Science likes to travel far in search of new phenomena, but nature keeps reminding us that, really, we’ve yet to discover many things much closer to us. That was a clear lesson for researchers who dug up the oldest ice on record—an ancient piece of Earth’s geological history from roughly 6 million years ago.
For a paper published on October 28 in Proceedings of the National Academy of Sciences, the Center for Oldest Ice Exploration, or COLDEX, describes the enigmatic composition of a 6-million-year-old ice core collected from the Allan Hills, a family of frigid hills in southeastern Antarctica. By carefully studying the composition of tiny air bubbles, permafrost, and other frozen deposits inside, the researchers derived an impressive reconstruction of Earth’s atmosphere from millions of years into the past.
The samples present “discontinuous climate ‘snapshots’ that are much older and extend back into a much warmer interval in Earth’s history,” Sarah Shackleton, study lead author and a geophysicist at Woods Hole Oceanographic Institution, told Gizmodo in an email.
The newest core is almost two times older than the previously oldest discontinuous ice core on record, dated at about 2.7 million years old, according to the paper.
Frozen time machines
Ice cores, as their name suggests, are solid, typically cylindrical samples drilled from ice sheets. East Antarctica harbors some of the oldest ice in the world, although reaching it requires drilling up to 6,500 feet (2,000 meters) underground—while, of course, abiding by international guidelines for environmental preservation.
“Traditionally, the deeper you drill, the older the ice gets,” Shackleton said, “but things are a bit more complicated at the Allan Hills, where very old ice outcrops at the surface.”
To be exact, the Allan Hills are very windy and cold, which enables older ice to persist nearer to the surface, at about 300 to 650 feet (100 to 200 meters), but also makes in-person expeditions very difficult, she added. Still, the team managed to retrieve three new ice cores over several years of camping out at Allan Hill for months at a time.
A window to the past
Once they collected the cores, the team took detailed measurements of the isotopes of argon for the trapped air bubbles inside the samples. This allowed the researchers to pin down the age of each sample. They also used laser spectroscopy to identify different oxygen isotopes in the meltwater, which revealed that the area corresponding to today’s Allan Hills experienced a gradual, long-term decrease in temperatures of about 22 degrees F (12 degrees C).
There was, however, one sample, dirty basal ice, that was basically gas-free. This made it impossible to date, Shackleton said. But analysis of the water isotopes for this sample suggests it formed at a much warmer temperature, and its position just below the oldest-dated sample strongly suggests it could be even older than 6 million years.
“Given that it’s gas-free, it’s likely refrozen liquid water,” Shackleton said. “We’ve speculated about what it represents and what it can tell us about the past conditions at this site, but it’s still somewhat of a mystery.”
Overall, the findings demonstrate the potential of ice cores in investigating and reconstructing climates long past—a critical insight, especially for warmer periods, since they could guide how scientists approach natural climate change. Either way, Shackleton and her colleagues are already at work to find how else ice cores may have frozen geological information in time.
“We think we’ve just scratched the surface of what’s possible, and much more data is forthcoming, both new measurements and new cores,” she said. “Based on what we’ve found so far, we think there may be even older ice out there to discover.”
