A team of U.S. scientists has discovered the oldest directly dated ice and air on the planet in the Allan Hills region of East Antarctica.
The 6-million-year-old ice and the tiny air bubbles trapped inside it provide an unprecedented window into Earth's past climate, according to a new study published today in the Proceedings of the National Academy of Sciences.
The oldest ice sample from Allan Hills dated by researchers clocks in at 6 million years, from a period in Earth's history where abundant geological evidence indicates much warmer temperatures and higher sea levels compared to today.
The research was led by Sarah Shackleton of Woods Hole Oceanographic Institution and John Higgins of Princeton University, who are affiliated with the National Science Foundation-funded Center for Oldest Ice Exploration, or COLDEX, a collaboration of 15 U.S. research institutions led by Oregon State University.
"Ice cores are like time machines that let scientists take a look at what our planet was like in the past," said Shackleton, who has participated in many seasons of ice core drilling at Allan Hills. "The Allan Hills cores help us travel much further back than we imagined possible."
This is the most significant discovery to date for COLDEX, an NSF Science and Technology Center funded in 2021 to explore the Antarctic ice sheet, which is the largest ice mass on the planet, said COLDEX Director Ed Brook, a paleoclimatologist in OSU's College of Earth, Ocean, and Atmospheric Sciences.
"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," Brook said.
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.
Research teams with COLDEX are exploring a different setting for old ice. Working in a remote field camp in the Allan Hills in East Antarctic for months at a time, the group drilled down one to two hundred meters 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 meters deep.
"We're still working out the exact conditions that allow such ancient ice to survive so close to the surface," said Shackleton. "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, Higgins explained, "the 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."
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, approximately 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.
A 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, 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," he said.
Additional co-authors on the paper are: Julia Marks Peterson, Christo Buizert and Jenna Epifanio of Oregon State; Valens Hishamunda, Austin Carter and Michael Bender of Princeton; Lindsey Davidge, Eric Steig and Andrew Schauer, University of Washington; Sarah Aarons, Jacob Morgan and Jeff Severinghaus of Scripps Institution of Oceanography at University of California, San Diego; Andrei V. Kurbatov and Douglas Introne of the University of Maine; Yuzhen Yan of Tongji University; and Peter Neff of the University of Minnesota.
COLDEX is supported by the NSF Office of Polar Programs; the Science and Technology Center Program at the NSF Office of Integrative Activities; and Oregon State University. Fieldwork in Antarctica is supported by the U.S. Antarctic Program and funded by NSF. Ice drilling support is provided by the NSF U.S. Ice Drilling Program and ice sample curation by the NSF Ice Core Facility in Denver, Colorado.
