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Updated:06/05/2022 01: 08h
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Most scientists are convinced that liquid water is essential to understanding how glacial ice behaves. Something that has a direct influence on the climate of the planet. It is known, for example, that meltwater lubricates the gravel bases of glaciers and accelerates their march towards the sea. In recent years, different investigations in the antarctica They have discovered, within the ice itself, hundreds of lakes and liquid rivers interconnected by an intricate network of channels. And they have also revealed the presence of large sediment basins just below the ice, which could contain the largest water reservoirs of all. But so far no one has been able to confirm the presence of large amounts of liquid water in these subterranean sediments, nor to study how they might interact with the ice itself.
Now, an international team of researchers has managed, for the first time, to locate a huge underground water system actively circulating through deep sediments in West Antarctica. In an article published today in ‘Science’, the scientists say that such systems, probably very common in Antarctica, may have as yet unknown implications for how the frozen continent reacts to climate change, or possibly even how it contributes to it.
“Many have hypothesized that there might be deep groundwater in these sediments,” says Chloe Gustafson of Columbia University’s Lamont-Doherty Earth Observatory and lead author of the paper, “but until now no one had obtained detailed images. The amount of groundwater we found is so significant that it probably influences ice flow processes. Now we need to find out more and figure out how to incorporate that into existing models.”
The importance of groundwater
For decades, numerous researchers have flown over the Antarctic ice sheet to obtain subsurface characteristics from the air with their radars. Among many other things, those missions revealed the existence of interbedded sediment basins between the ice and bedrock. But with few exceptions, the aerial survey is only able to show the approximate contours of the basins, and says nothing about the water content and other important characteristics.
In one such exception, a 2019 study of the McMurdo Dry Valleys used helicopter-borne instruments to document a few hundred meters of subglacial groundwater beneath about 350 meters of ice. Very little, considering that most of Antarctica’s known sedimentary basins are much deeper and most of their ice is much thicker, beyond the reach of airborne instruments.
Elsewhere, researchers have drilled through the ice to the sediments, but only managed to penetrate the top few meters. Therefore, current models of ice sheet behavior include only surface hydrological systems, which are within or just below the ice. This is a major handicap, since most of Antarctica’s sedimentary basins lie below current sea level, wedged between bedrock-bound land ice and floating sea-ice shelves that fringe the continent. They are believed to have formed on the seabed during warm periods, when sea levels were higher. If this were to happen and the ice shelves were to recede with a warmer climate, the ocean waters could re-invade the sediments, and the glaciers behind them could advance and raise sea levels around the world.
A map of the underground
In the new study, the researchers focused on the 60-mile-wide Whillans Ice Stream, one of a half-dozen fast-moving streams that feed the Ross Ice Shelf, the world’s largest, almost as as big as the Iberian Peninsula. According to Gustafson, “Ice streams are important because they channel about 90 percent of Antarctica’s ice from the interior to the margins.”
Previous investigations have already revealed the presence of a subglacial lake within the ice, and also a sedimentary basin that extends below it. Shallow drilling of sediments (down to just about 30 cm) already showed that there was liquid water and a thriving community of microbes there. But what lies below remained a mystery.
Seeking to gather more data, in late 2018 a US Air Force LC-130 ski plane dropped Gustafson in the area, along with Lamont-Doherty geophysicist Kerry Key, Colorado School of Mines Matthew Siegfried and mountain climber Meghan Seifert at 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, for six grueling weeks the team dug in the snow, planted instruments and carried out countless other tasks.
On the ground, the researchers used a technique called magnetotelluric imaging, which measures the penetration of natural electromagnetic energy generated high 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, 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.
plenty of water
Their analyzes 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 all of it were 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, and perhaps even 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. The researchers think 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. When the ice advanced again, fresh melt water produced by pressure from above and friction at the base of the ice was evidently forced into the upper sediments, and probably continues to percolate and mix today.
Gustafson and his team believe that this slow drainage of fresh water into the sediments could prevent water from accumulating at the base of the ice. Which 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. And that suggests that fresh water is flowing through the sediments into the ocean, making room for more melt water to enter and keeping the system stable.
Potential risks
However, that stability could only be temporary. If the ice cover were to 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,” says Gustafson, “we don’t have a lot of data on the permeability of the sediments or how fast the water will flow. Would it make a big difference if he was capable of generating a runaway reaction? Or is groundwater a minor player in the grand scheme of ice flow?
Furthermore, the known presence of microbes in the shallow sediments adds another problem, 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. And that would possibly make Antarctica a previously uncounted source of carbon in a world where carbon is already too much. But again, Gustafon said, the question is whether or not this would be capable of producing any significant effect.
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