Scientists discover massive groundwater system in sediments beneath Antarctic ice

Lead author Chloe Gustafson and mountaineer Meghan Seifert set up geophysical instruments to measure groundwater beneath the Whillans Ice Stream of West Antarctica. Credit: Kerry Key/Lamont-Doherty Earth Observatory

Previously unmapped reservoirs could accelerate glaciers and release carbon.

Many researchers believe that liquid water is key to understanding the behavior of the frozen form found in glaciers. Meltwater is known to lubricate their gravel bases and hasten their march toward the sea. In recent years, scientists in Antarctica have discovered hundreds of interconnected liquid lakes and rivers cradled within the same ice. And they have captured thick basins of sediment under the ice, potentially containing the largest reservoirs of water of all. But so far, no one has confirmed the presence of large amounts of liquid water in the sediments below the ice, or investigated how it might interact with the ice.

Now, a research team has for the first time mapped a huge actively circulating groundwater system in sediments deep in West Antarctica. They say such systems, likely common in Antarctica, may have yet-unknown implications for how the frozen continent reacts, or possibly even contributes to climate change. The research was published in the journal Science on May 5, 2022.

Survey Locations at Whillans Ice Stream

Survey locations in Whillans Ice Stream. The electromagnetic imaging stations were installed in two general areas (yellow markings). The team traveled to larger areas to perform other tasks, shown by red dots. Click on the image to see a larger version. Credit: Courtesy of Chloe Gustafson

“People have hypothesized that there might be deep groundwater in these sediments, but so far no one has gotten detailed images,” said study lead author Chloe Gustafson, who conducted the research as a graduate student at[{” attribute=””>Columbia University’s Lamont-Doherty Earth Observatory. “The amount of groundwater we found was so significant, it likely influences ice-stream processes. Now we have to find out more and figure out how to incorporate that into models.”

Scientists have for decades flown radars and other instruments over the Antarctic ice sheet to image subsurface features. Among many other things, these missions have revealed sedimentary basins sandwiched between ice and bedrock. But airborne geophysics can generally reveal only the rough outlines of such features, not water content or other characteristics. In one exception, a 2019 study of Antarctica’s McMurdo Dry Valleys used helicopter-borne instruments to document a few hundred meters of subglacial groundwater below about 350 meters of ice. But most of Antarctica’s known sedimentary basins are much deeper, and most of its ice is much thicker, beyond the reach of airborne instruments. In a few places, researchers have drilled through the ice into sediments, but have penetrated only the first few meters. Thus, models of ice-sheet behavior include only hydrologic systems within or just below the ice.

Matthew Siegfried Pulls Buried Electrode Wire

Coauthor Matthew Siegfried pulls up a buried electrode wire. Credit: Kerry Key/Lamont-Doherty Earth Observatory

This is a big deficiency; most of Antarctica’s expansive sedimentary basins lie below current sea level, wedged between bedrock-bound land ice and floating marine ice shelves that fringe the continent. They are thought to have formed on sea bottoms during warm periods when sea levels were higher. If the ice shelves were to pull back in a warming climate, ocean waters could re-invade the sediments, and the glaciers behind them could rush forward and raise sea levels worldwide.

The researchers in the new study concentrated on the 60-mile-wide Whillans Ice Stream, one of a half-dozen fast-moving streams feeding the Ross Ice Shelf, the world’s largest, at about the size of Canada’s Yukon Territory. Prior research has revealed a subglacial lake within the ice, and a sedimentary basin stretching beneath it. Shallow drilling into the first foot or so of sediments has brought up liquid water and a thriving community of microbes. But what lies further down has been a mystery.

In late 2018, a US Air Force LC-130 ski plane dropped Gustafson, along with Lamont-Doherty geophysicist Kerry Key, Colorado School of Mines geophysicist Matthew Siegfried and mountain climber Meghan Seifert in the Whillans. Their mission: to better map sediments and their properties using geophysical instruments placed directly on the surface. Far from any help if something went wrong, it would take them six grueling weeks of travel, digging in the snow, planting tools, and countless other tasks.

The team used a technique called magnetotelluric imaging, which measures the penetration of natural electromagnetic energy generated in the planet’s atmosphere into the earth. Ice, sediment, fresh water, salt water, and bedrock all conduct electromagnetic energy to varying degrees; By measuring the differences, researchers can create maps of the different elements similar to those in an MRI. The team planted their instruments in snow pits for a day or so, then dug them up and relocated them, eventually taking readings at about four dozen locations. They also re-analyzed natural seismic waves emanating from the ground that had been collected by another team, to help distinguish bedrock, sediment and ice.

Their analysis showed that, depending on location, sediments extend below the ice base from half a kilometer to almost two kilometers before touching bedrock. And they confirmed that the sediments are loaded with liquid water to the bottom. The researchers estimate that if it were all removed, it would form a column of water 220 to 820 meters high, at least 10 times higher than in the shallow hydrological systems within and at the base of the ice, perhaps much higher than that. .

Salt water conducts energy better than fresh water, so they were also able to show that groundwater becomes more saline with depth. Key said this makes sense, because the sediments are thought to have formed in a marine environment a long time ago. Ocean waters probably last 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 advanced again, fresh meltwater produced by pressure from above and friction at the base of the ice was evidently forced into the upper sediments. It probably continues to seep and mix today, Key said.

The researchers say this slow drainage of freshwater into the sediments could prevent water from accumulating at the base of the ice. This could act as a brake on the forward movement of the ice. Measurements by other scientists at the ice stream ground line, the point where the land ice stream meets the floating ice shelf, show that the water there is somewhat less salty than seawater. normal. This suggests that freshwater flows through the sediments into the ocean, making room for more meltwater to enter and keeping the system stable.

However, the researchers say, if the ice surface were too thin — a distinct possibility as the climate warms — the direction of water flow could reverse. Overlying pressures would decrease and deeper groundwater could begin to well up into the ice base. This could further lubricate the base of the ice and increase its forward motion. (The Whillans already moves ice seaward about a meter a day, very fast for glacial ice.) Also, if deep groundwater flows up, it could transport naturally generated geothermal heat in bedrock; this could further thaw the base of the ice and propel it forward. But whether and to what extent that will happen is unclear.

“Ultimately, we don’t have huge constraints on the permeability of the sediments or how fast the water will flow,” Gustafson said. “Would it make a big difference that it would create a runaway reaction? Or is groundwater a minor player in the grand scheme of ice flow?

The known presence of microbes in the shallow sediments adds another wrinkle, the researchers say. It is likely that this basin and others are inhabited further downstream; and if the groundwater starts to move up, it would take out the dissolved carbon used by these organisms. Lateral groundwater flow would send some of this carbon into the ocean. This would possibly make Antarctica a hitherto unconsidered source of carbon in a world that already swims in it. But again, the question is whether this would produce any significant effect, Gustafson said.

The new study is just a start in addressing these questions, the researchers say. “Confirmation of the existence of deep groundwater dynamics has transformed our understanding of ice stream behavior and will force modification of subglacial water models,” they write.

The 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 Sciences. Chloe Gustafson is now a postdoctoral researcher at Scripps.

Reference: “A dynamic saline groundwater system mapped beneath an Antarctic ice stream” by Chloe D. Gustafson, Kerry Key, Matthew R. Siegfried, J. Paul Winberry, Helen A. Fricker, Ryan A. Venturelli, and Alexander B Michaud, May 5, 2022, Science.
DOI: 10.1126/science.abm3301

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