Many researchers believe that liquid water is key to understanding the behavior of frozen forms found in glaciers. Melting water is known to lubricate their gravel bases and accelerate their journey toward the sea. In recent years, scientists in Antarctica have discovered hundreds of interconnected liquid lakes and rivers that lie among ice sheets. What’s more, they also photographed thick sediment basins under the ice that may contain all the largest reservoirs. But so far, no one has confirmed the presence of large amounts of liquid water in subglacial sediments, nor has anyone investigated its possible interactions with ice. Now, a research teamDraw for the first timeA large, actively circulating groundwater system in the deep sediments of west Antarctica.
Such a system, which may be common in Antarctica, may have effects on how the frozen continent responds to climate change, and may even contribute to climate change, with effects that are still unknown. The study was published in May 5《sciencemagazine.
For decades, scientists have been flying radar and other instruments over the Antarctic ice sheet, imaging subsurface features. In many other ways, these missions reveal sedimentary basins sandwiched between ice and bedrock. But airborne geophysics can usually only reveal a rough outline of these features, not water content or other features. In one exception, a 2019 study of Antarctica’s McMuddo Dry Valley used instruments carried by helicopters to record groundwater beneath glaciers, hundreds of meters below the ice. But most of Antarctica’s known sedimentary basins are much deeper, and most of their ice is so thick that airborne instruments cannot reach them. In a few places, researchers have drilled through the ice to enter the sediment, but only the first few meters. Thus, models of ice behavior include only hydrological systems within or under ice.
This is a big flaw; much of Antarctica’s vast sedimentary basin lies below current sea level, sandwiched between bedrock-covered land ice and ocean ice shelves floating on the continental edge. They are thought to have formed on the seafloor during warm periods of higher sea level. If the ice shelf is pulled back in a warming climate, seawater may re-invade the sediment, while glaciers behind the ice shelf may rush forward, raising sea levels around the world.
The researchers for the new study focused on the 60-mile-wide Whalens Ice Stream, one of several rapid currents that provide water to the world’s largest Ross Ice Shelf, which is about the size of Canada’s Yukon. Previous studies have found that within the ice there is a lake under the glacier and a sedimentary basin below it. Shallow drilling of sediment around the first foot revealed liquid water and a thriving microbial community. But what further down has always been a mystery.
The team used a technique called magnetoelectric imaging, which measures the penetration of natural electromagnetic energy generated high in Earth’s atmosphere. Ice, sediment, freshwater, brackish water, and bedrock all conduct electromagnetic energy to varying degrees; by measuring differences, researchers can create NMR-like maps of different elements. The team placed their instruments in snow pits for a day or so at a time, then dug it out and relocated it, eventually taking readings at about forty sites. They also reanalyzed natural seismic waves emanating from Earth that were collected by another team to help distinguish between bedrock, sediment and ice.
Their analysis showed that depending on the location, the sediment extends from half a kilometer to nearly two kilometers below the bottom of the ice before touching the bedrock. And they confirmed that the sediments were filled with liquid water all the way down. The researchers estimate that if all the water were extracted, it would form a column of water from 220 meters to 820 meters high, at least 10 times more than the shallow hydrological systems inside and at the bottom of the ice, and perhaps even more than that.
Brackish water conducts energy better than fresh water, so they were also able to show that groundwater became saltier as the depth increased. Because these sediments are thought to have formed in the marine environment a long time ago. Ocean water may have reached the area now covered by Whelans for the last time during a warm period about 5,000 to 7,000 years ago, saturating the sediment with salt water. As the ice re-moves, fresh meltwater generated by the pressure above and friction at the bottom of the ice is apparently forced into the upper sediment.
The researchers say this slow drainage of freshwater into sediment prevents water from accumulating at the bottom of the ice. This may act as a brake on the forward movement of the ice. Measurements taken by other scientists at the grounding line of the ice flow — where the ice flow on land meets the floating ice shelf — show that the water there is somewhat less salinity than normal seawater. This suggests that freshwater is moving toward the ocean through sedimentary streams, making room for more meltwater to enter and keeping the system stable.
However, the researchers say that if the ice surface is too thin — a clear possibility as the climate warms — the direction of water flow could be reversed. The pressure on the upper layers will decrease and deeper groundwater may begin to gush out into the ice base. This further lubricates the bottom of the ice and increases its forward movement. In addition, if deep groundwater flows upwards, it can carry geothermal heat naturally generated in the bedrock; this can further thaw the bottom of the ice and propel it forward. However, whether this will happen, and to what extent, is unclear.
The researchers say the presence of known microbes in shallow sediments adds to another problem. This and other basins are likely to be suitable for habitat farther afield; if groundwater begins to move upwards, it will bring in dissolved carbon used by these organisms. Lateral groundwater flows will send this carbon into the ocean. This could turn Antarctica into a hitherto unconsidered source of carbon. But the question remains whether this will have some significant impact.
The researchers say the new study is just the beginning of solving these problems. The confirmation of the dynamic existence of deep groundwater changes our understanding of glacier behavior and will force changes to the glacier sewage model.