A little over 500 kilometers from McMurdo Station, nestled in the Transantarctic Mountain Range, is the Ong Valley. This small, arid valley is about 6 kilometers long by 2.5 kilometers wide, and records suggest it has been visited by fewer people than the Moon.
Jaakko Putkonen, associate professor and director of the Harold Hamm School of Geology and Geological Engineering at the University of North Dakota, Grand Forks, has been to the Ong Valley three times. He loves being one of the few: “You never know what’s behind the big rock because nobody’s ever looked there,” he said. “There are no footprints anywhere. Nothing.”
Marie Bergelin, Putkonen’s Ph.D. student, joined him on his 2017–2018 expedition, in which a critical research team from multiple universities drilled into the Ong Valley’s ice bed and recovered two 10-meter ice cores. Putkonen and Bergelin presented their findings during a poster session at AGU’s Fall Meeting 2019 in San Francisco, Calif.
One of the oldest sections of the cores, according to Bergelin, is likely to be around 2.6 million years old and at least no younger than the dirt above it, which was dated at 1.6 million years old. Putkonen and Bergelin are quick to note that the core may be older or younger than 2.6 million years, however. Determining the date of ice is a complex process, Putkonen said, and the numbers come out more “as a range of scenarios” rather than one specific date.
The trick to dating ice cores is not the ice itself, but quartz grains embedded in or around it. And the trick to preserving ice is the layer of dirt on top of it.
Dating the Ice
Scientists use cosmogenic nuclide dating, which analyzes isotopes produced in quartz by cosmic rays at or near Earth’s surface. “The longer time it’s exposed to the surface and sitting at the surface, the higher concentrations of isotopes build up,” Bergelin said.
The cosmic rays that produce these isotopes penetrate only a few meters. Below that, the isotopes stop building up, which helps scientists predict the age of a subsection of Earth or ice containing the debris.
Bergelin extracted quartz grains from the full length of a core to determine the age of the ice inside. As suspected, the oldest section was at the bottom of the core.
Preserving the Ice
It’s well known that a sufficient amount of debris acts as a shield for ice, preventing it from melting or sublimation, the process by which ice bypasses a liquid stage and turns directly into vapor. (Sublimation is common in extremely arid climates like Antarctica.) “The soil cover is critical to preserving [the ice],” Putkonen said, “but even then we don’t fully understand the [preservation] process.”
What scientists do know is that as ice sublimates and disappears, dirt dispersed in the ice will be left behind. The more ice sublimates, the more layers of dirt and debris will build up. Eventually, this layer of dirt will become thick enough that it inhibits the ice underneath from sublimating. Less than 5 centimeters of the right kind of dirt will actually enhance the melting of ice, but a thicker layer, maybe around 30 centimeters or more, said Putkonen, “will act as an insulating blanket and preserve the ice.”
Without the protection of 60 centimeters of dirt on top of the ice, the 10-meter cores collected by Putkonen and Bergelin might have sublimated away in just over 100 years. Instead, the ice in the core is over a million years old.
Brenda Hall, a professor in the School of Earth and Climate Sciences at the University of Maine, wrote in an email to Eos, “Bergelin and Putkonen have demonstrated the great antiquity of the buried ice and its potential for providing a glimpse into an environment that existed in the distant past. Perhaps more exciting, their work implies that this site may not be a ‘one off’ location, but rather that there is potential for old ice throughout the Transantarctic Mountains that can be used to reconstruct Earth’s past.”
Bergelin and Putkonen have already found pollen, DNA, dust, and atmospheric gases trapped inside of the ice cores, which they continue to analyze.
“In a way this is like opening up a window into a snapshot of the past conditions,” Putkonen said. “Once we start understanding the system better, there could be opportunities for a whole new way of looking into paleoconditions through pockets of preserved ice.”
Hendricks, J. (2020), Antarctic ice cores might be older than dirt, Eos, 101, https://doi.org/10.1029/2020EO139799. Published on 06 February 2020.
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