Researchers have discovered 6-million-year-old ice and air in shallow ice cores drilled in the Allan Hills region of East Antarctica. They suggest that this ancient ice reflects surface snow cover or permafrost that was preserved by the growth of the East Antarctic ice sheet during the Middle and Late Miocene era.
Ancient ice from ice core ALHIC1902. Image credit: Shackleton etc.., doi: 10.1073/pnas.2502681122.
“Ice cores are like time machines, giving scientists a glimpse into what our planet was like in the past,” said Dr. Sarah Shackleton, a researcher at Woods Hole Oceanographic Institution.
“The Allan Hills cores are helping us travel much further back than we ever imagined.”
“This is the most significant discovery to date for the NSF-funded Center for the Study of Ancient Ice (COLDEX),” said Dr. Ed Brook, COLDEX director and paleoclimatologist at Oregon State University.
“We knew the ice in this region was old. We initially hoped to find ice up to 3 million years old or maybe a little older, but this discovery far exceeded our expectations.”
COLDEX is one of several teams around the world currently engaged in a friendly competition to extend the record for collecting ice cores beyond the previous limit of 800,000 years.
A European team recently announced the discovery of a deep, continuous ice core dating back 1.2 million years in the interior of East Antarctica.
Working for months at a remote field camp in the Allan Hills in East Antarctica, the team drilled 100-200 meters deep along the edges of the ice sheet in several places where ice flow and rugged mountain topography combine to preserve old ice, bring it closer to the ice surface and make it more accessible.
In contrast, extracting the oldest solid ice cores from sites in East Antarctica requires drilling to depths of more than 2,000 m.
“We are still trying to determine the exact conditions that allow such ancient ice to persist so close to the surface,” Dr Shackleton said.
“Along with the topography, it's likely a mixture of high winds and extreme cold.”
“The wind blows away the fresh snow, and the cold slows the ice to almost a standstill.”
“This makes the Allan Hills one of the best places in the world to find shallow old ice, and one of the toughest places to field a field season.”
The trapped air in these new cores allows scientists to directly date the ice through careful measurements of the noble gas isotope argon.
Direct dating means that scientists measured things in the ice that indicate age, rather than drawing conclusions based on associated features or sediments.
Although records of this old ice are not continuous, its antiquity is unprecedented, the researchers say.
“By dating multiple samples, our team has created a library of what we call 'climate snapshots' about six times older than any previously reported ice core data, complementing more detailed younger data from cores in the Antarctic interior,” Dr Higgins said.
Temperature records derived from measurements of oxygen isotopes in the ice show that the area experienced a gradual and sustained cooling of about 12 degrees Celsius (22 degrees Fahrenheit).
This is the first direct measurement of the degree of cooling in Antarctica over the past 6 million years.
Ongoing research on these ice cores aims to reconstruct atmospheric greenhouse gas levels and ocean heat content, which has 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 further drilling with the possibility of obtaining more detailed images and even older ice,” Dr Brook said.
“Given the impressively old ice we found in the Allan Hills, we have also developed a comprehensive long-term new study of the region to try to extend the record even further back in time, which we hope to take between 2026 and 2031.”
teams paper was published today in the magazine Proceedings of the National Academy of Sciences.
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S. Shackleton etc.. 2025. Miocene and Pliocene ice and air from the Allan Hills blue ice region, East Antarctica. PNAS 122 (44): e2502681122; doi: 10.1073/pnas.2502681122
