A team of scientists has achieved something extraordinary in the frozen Allan Hills, east of Antarctica: extracting 6 million year old ice samples, the oldest ever directly dated. Trapped inside are air bubbles that date back to Earth’s Miocene atmosphere, when our planet was much warmer and sea level considerably higher than today.
A time capsule in the form of ice. The discovery, published in the journal PNAS on October 28 and led by Sarah Shackleton of the Woods Hole Oceanographic Institute and John Higgins of Princeton University, more than doubles the age of the oldest known ice so far, which dated to about 2.7 million years ago. “Ice cores are like time machines that allow scientists to take a look at what our planet was like in the past,” explains Shackleton. “The Allan Hills cores help us travel much further back than we thought possible.”
How they found it. Between 2019 and 2023, the Center for the Exploration of Older Ice (COLDEX) team drilled between 100 and 200 meters deep into the ice sheet in the Allan Hills region, located about 2,000 meters above sea level. As reported by Space, this area is especially valuable because the topography of the terrain and the ice flow patterns allow extremely old ice to be preserved closer to the surface, unlike the Antarctic interior where it would be necessary to drill more than 2,000 meters to reach similar ages.
Data. The researchers determined the age of the ice by measuring the radioactive decay of argon isotopes present in trapped air bubbles. This method allows ice to be dated directly, without the need to examine the rocks or soil around it. The result: 6 million years, a time when the Earth was home to now-extinct creatures such as saber-toothed tigers, Arctic rhinos and the first mammoths.
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Cooling. Analysis of oxygen isotopes in the cores revealed that the Allan Hills region has cooled by approximately 12°C over the past 6 million years. It is the first direct evidence that quantifies how much the Antarctic climate has cooled since that ancient warm period. Ed Brook, director of COLDEX and paleoclimatologist at Oregon State University, notes that “the team has built a library of what we call ‘climate snapshots’ about six times older than any previously reported ice core data.”
Why does it matter? While Antarctica and the Earth as a whole have progressively cooled for millennia, humans are now rapidly increasing global temperatures by releasing large amounts of greenhouse gases into the atmosphere. Studying these bubbles of ancient air will allow scientists to reconstruct past greenhouse gas concentrations and ocean heat levels, which could give us insight into what natural factors have contributed to climate change throughout our planet’s history.
Surviving extreme conditions. “We’re still discovering the exact conditions that allow such ancient ice to survive so close to the surface,” Shackleton notes. “Along with the topography, it’s likely a mix of strong winds and intense cold. The wind blows fresh snow and the cold slows the ice almost to a stop. That makes Allan Hills one of the best places in the world to find shallow old ice, and one of the toughest to spend a season in the field,” he continued.
Next steps. The COLDEX team plans to return to Allan Hills in the coming months to conduct further drilling. They hope to recover even older samples and produce a more detailed record of Earth’s ancient atmosphere. “Given the spectacularly old ice we have discovered in Allan Hills, we have also designed a new comprehensive long-term study of this region to try to extend the records even further in time, which we hope to carry out between 2026 and 2031,” Brook concludes.
Images | COLDEX
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