In the sediment beneath the Antarctic ice sheet, scientists discovered a huge underground water system – Floating thanks to that?

Previously unmapped reservoirs could accelerate glaciers, release carbon

Peer-reviewed publications

COLUMBIA CLIMATE SCHOOL

Many scientists say that liquid water is key to understanding the behavior of the frozen form found in glaciers. Molten water is known to lubricate their gravel bases and promote their march out to sea. In recent years, researchers in Antarctica have discovered hundreds of connections liquid river lake cradled in the ice itself. And, they photographed the thick sedimentary basins under the ice, potentially containing the largest reservoirs of water. But so far, no one has confirmed the presence of large amounts of liquid water in the sediments beneath the ice, nor studied how it might interact with the ice.

Now, a research team has mapped for the first time a massive, actively circulating groundwater system in the deep sediments of West Antarctica. They say such systems, which may be common in Antarctica, could have unknown effects on how the frozen continent responds to, or could even contribute to, climate change. . The study appeared today in the journal Science.

Lead author of the study, Chloe Gustafson, who conducted the study as a PhD student at Columbia University, said: “People have hypothesized that there may be groundwater deep in the sediments. this, but so far, no one has done any detailed visualization. Lamont-Doherty . Earth Observatory. “The amount of groundwater that we found is so large, it has the potential to affect ice-forming processes. Now we have to learn more and find a way to incorporate that into the models.”

Scientists have for decades flying radar and other tools through the Antarctic ice sheet to subsurface imaging features. Among many other things, these missions have revealed sedimentary pools located between ice and bedrock. But atmospheric geophysics can generally reveal only rough outlines of those features, not water content or other features. In one exception, a 2019 study of the McMurdo Dry Valley in Antarctica used helicopter instruments to record a few hundred meters of subglacial groundwater about 350 meters below the ice. But most of Antarctica’s known sedimentary basins are much deeper, and much of its ice much thicker, beyond the reach of airborne devices. In some places, the researchers drilled through the ice into layers of sediment, but only penetrated the first few meters. Therefore, ice sheet behavior models include only hydrological systems within or just below the ice sheet.

This is a major shortcoming; Most of Antarctica’s extensive sedimentary basins lie below current sea level, lying between the land ice and the bedrock and floating sea ice shelves surrounding the continent. They are thought to have formed on the seafloor during warm periods when sea levels were higher. If the ice shelves were pulled back in a warming climate, seawater could re-invade the sediments, and the glaciers behind them could plunge forward and raise sea levels worldwide.

Researchers in new study focused on distances as wide as 60 miles Whillans băng Ice Line, one of half a dozen fast-moving currents that feed the Ross Ice Shelf, the largest in the world, about the size of Canada’s Yukon Territory. Earlier research had discovered a subglacial lake within the ice and a sedimentary basin stretching below it. Shallow drilling into the first foot of the sediment created liquid water and a thriving community of bacteria. But what lies deep below remains a mystery.

At the end of 2018, a US Air Force LC-130 jet ski dropped Gustafson, along with geophysicist Lamont-Doherty Kerry Key, geophysicist Matthew Siegfried of the Colorado School of Mines, and mountaineer Meghan Seifert on the Whillans. Their mission: to better map sediment layers and their characteristics using geophysical instruments placed directly on the surface. Without any help if something goes wrong, they will have to lose six tiring weeks traveling, digging in the snow, planting tools and countless other housework.

(Watch videos and photos of the expedition)

The team used a technique called magnetic-cell imaging, which measures the penetration of naturally occurring electromagnetic energy into the planet’s atmosphere. Ice, sediment, freshwater, saltwater, and bedrock all conduct electromagnetic energy to varying degrees; By measuring differences, researchers can create MRI-like maps of different elements. The team planted their devices in snow holes for a day or so, then dug them out and relocated them, eventually obtaining measurements at about forty locations. They also reanalyzed seismic waves naturally emanating from the earth that had been collected by another group, to help distinguish bedrock, sediment, and ice.

Their analysis shows that, depending on location, layers of sediment extend below the ice base from half a kilometer to nearly two kilometers before hitting the bedrock. And they confirmed that the sediments are loaded with liquid water all the way down. The researchers estimate that if all of them were extracted, it would form a head of water between 220 and 820 meters high – at least 10 times more than shallow hydrological systems within and under the ice – maybe much more than that.

Saltwater conducts energy better than fresh water, so they were also able to demonstrate that groundwater becomes saltier with depth. This makes sense, says Key, because the sediments are thought to have formed in the marine environment long ago. Ocean water could have reached what is now the area covered by the Whillans during a warm period about 5,000 to 7,000 years ago, saturating the sediments with salt water. As the ice moved in equilibrium, newly melted water was created by the pressure from above and the friction at the base of the ice was clearly forced into the sediments above. Key can continue to filter and mix today.

This slow draining of water into the sediments could prevent water from accumulating at the bottom of the ice, the researchers say. This can act as a brake on the forward motion of the ice. Measurements by other scientists at the ground line of the ice stream — the point where the inland ice stream meets the floating ice shelf — showed that the water there was less salty than normal seawater. This suggests that fresh water is flowing through the sediments to the ocean, making room for more meltwater and keeping the system stable.

However, the researchers say, if the surface of the ice thins – a clear possibility as the climate warms – the direction of water flow could be reversed. The pressure below will drop, and deeper groundwater can begin to rise to the ice. This can further lubricate the base of the tape and increase its forward motion. (The Whillans moved the ice toward the sea about a meter a day – very fast for ice.) Furthermore, if deep groundwater were to flow upward, it could carry geothermally generated naturally in the bedrock; this could melt the ice further and push it forward. But whether that will happen, and to what extent, remains unclear.

“Ultimately, we don’t have major limitations on sediment permeability or water flow rates,” says Gustafson. “Will it make a big difference that will generate a runaway reaction? Or is groundwater a smaller factor in the grand scheme of ice flow? “

The known presence of bacteria in shallow sediments adds another wrinkle, the researchers say. This and other basins may be inhabited deeper; and if groundwater starts to move upwards, it will bring in the dissolved carbon used by these organisms. The nearby groundwater flow then sends some of this carbon out into the ocean. This could turn Antarctica into a hitherto unused source of carbon in a world that swam in it. But again, the question is whether this produces some significant effect, Gustafon said.

The study is just the beginning of addressing these questions, the researchers say. Confirmation of the existence of deep groundwater dynamics has changed our understanding of ice flow behavior and will force modifications to subglacial water models, they write.

Other authors are Helen Fricker of the Scripps Institution of Oceanography, J. Paul Winberry of Central Washington University, Ryan Venturelli of Tulane University, and Alexander Michaud of the Bigelow Laboratory for Ocean Science. Chloe Gustafson is currently a postdoctoral researcher at Scripps.

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Videos and photos of the expedition

JOURNEYS

Science

RESEARCH METHODS

Observational research

RESEARCH SUBJECTS

Do not apply

ARTICLE TITLE

A dynamic saline groundwater system mapped beneath the Antarctic ice line

ARTICLE PUBLICATION DATE

May 6, 2022

From EurekAlert!