios would have increased the amount of beryllium-10 that fell to Earth without affecting its geographic distribution.
Epoch or event? Defining the Anthropocene
To distinguish between these competing hypotheses, the researchers now plan to analyse additional samples from different locations on Earth. “If the anomaly were found everywhere, then the astrophysics hypothesis would be supported,” Koll says. “But if it were detected only in specific regions, the explanation involving altered ocean currents would be more plausible.”
Whatever the reason for the anomaly, Koll suggests it could serve as a cosmogenic time marker for periods spanning millions of years, the likes of which do not yet exist. “We hope that other research groups will also investigate their deep-ocean samples in the relevant period to eventually come to a definitive answer on the origin of the anomaly,” he tells Physics World.
