Indeed, the oldest directly dated ice ever retrieved from Earth has been pulled from the frozen expanse of East Antarctica, and it is far older than scientists had dared to hope. At 6 million years old, this Allan Hills ice shatters previous records by more than doubling the age of the oldest known samples and offers a rare, unaltered archive of atmospheric and climatic conditions from the Miocene epoch.
This extraordinary find was made possible through a combination of geological serendipity and engineering precision. The Allan Hills blue ice area lies at approximately 2,000 meters above sea level, in a place where rugged mountain topography and near-static ice flow have locked ancient ice close to the surface. Strong katabatic winds scour away fresh snow, slowing accumulation and exposing compressed, crystal-blue ice that has survived for millions of years. “The wind blows away fresh snow, and the cold slows the ice to almost a standstill,” said Sarah Shackleton of Woods Hole Oceanographic Institution. “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.”
From 2019 to 2023, Shackleton’s team drilled 100–200 meters and pulled up cores laced with microscopic air bubbles, sealed time capsules of ancient atmosphere. To date those samples, the team had to make an exact measurement of the radioactive decay of argon isotopes, refining a technique that has evolved from earlier noble-gas dating methods. Unlike stratigraphic matching or sediment-based inference, argon isotope dating produces a direct age from the ice itself, rather than from external markers, eliminating their attendant uncertainties. This approach builds upon advances in the ATTA, a laser-based method able to detect isotopes such as 39Ar at abundances as low as 10⁻¹⁷, reducing sample sizes to merely a few kilograms of ice.
The oxygen isotope ratios in the Allan Hills cores gave a continuous cooling of about 12°C over the past 6 million years, the first direct quantification of Antarctica’s cooling from a warm Miocene world to today’s frigid polar climate. The cooling trend correlated with geological evidence of shrinking polar seas and growing ice sheets, but the Allan Hills data provided a finer resolution. “Ice cores are like time machines that let scientists take a look at what our planet was like in the past,” Shackleton said. “The Allan Hills cores help us travel much further back than we imagined possible.”
The trapped air can provide much more than temperature records. Future analyses are planned for the reconstruction of ancient concentrations of greenhouse gases and ocean heat content, key variables for understanding natural drivers of climate. Similar work on younger cores has revealed tight correlations between levels of carbon dioxide and global temperature; the Miocene samples will extend that relationship into a climate regime where CO₂ and sea levels were markedly higher than today.
Critical to this achievement has been the technological progress made in the process of drilling ice cores. Traditional deep-core programs in Antarctica have needed to penetrate upwards of more than 2,000 meters of ice to reach material even approaching a million years old. In contrast, the COLDEX team’s intermediate-depth drills-designed for blue ice areas-can recover ancient ice at a fraction of that depth. The cores are discontinuous but form a sort of ” library of climate snapshots” that complement the continuous records from the Antarctic interior. “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.”
The Allan Hills find also contributes to climate modelling: paleoclimate data from the ice cores feeds directly into simulations predicting the behaviour of ice sheets under warming scenarios. The Miocene epoch is considered a partial analogue for projected future climates in the case of continued high greenhouse gas emissions, and these samples are a benchmark in testing model sensitivities to elevated CO₂ and ocean heat. In engineering terms, this data provides boundary conditions for the coupled ice–ocean–atmosphere models, which will improve forecasts of ice sheet stability and sea level rise.
A team will return in the months ahead to Allan Hills, with the aim of pushing the record even further back in time. Between 2026 and 2031, a longer-term program will target even older ice – possibly well into the Miocene. Each core extracted is more than a scientific curiosity – a precision-engineered archive, preserved by nature and unlocked by technology, holding answers to how Earth’s climate system has evolved, and how it may respond in the centuries ahead.
