A Japanese research team has studied the variations in beryllium-7 concentrations in the surface air over the Antarctic regions of Southern Ocean. Beryllium-7 is a radioactive isotope of beryllium produced by cosmic rays in the atmosphere. The team explored, over space and time, how the beryllium-7 is transported from the atmosphere to the Earth's surface. Their goal was to better understand the mechanisms of atmospheric mixing on Earth.
Their research is published in the Journal of Geophysical Research: Atmospheres on October 14, 2025.
"We aimed to clarify where and by what atmospheric flows the radioactive isotope beryllium-7, produced in the stratosphere and upper troposphere, is transported to the Earth's surface. To achieve this, we undertook the collection of daily continuous data in the Antarctic region — something that had not been done before," said Naohiko Hirasawa, National Institute of Polar Research, Research, Organization of Information and Systems.
Beryllium-7 is a rare isotope produced when high-energy cosmic rays collide with atoms in the atmosphere, mainly in the lower stratosphere and the upper troposphere. Immediately after its production, it attaches to nearby aerosol particles, allowing it to be transported through atmospheric circulation. High concentrations of beryllium-7 in the air indicate that air from the stratosphere has been transported down into the troposphere. By investigating the atmospheric circulation of beryllium-7 concentrations, researchers can better understand the mechanisms that drive air transport from the stratosphere through the troposphere to the surface of the Antarctic ice sheet.
The team gathered observations over three summers from 2014 to 2018, as part of the Japanese Antarctic Research Expedition. They examined the geographical characteristics of beryllium-7 concentrations over high latitudes in the Indian sector of the Southern Ocean, including two Japanese coastal stations. They collected aerosol particles using a glass fiber filter that trapped particles greater than 0.6 µm in diameter. The team conducted their experiments aboard the Japanese icebreaker Shirase, used for Antarctic expeditions.
They had several goals in undertaking the study. They wanted to describe the spatial distribution of beryllium-7 concentrations over a wider area of the Antarctic region than has been conducted in past studies. They also wanted to investigate variations in beryllium-7 concentrations in relation to synoptic‐scale atmospheric circulation. These are large-scale disturbances, spanning hundreds to thousands of kilometers, that typically move across the Antarctic region with a period of about one week. The team also wanted to examine diurnal variations in beryllium-7 concentrations associated with katabatic winds. These katabatic winds are winds that blow down the Antarctic ice sheet slope because of gravity. The team hopes their results could potentially be used to validate beryllium-7 transport models.
"The greatest challenge was detecting beryllium-7 at extremely low concentrations — a result of the short sampling duration and the long delay between collection and measurement — with sufficient precision to capture its variability. For this reason, each filter analysis required 8 to 12 hours," said Hirasawa.
Their findings showed that variations in the beryllium-7 concentration were connected to synoptic-scale disturbances. These disturbances also deposit other stratospheric materials, like volcanic material from the troposphere, onto the Antarctic ice sheet. These stratospheric materials can be used as climatic markers in ice cores. The team's findings in this study also contribute to decoding paleoclimate atmospheric circulation patterns in ice core investigations.
"We found that beryllium-7 is periodically transported down to near the surface through tropopause foldings associated with synoptic-scale low- and high-pressure systems. In addition, there was another mechanism in which katabatic winds blowing down the Antarctic ice sheet slopes entrained beryllium-7 present in the mid-troposphere over the ice sheet and transported it to coastal regions," said Hirasawa.
The team noted other findings besides the beryllium-7 transport. "While beryllium-7 is supplied from the upper troposphere, another radioactive isotope, radon-222, is emitted from the land surface of continents through soil and rocks. By elucidating the transport processes of these two substances, we aim to deepen our understanding of the mechanisms of atmospheric mixing on Earth," said Hirasawa.
The research team includes Naohiko Hirasawa from the National Institute of Polar Research, Research Organization of Information and System and SOKENDAI (The Graduate University for Advanced Studies); Taku Nakamura, Shigeki Tasaka, and Miyoko Miwa from Gifu University; Tetsuro Ojio from Nagoya City Science Museum; and Kyohei Yamada from National Institute of Polar Research, Research Organization of Information and System.
The research is funded by the Japanese Antarctic Research Expedition and the National Institute of Polar Research.
