Beryllium-10, a rare radioactive isotope produced by cosmic rays in the atmosphere, provides valuable insights into the Earth’s geological history. A research team from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), in collaboration with the TUD Dresden University of Technology and the Australian National University (ANU), has discovered an unexpected accumulation of this isotope in samples taken from the Pacific seabed. Such an anomaly may be attributed to shifts in ocean currents or astrophysical events that occurred approximately 10 million years ago. The findings hold the potential to serve as a global time marker, representing a promising advancement in the dating of geological archives spanning millions of years.
Radionuclides are types of atomic nuclei (isotopes) that decay into other elements over time. They are used to date archaeological and geological samples, with radiocarbon dating being one of the most well-known methods. In principle, radiocarbon dating is based on the fact that living organisms continuously absorb the radioactive isotope carbon-14 (14C) during their lifetime. Once an organism dies, the absorption ceases, and the 14C content starts to decrease through radioactive decay with a half-life of approximately 5,700 years. By comparing the ratio of unstable 14C to stable carbon-12 (12C), researchers can determine the date of the organism’s death.
Archaeological finds, such as bones or remnants of wood, can be dated quite accurately in this way. “However, the radiocarbon method is limited to dating samples no more than 50,000 years old,” explains HZDR physicist Dr. Dominik Koll. “To date older samples, we need to use other isotopes, such as cosmogenic beryllium-10 (10Be).” This isotope is created when cosmic rays interact with oxygen and nitrogen in the upper atmosphere. It reaches the Earth through precipitation and can accumulate on the seabed. With a half-life of 1.4 million years, 10Be decays into boron, allowing geological dating that can extend back over 10 million years.
Conspicuous accumulation of beryllium
Some time ago, Koll’s research group examined unique geological samples retrieved from the Pacific Ocean at a depth of several kilometers. The samples consisted of ferromanganese crusts, primarily composed of iron and manganese, which had formed slowly but steadily over millions of years. To date the samples, the team analyzed the 10Be content using a highly sensitive method – Accelerator Mass Spectrometry (AMS) at HZDR. In this process, the sample is chemically purified before undergoing analysis for trace isotopes. Individual atoms from the sample are accelerated by high voltage, deflected by magnets, and then registered by specialized detectors. This method allows for the precise identification of 10Be, distinguishing it from other beryllium isotopes as well as molecules and isotopes with the same mass, such as boron-10.
When the research group evaluated the collected data, they were in for a surprise. “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.” To eliminate any possibility of contamination, the experts analyzed additional samples from the Pacific, which also exhibited the same anomaly. This consistency allows the team to conclude that it is indeed a real phenomenon.
Read more on HZDR website
Image: Schematic depiction of production and incorporation of cosmogenic 10Be into ferromanganese crusts. A pronounced anomaly in 10Be concentration about 10 million years ago was discovered. This anomaly has great potential as time marker for the Late Miocene.
Credit: HZDR / blrck.de
