In sediments under Antarctic ice, scientists discover a massive groundwater system

In sediments under Antarctic ice, scientists discover a massive groundwater system

Previously unmapped reservoirs could speed up glaciers, release carbon

Many scientists say that liquid water is the key to understanding the behavior of frozen forms in glaciers. Melting water is known to lubricate their gravel bases and accelerate their march to the sea.In recent years, researchers in Antarctica have discovered hundreds of interconnected liquid lakes and rivers Encased in ice cubes themselves. What’s more, they photographed thick sediment basins beneath the ice, which may contain the largest reservoirs. But so far, no one has confirmed the presence of large amounts of liquid water in subglacial sediments, nor has they studied how it interacts with ice.

Now, for the first time, a team from six research institutions has mapped a large, active circulating groundwater system in deep sediments in West Antarctica. Such systems, which may be common in Antarctica, could have unknown effects on how the frozen continent responds to, and may even have, climate change, they say.The study was published today in the journal science.

“It was hypothesized that there might be deep groundwater in these sediments, but so far, no one has done any detailed imaging,” said study lead author Chloe Gustafson, a graduate student at Columbia University. . Lamont-Doherty Earth Observatory“The amount of groundwater we found is very large, and it could be influencing ice flow processes. Now we have to find more information and figure out how to incorporate that into the model.”

scientists for decades Flight radar and other instruments Imaging subsurface features over the Antarctic ice sheet. Among many other things, the missions revealed sedimentary basins sandwiched between ice and bedrock — whose porous nature provides the potential to store groundwater. But aerial geophysics usually only reveals rough outlines of these features, not water content or other features. In one exception, A 2019 study The McMurdo Dry Valleys of Antarctica have recorded subglacial groundwater several hundred meters below an ice sheet of about 350 meters using instruments carried by helicopters. But most of Antarctica’s known sedimentary basins are deeper, and most of the ice is thicker, beyond the reach of aerial instruments. In some places, researchers have drilled through the ice and into the sediment, but only for the first few meters. Therefore, models of ice sheet behavior only include hydrological systems within or under ice.

That’s a big flaw; most of Antarctica’s vast sedimentary basins lie below current sea level, sandwiched between bedrock-bound land ice and floating ocean ice shelves on continental margins. They are thought to have formed on the ocean floor during warm periods when sea levels were higher. If ice shelves retreat in a warming climate, ocean water could re-intrude sediments, and glaciers behind them could rush forward, raising global sea levels.

The researchers in the new study focused on the 60-mile-wide Whelans Ice Creek, is one of six fast-flowing streams that feed the world’s largest Ross Ice Shelf, which is about the size of Canada’s Yukon Territory. Previous research revealed a subglacial lake in the ice, and a sedimentary basin extending beneath it. Shallow drilling in the first foot or so of sediment brought liquid water and thriving microbial communities. But further afield is a mystery.

Survey site on the Whelans Ice Stream. Electromagnetic imaging stations were set up in two general areas (marked in yellow). The team traveled to the wider area for other missions, as indicated by the red dots. Click on the image for a larger view. (Courtesy Chloe Gustafson)

A U.S. Air Force LC-130 ski plane with Lamont-Doherty geophysicists crashed into Gustafson in late 2018 Crickey, Colorado School of Mines geophysicist Matthew Siegfried and Whelans mountaineer Megan Seifert. Their mission: to better map sediments and their properties using geophysical instruments placed directly on the surface.If something goes wrong they are of no help an exhausting six weeks Travel, dig in the snow, grow musical instruments, and countless other chores.

(See video and images of the expedition)

The team used a technique called magnetotelluric imaging, which measures the penetration of the Earth by natural electromagnetic energy generated in the Earth’s atmosphere. Ice, sediment, fresh water, salt water and bedrock all conduct electromagnetic energy to varying degrees; by measuring the difference, researchers can create MRI-like maps of different elements. The team placed their instruments in snow pits for a day or so at a time, then dug them out and relocated them, eventually taking readings at about four dozen locations. They also reanalyzed natural seismic waves emanating from Earth collected by another team to help distinguish between bedrock, sediment and ice.

Their analysis showed that, depending on the location, the sediment stretched from half a kilometer to nearly two kilometers below the bottom of the ice, before hitting the bedrock. They confirmed that the sediments were filled with liquid water all the way down. The researchers estimate that if it were all extracted, it would form a water column 220 to 820 meters high — at least 10 times larger than the shallow water systems within and at the bottom of the ice — and possibly much higher than that . .

Saltwater conducts energy better than freshwater, so they were also able to show that groundwater becomes saltier with depth. This makes sense, Key said, because the deposits are thought to have formed long ago in the marine environment. During a warm period about 5,000 to 7,000 years ago, seawater may have last reached the area now covered by the Whillans, filling the sediments with brine. Fresh meltwater from pressure from above and friction from the ice base was apparently forced into the upper sediments as the ice re-propelled. It will likely continue filtering and blending today, Key said.

The slow flow of fresh water into the sediments prevents water from accumulating on the bottom of the ice, the researchers said. This acts as a brake on the ice’s forward movement. Measurements by other scientists at the ice flow grounding line, where the onshore ice flow meets the floating ice shelf, suggest that the water there is somewhat less salinity than normal seawater. This suggests that fresh water is flowing into the ocean through sediments, making room for more meltwater to enter and keeping the system stable.

Co-author Matthew Siegfried of the Colorado School of Mines pulls up the buried electrode wire. (Criki/Lamont-Doherty Earth Observatory)

However, the researchers say that if the ice surface thins — a clear possibility as the climate warms — the direction of water flow could reverse. Overlying pressure will decrease, and deeper groundwater may begin to flood the ice base. This further lubricates the bottom of the ice and increases its forward movement. (Whillans already move the ice to the sea by about 1 meter per day—very fast for glacial ice.) Also, if deep groundwater flows upwards, it can carry away naturally occurring geothermal heat from the bedrock; this could melt the ice further. bottom and push it forward. But whether that will happen, and to what extent, is unclear.

“Ultimately, we don’t have much constraints on the permeability of sediments or the speed of water flow,” Gustafson said. “Does it make a big difference, creating a runaway response? Or is groundwater just a smaller role in the ice flow’s grand scheme?”

The constant cold, dryness and hard labor split co-author Kerry Key’s hands. Solution to close the wound: Use superglue. efficient. (Criki/Lamont-Doherty Earth Observatory)

The known presence of microbes in shallow sediments adds another wrinkle, the researchers said. This and other basins may inhabit further afield; if groundwater begins to move upwards, it will bring out the dissolved carbon that these organisms use. Lateral groundwater flow then releases some of the carbon into the ocean. In a world already swimming in it, this could turn Antarctica into a hitherto unconsidered source of carbon. But again, the question is whether this will have some significant impact, Gustafon said.

The new study is just the beginning of addressing these questions, the researchers said. “Confirmation of the existence of deep groundwater dynamics changes our understanding of ice current behavior and will force revisions to subglacial water models,” they wrote.

Additional authors are Helen Fricker of Scripps Institution of Oceanography, J. Paul Winberry of Central Washington University, Ryan Venturelli of Tulane University and Alexander Michaud of Bigelow Marine Science Laboratory. Chloe Gustafson is now a Postdoctoral Fellow at Scripps.