Lead author Chloe Gustafson and climber Meghan Seifert are installing geophysical instruments to measure groundwater under the Whillans Icy Stream in West Antarctica. Credit: Kerry Key / Lamont-Doherty Earth Observatory
Previously unmapped tanks could accelerate glaciers and release carbon.
Many researchers believe that liquid water is the key to understanding the behavior of the frozen form found in glaciers. It is known that molten water lubricates their gravel bases and accelerates their march to the sea. In recent years, scientists in Antarctica have discovered hundreds of interconnected liquid lakes and rivers located in the ice itself. And they have depicted thick pools of sediments under the ice, potentially containing the largest water reservoirs of all. But so far no one has confirmed the presence of large amounts of liquid water in the sediments under ice, nor has it studied how it can interact with ice.
Now, for the first time, a research team is mapping a vast, actively circulating system of groundwater in deep sediments in West Antarctica. They say such systems, probably common in Antarctica, could have as yet unknown effects on how the frozen continent reacts or may even contribute to climate change. The study was published in the journal Science on May 5, 2022.
Explore the locations of the Whillans Ice Stream. The electromagnetic imaging stations were built in two common areas (yellow marking). The team travels to wider areas to perform other tasks shown with red dots. Click on the image to see a larger version. Credit: Courtesy of Chloe Gustafson
“People have speculated that there may be deep groundwater in these sediments, but so far no one has taken any detailed images,” said lead author Chloe Gustafson, who conducted the study as a graduate student at Columbia University’s Lamonta. Doherty Earth Observatory. “The amount of groundwater we found was so significant that it probably affects the processes of ice flow. Now we need to find out more and figure out how to incorporate this into the models. “
For decades, scientists have flown radar and other instruments over the ice sheet of Antarctica to depict underground features. Among many other things, these missions revealed sedimentary basins sandwiched between ice and rock. But aerial geophysics can usually only reveal the rough outlines of such characteristics, not the water content or other characteristics. With one exception, a 2019 study of the McMurdo Dry Valleys in Antarctica used helicopter tools to document several 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 air tools. In several places, researchers have drilled ice into sediments, but only penetrated the first few meters. Thus, the patterns of ice cover behavior include only hydrological systems in or just below the ice.
Co-author Matthew Siegfried pulls a buried electrode wire. Credit: Kerry Key / Lamont-Doherty Earth Observatory
This is a big deficit; most of Antarctica’s vast sedimentary basins lie below the current sea level, sandwiched between rocky ground ice and floating sea ice shelves that surround the continent. They are believed to have formed on the seabed during warm periods when sea levels were higher. If the ice shelves recede in a warming climate, ocean waters could invade the sediments again, and the glaciers behind them could rush forward and raise sea levels around the world.
Researchers in the new study are focusing on the 60-mile-wide Whillans Ice Stream, one of half a dozen fast-moving streams feeding the world’s largest ice shelf, Ross, about the size of the Canadian Yukon Territory. Previous research has revealed an subglacial lake in the ice and a sedimentary basin that stretches beneath it. Shallow drilling in the first legs of the sediments resulted in liquid water and a thriving microbial community. But what lies below is a mystery.
At the end of 2018, the US Air Force’s LC-130 ski plane shot down Gustafson, along with geophysicist from Lamont-Doherty Kerry Key, geophysicist from the Colorado Mining School Matthew Siegfried and climber Megan Seifert of Wheeland. Their mission: to better map sediments and their properties with the help of geophysical tools placed directly on the surface. Far from any help, if something goes wrong, it will take them six grueling weeks of traveling, digging in the snow, planting tools and countless other responsibilities.
The team uses a technique called magnetotelluric imaging, which measures the penetration into the earth of natural electromagnetic energy generated high in the planet’s atmosphere. Ice, sediments, fresh water, salt water and rock base all conduct electromagnetic energy to varying degrees; by measuring differences, researchers can create MRI-like maps of different elements. The team put their instruments in snow pits for about a day, then dug them up and moved them, eventually testifying in about four dozen places. They also re-analyzed natural seismic waves emitted from the ground, which were collected by another team to help distinguish rocks, sediments and ice.
Chloe Gustafson Kerry Key
Their analysis showed that, depending on the location, the sediments stretched under the ice base from half a kilometer to almost two kilometers before hitting the rock. And they confirmed that the sediments were filled with liquid water all the way down. Researchers estimate that if all this is extracted, it will form a water column 220 to 820 meters high – at least 10 times more than in shallow hydrological systems in and at the base of the ice – perhaps much more than that.
Salt water conducts energy better than fresh water, so they also managed to show that groundwater becomes saltier with depth. Ki said it made sense, as sediments are thought to have formed in the marine environment long ago. Ocean waters probably last reached the current area covered by Wilens during a warm period about 5,000 to 7,000 years ago, saturating the sediments with salt water. When the ice moved again, fresh molten water produced by top pressure and friction at the base of the ice was apparently pushed into the upper sediments. It probably continues to filter and mix today, Kee said.
Researchers say that this slow flow of fresh water into the sediments can prevent water from accumulating at the base of the ice. This can act as a brake on the movement of the ice forward. Measurements by other scientists on the ground line of the ice stream – the point where land-related ice flow meets the floating ice shelf – show that the water there is slightly less salty than normal seawater. This suggests that fresh water flows through the sludge to the ocean, making room for more molten water to enter and keeping the system stable.
Matt Siegfried Megan Seifert
However, researchers say if the ice surface was too thin – a different option as the climate warms – the direction of water flow could be reversed. Pressures will decrease and deeper groundwater may begin to rise to the ice base. This can further crush the base of the ice and increase its forward movement. (Whillans already move ice toward the sea about a meter a day – very fast for glacial ice.) In addition, if deep groundwater flows upward, it can transfer geothermal heat naturally generated at the rock base; this can further thaw the base of the ice and move it forward. But whether this will happen and to what extent is unclear.
“After all, we don’t have big limits on sediment permeability or how fast water will flow,” Gustafson said. “Will there be a big difference that it would provoke a quick reaction?” Or is groundwater a minor player in the big ice flow scheme?
The known presence of microbes in shallow sediments adds another wrinkle, researchers say. This pool and others are probably inhabited below; and if groundwater begins to move upward, it will lead to dissolved carbon used by these organisms. Then the lateral flow of groundwater will send some of this carbon into the ocean. This would probably make Antarctica an unprecedented source of carbon in a world that is already floating in it. But again, the question is whether this will have any significant effect, Gustafson said.
The new study is just the beginning to address these issues, researchers say. “Confirmation of the existence of deep groundwater dynamics is transforming our understanding of ice flow behavior and will require modification of groundwater patterns,” they wrote.
The other authors are Helen Fricker of the Scripps Institute of Oceanography, J. Paul Winbury of Central Washington University, Ryan Ventureli of Tulane University and Alexander Misho of the Bigelow Ocean Science Laboratory. Chloe Gustafson is now a postdoctoral fellow at Scripps.
Reference: “Dynamic Groundwater Salt System Mapped Under Antarctic Ice Stream” by Chloe D. Gustafson, Kerry Key, Matryw 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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