Scientists have mapped a huge, circulating groundwater system in deep below-ice sediments in Western Antarctica.
These systems, which are probably common across Antarctica, may have unknown implications for how the frozen continent reacts to, or even possibly contributes to, climate change.
Their findings, published in a new study in Science, indicate that groundwater could play a potentially critical role in understanding the effect of water flow on the movement of Antarctic ice sheets into the ocean.
“People have hypothesised that there could be deep groundwater in these sediments, but up to now, no one has done any detailed imaging,” says lead author Chloe Gustafson, who did the research as a graduate student in the Lamont-Doherty Earth Observatory at Columbia University, US. “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.”
Subglacial water moves along the base of the Antarctic ice sheet through an under-ice plumbing network known as the “subglacial hydrologic system”. This flow plays an important role in the motion of glacial ice by providing lubrication between the ice sheet and the bedrock, or by causing the deformation of wet sediments beneath glaciers.
Through these two mechanisms, water at the base of the ice sheet controls Antarctica’s ice sheet dynamics and, potentially, its contribution to sea-level rise.
Before now, this subglacial hydraulic system was thought to be a shallow system where water only exists at or very near the base of the ice sheet. But now scientists suggest deep groundwater in below-ice sediments could also contribute.
Trying a different technique to image groundwater
For decades, scientists have flown radars and other instruments over the Antarctic ice sheet to image its subsurface features, but these methods aren’t powerful enough to reach through the thick ice and deep sediments to measure their water contents.
This team used a different on-ground technique called magnetotelluric imaging instead, which measures the penetration into the Earth of natural electromagnetic energy generated high in the planet’s atmosphere,
Ice, sediments, fresh water, salty water and bedrock all conduct electromagnetic energy to different degrees. So, by measuring the differences, researchers can create MRI-like maps of these different components.
They found that the sediments in the Whillans Ice Stream in Western Antarctica are loaded with liquid water, at least 10 times more than in the shallow hydrologic systems within and just below the ice.
The groundwater becomes more saline with depth as fresh melt water, produced by pressure from above and friction from the ice base, is forced into the upper sediments and filters down.
The researchers say that the slow draining of fresh water into the sediments could prevent it from building up at the base of the ice and act as a brake on the ice’s forward motion. But they suggest that if the surface ice were to thin due to climate change, the direction of water flow could be reversed, and deep groundwater could begin to well up toward the ice base instead.
This could instead accelerate the forward motion of the ice towards the sea. But the probability of that happening, and to what extent, is currently unclear.
“Ultimately, we don’t have great constraints on the permeability of the sediments or how fast the water would flow,” says 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?”
According to the researchers, this new study is just a start to addressing these questions. “The confirmation of the existence of deep groundwater dynamics has transformed our understanding of ice-stream behaviour, and will force modification of subglacial water models,” they write.