om an ancient warm climate.JULIA MARKS PETERSON
SCIENCE 24 APRIL 2024 Goudschmit conference report
VIENNA—Samples of eerie blue glacial ice from Antarctica are a staggering 6 million years old, scientists announced last week, doubling the previous record for Earth’s oldest ice. The ice opens a new window on Earth’s ancient climate—one that isn’t exactly what scientists expected.
Bubbles in the ice trap air from the Pliocene epoch, a time before the ice ages when the planet was several degrees warmer than today and carbon dioxide (CO2) levels may have been just as high as they are now. But an initial analysis of the bubbles suggests CO2 levels were rather low in the late Pliocene and only sank slightly between 2.7 million and 1 million years ago as the Pliocene ended, the ice ages began, and Earth headed toward a dramatic climate shift that caused ice ages to grow longer and deeper.
The results are preliminary, stresses Ed Brook, a geochemist at Oregon State University (OSU) and leader of the U.S. Center for Oldest Ice Exploration (COLDEX), which presented the discovery last week here in multiple talks at the European Geosciences Union General Assembly. But if even a tiny drop in CO2can kick off a major climate change, Brook adds, “you know, we probably care about that.”
Finding ice this old is “fantastic,” says Eric Wolff, a paleoclimatologist at the University of Cambridge who wasn’t involved in the work... Wolff adds. “Nothing’s quite as direct as actually taking a bubble, snapping it open, and putting it straight into a mass spectrometer.”...
To date the ice, Sarah Shackleton, a paleoclimatologist at the Woods Hole Oceanographic Institution, and colleagues at Princeton University analyzed the argon isotopes contained in its air bubbles. But the technique consumes a lot of the ice, leaving little of a standard 8-centimeter core left over for other analyses of the same ice layer. For now, the team has only drilled small cores of the 6-million-year-old ice, so its age is all they know, Brook says. They’re heading back to Antarctica next austral summer to retrieve larger samples.
But last season, Brook and his colleagues did manage to drill jumbo-size cores of ice as old as 3 million years. These cores, as wide as a dinner plate, yielded hundreds of samples of ancient air—including the first ever from the Pliocene, which ended about 2.6 million years ago with the start of the ice ages
Scientists think high levels of CO2 were responsible for the Pliocene’s warmth. Proxy data from sediment cores, such as the chemical compositions of the shells of tiny marine algae and plant leaf waxes, suggest CO2 was probably about as high as today’s unnaturally elevated level, 425 parts per million (ppm). But not one blue ice sample older than 1 million years exceeded 300 ppm, says Julia Marks Peterson, a paleoclimatologist at OSU who performed the greenhouse gas analysis.
The greenhouse gas data also raise questions about a mysterious climate shift that began about 1.2 million years ago. At this time, something caused the ice ages to grow longer and more intense, stretching out from mild 40,000-year cycles to deeper 100,000-year cycles. The leading theory for this flip is that CO2levels dropped, allowing ice sheets to grow too thick to melt away on a 40,000-year cycle. A new climate record from clues preserved in sediment cores, reported in February, supports that picture. But the snapshots across the transition found in the blue ice suggest CO2 levels held steady between about 220 ppm and 250 ppm. “We don’t see much change in CO2,” Marks Peterson says. “That doesn’t mean there wasn’t one. But it might be smaller than we expected.”
