Many scientists believe that water is a key to understanding the behavior of the frozen form of glaciers. Melt water is known to lubricate the base of a glacier and hasten movement towards the sea. Previously, unmapped reservoirs could speed glaciers, and as a result, release carbon.
In recent years, researchers in Antarctica have discovered hundreds of interconnected liquid lakes and rivers within the ice itself. Images of thick basins of sediments under the ice have been captured, and these potentially contain the biggest water reservoirs of all.
For the first time, a team has mapped a huge, actively circulating groundwater system in deep sediments in West Antarctica. These systems may have unknown implications for how Antarctica reacts to, or even contributes to, climate change.
“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.” said study lead author Chloe Gustafson, who did the research as a graduate student at Columbia University’s Lamont-Doherty Earth Observatory.
Most of Antarctica’s sedimentary basins lie below current sea level, and are thought to have formed on sea bottoms during warm periods when sea levels were higher. Under a warming climate, these ice shelves could pull back, allowing ocean waters to re-invade the sediments, and the glaciers behind them could raise sea levels worldwide.
The 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. Shallow drilling into the first foot of sediments brought up liquid water and a thriving community of microbes. But what lies further down has been a mystery.
In 2018, a U.S. Air Force ski plane dropped Gustafson, along with Lamont-Doherty geophysicst Kerry Key, geophysicist Matthew Siegfried, and mountaineer Meghan Seifert on the Whillans. Their mission was to better map the sediments using geophysical instruments over six exhausting weeks of work.
The team planted their instruments in snow pits, collecting readings at four dozen locations. Their analysis showed that, depending on location, the sediments extend below the base of the ice from a half kilometer to nearly two kilometers before hitting bedrock. It was confirmed that the sediments are loaded with liquid water all the way down. If all of it were extracted, it could form a water column from 220 to 820 meters high, or higher.
The research team was able to show that the groundwater becomes more saline with depth. This is because the sediments are believed to have formed 5,000 to 7,000 years ago, when ocean waters last reached the area, saturating the sediments with salt water. When the ice readvanced, fresh melt water was forced into the upper sediments. It probably continues to filter down and mix in today, said Key.
This slow draining of fresh water into the sediments could prevent water from building up at the base of the ice, acting as a brake on the ice’s forward motion. It is likely that fresh water is flowing through the sediments to the ocean, making room for more melt water to enter, and keeping the system stable.
However, a warming climate could lead to thinning of the ice surface, causing the direction of water flow to be reversed. This could further lubricate the base of the ice and increase its forward motion. This is concerning as the Whillans already moves ice seaward about a meter a day, considered fast for glacial ice.
Furthermore, if deep groundwater flows upward, it could carry up geothermal heat naturally generated in the bedrock, thawing the base of the ice and propelling it forward. But if that will happen, and to what extent, is not clear.
“Ultimately, we don’t have great constraints on the permeability of the sediments or how fast the water would flow,” said Gustafson. “Would it make a big difference that would generate a runaway reaction? Or is groundwater a more minor player in the grand scheme of ice flow?”
Another potential concern is the known presence of microbes in the shallow sediments. If groundwater begins moving upward, it would bring up the dissolved carbon used by these organisms, sending some of this carbon to the ocean. This could turn Antarctica into a source of carbon in a world already swimming in it. But again, the question is whether this would produce some significant effect, said Gustafon.
The new study is just a start to addressing these questions, said the researchers. “The confirmation of the existence of deep groundwater dynamics has transformed our understanding of ice-stream behavior, and will force modification of subglacial water models.”
The research is published in the journal Science.