Researchers have discovered 6-million-year-old-ice and air in shallow ice cores drilled in the Allan Hills region of East Antarctica. They speculate that this ancient ice reflects surface snowpack or permafrost that was preserved by the growth of the East Antarctic ice sheet in the Middle to Late Miocene epoch.
Ancient ice from the ALHIC1902 ice core. Image credit: Shackleton et al., doi: 10.1073/pnas.2502681122.
“Ice cores are like time machines that let scientists take a look at what our planet was like in the past,” said Dr. Sarah Shackleton, a researcher with Woods Hole Oceanographic Institution.
“The Allan Hills cores help us travel much further back than we imagined possible.”
“This is the most significant discovery to date for the NSF-funded Center for Oldest Ice Exploration (COLDEX),” said Dr. Ed Brook, COLDEX director and a paleoclimatologist at Oregon State University.
“We knew the ice was old in this region. Initially, we had hoped to find ice up to 3 million years old, or maybe a little older, but this discovery has far exceeded our expectations.”
COLDEX is one of several teams around the world currently in a friendly competition to extend the ice core record beyond its previous 800,000-year limit.
Recently a European team announced finding a deep continuous ice core that reached 1.2 million years in the interior of East Antarctica.
Working in a remote field camp in the Allan Hills in East Antarctic for months at a time, the team drilled down 100-200 m on the edges of the ice sheet in several locations where ice flow and rugged mountain topography combine to preserve the old ice and bring it nearer to the ice surface and easier to reach.
In contrast, recovering the oldest continuous ice cores from sites in east Antarctica requires drilling more than 2,000 m deep.
“We’re still working out the exact conditions that allow such ancient ice to survive so close to the surface,” Dr. Shackleton said.
“Along with the topography, it’s likely a mix of strong winds and bitter cold.”
“The wind blows away fresh snow, and the cold slows the ice to almost a standstill.”
“That makes Allan Hills one of the best places in the world to find shallow old ice, and one of the toughest places to spend a field season.”
The trapped air in these new cores allows scientists to directly date the ice through careful measurements of an isotope of the noble gas argon.
Direct dating means scientists measured things in the ice itself that indicate age rather than making an inference based on an associated feature or deposit.
Although the records from this old ice are not continuous, their antiquity is unprecedented, the researchers said.
“By dating many samples, our team has built up a library of what we call ‘climate snapshots’ roughly six times older than any previously reported ice core data, complementing the more detailed younger data from cores in the interior of Antarctica,” Dr. Higgins said.
Temperature records from measurements of oxygen isotopes in the ice reveal that this area experienced a gradual, long-term cooling of about 12 degrees Celsius (22 degrees Fahrenheit).
This is the first direct measure of the amount of cooling in Antarctica over the last 6 million years.
Ongoing research into these ice cores seeks to reconstruct levels of atmospheric greenhouse gases and ocean heat content, which have important implications for understanding the causes of natural climate change.
“The COLDEX team will be heading to the Allan Hills in the coming months for more drilling, with the potential for obtaining more detailed snapshots and even older ice,” Dr. Brook said.
“Given the spectacularly old ice we have discovered at Allan Hills, we also have designed a comprehensive longer-term new study of this region to try to extend the records even further in time, which we hope to conduct between 2026 and 2031.”
The team’s paper was published today in the Proceedings of the National Academy of Sciences.
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S. Shackleton et al. 2025. Miocene and Pliocene ice and air from the Allan Hills blue ice area, East Antarctica. PNAS 122 (44): e2502681122; doi: 10.1073/pnas.2502681122
