Tuesday, November 29, 2022

Giant groundwater system discovered in sediments below Antarctic ice

Previously unmapped reservoirs could speed glaciers, and release carbon.

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Shallow, dynamic subglacial water systems provide lubrication that facilitates the movement of overlying ice. Antarctica’s fast-flowing ice streams drain the ice sheet, controlling the velocity of subglacial water systems. The current understanding of these water systems is confined to the shallow parts around the ice-bed interface, although deeper groundwater could also influence ice streaming.

A team from six research institutions has, for the first time, confirmed the presence of large amounts of liquid water in below-ice sediments. They mapped a huge, actively circulating giant groundwater system in deep sediments in West Antarctica.

The amount of groundwater they found was so significant. It likely influences ice-stream processes claims scientists.

The majority of Antarctica‘s known sedimentary basins are substantially deeper, and the majority of its ice is much thicker, beyond the reach of airborne instruments. Scientists had drilled through the ice into sediments in a few places, but their instruments could only reach the first few meters. Hence, models of ice-sheet behavior include only hydrologic systems within or just below the ice.

In this new study, scientists concentrated on the 60-mile-wide Whillans Ice Stream. Whillans Ice Stream is 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. A previous study has revealed a subglacial lake within the ice and a sedimentary basin stretching beneath it.

Ice, sediments, freshwater, salty water, and bedrock conduct electromagnetic energy to different degrees. Scientists used magnetotelluric imaging and passive seismic data from Whillans Ice Stream to measure the penetration of natural electromagnetic energy into the earth. Seismic data help them distinguish bedrock, sediment, and ice.

According to their findings, the sediments stretch from a half-kilometer to nearly two kilometers below the ice’s base before striking bedrock, depending on location. They also confirmed that the sediments are saturated with liquid water throughout. According to the scientists, if all of it were extracted, it would form a water column ranging from 220 to 820 meters high, at least ten times higher than the shallow hydrologic systems found within and at the base of the ice, maybe much higher.

Lamont-Doherty geophysicist Kerry Key said, “Salty water conducts energy better than freshwater, so they were also able to show that the groundwater becomes more saline with depth. This makes sense because the sediments are believed to have been formed in a marine environment long ago.”

“Ocean waters probably last reached what is now the area covered by the Whillans during a warm period some 5,000 to 7,000 years ago, saturating the sediments with saltwater. When the ice readvanced, fresh meltwater produced by pressure from above and friction at the ice base was evidently forced into the upper sediments. It probably continues to filter down and mix in today.”

“This slow draining of freshwater into the sediments could prevent water from building up at the ice base. This could act as a brake on the ice’s forward motion. Measurements by other scientists at the ice stream’s grounding line—the point where the landbound ice stream meets the floating ice shelf—show that the water there is somewhat less salty than normal seawater. This suggests that freshwater is flowing through the sediments to the ocean, making room for more meltwater to enter and keeping the system stable.”

Scientists noted, “However, if the ice surface were to thin—a distinct possibility as the climate warms—the direction of water flow could be reversed. Overlying pressures would decrease, and deeper groundwater could begin welling up toward the ice base. This could further lubricate the base of the ice and increase its forward motion.”

“Furthermore, if deep groundwater flows upward, it could carry up geothermal heat naturally generated in the bedrock; this could further thaw the base of the ice and propel it forward. But if that will happen, and to what extent, is not clear.”

Chloe Gustafson, who researched as a graduate student at Columbia University‘s Lamont-Doherty Earth Observatory, said, “Ultimately, we don’t have great constraints on the permeability of the sediments or how fast the water would flow. Would it make a big difference that would generate a runaway reaction? Or is groundwater a minor player in the grand scheme of ice flow?”

Scientists said“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.”

Journal Reference:

  1. Chloe D. Gustafson et al. A dynamic saline groundwater system mapped beneath an Antarctic ice stream. DOI: 10.1126/science.abm3301
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