Privileged Planet: oldest Antarctic ice studied (Introduction)

by David Turell , Friday, April 26, 2024, 15:30 (11 days ago) @ David Turell

Deep bores into blue ice:

https://www.sciencemagazinedigital.org/sciencemagazine/library/item/26_april_2024/41907...

"Samples of eerie blue glacial ice from Antarctica are a staggering 6 million years old, scientists announced last week, more than 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 bubbles between 2.7 million and 1 million years old suggests CO2 levels were rather low in the late Pliocene and only sank slightly as the ice ages began, and Earth headed toward a dramatic climate shift that caused ice ages to grow longer and deeper.

***

"Finding ice this old is “fantastic,” says Eric Wolff, a paleoclimatologist at the University of Cambridge who wasn’t involved in the work. The climate clues in the oldest ice samples—those between 3 million and 6 million years old—might have been corrupted when the ice interacted with bedrock. But younger samples offer an unprecedented set of climate snapshots from an ancient world, Wolff adds. “Nothing’s quite as direct as actually taking a bubble, snapping it open, and putting it straight into a mass spectrometer.”

"Most scientific drillers target ice in the deep Antarctic interior, where snow accumulates year after year and, crushed under its own weight, gets squeezed into tidy layers of ice that preserve a continuous archive of ancient air bubbles. But the oldest of these continuous cores, exhumed in 2004, taps out at 800,000 years.

***

"In 2019, the COLDEX team reported on ice as old as 2.7 million years, including an analysis of greenhouse gases in air bubbles as old as 1.5 million years. Now, the team has gone back to Allan Hills and retrieved even older ice. 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 the remaining air bubbles. 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 managed to drill jumbo 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. “It’s a unique snapshot,” says paleoclimatologist Hubertus Fischer of the University of Bern, who was not involved in the work.

"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 CO2 levels 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.”

"To find out what really triggered the ice age shift, researchers want a continuous core that covers the transition. Finding such a core “is kind of the holy grail of understanding whether the CO2 was part of this change,” Wolff says.

***

"...few think the center of the continent holds continuous ice much older than 1.5 million years. “You’re certainly not going to get many millions of years,” Fischer says. That means the warmer Pliocene climate—and its clues to our warming future—will be the sole preserve of blue ice deposits.

"That alone makes this blue ice special, Marks Peterson says. “It’s really rare to be an ice core scientist who’s studying the Pliocene.'”

Comment: this is exciting new research opening up the past climate history of this planet. It is amazing how humans invent new ways to make these studies. All based on older techniques. Mass spectrometers date back to initial discovery work in the 1880's.