Even though parts of West Antarctica are melting rapidly, raising sea levels, large swathes of ice remain stable for now. Scientists have now explored one of those stable spots – an isolated corner where the ocean meets the ice. There, the team found the underside of the ice carved into strange grooves, ripples and globes.
This environment is “really on the border” between melting and freezing, says planetary scientist Justin Lawrence. The delicate balance between these two processes shapes the ice into these strange textures – similar to the way minerals dissolve and recrystallize to form the beautiful shapes inside limestone caverns.
The result, at Kamb Ice Stream, is that massive cracks under the ice appear to be refreezing as beach ball-sized globes fill crevices from above, Lawrence and his colleagues report March 2 to nature geoscience.
This refreeze differs from what is happening at the Thwaites Glacier in Antarctica. There, scientists recently reported that these cracks, called basal crevasses, are instead sites of quick merge (SN: 02/15/23).
Understanding what is happening at Kamb will help scientists predict how large parts of the Antarctic coast that are not currently vulnerable might react as the world continues to warm due to human-induced climate change. Here’s what’s different about Kamb.
Super cold water underpins ice at Kamb, slowing melt
In December 2019, two teams of researchers from New Zealand and the United States visited the Kamb Ice Stream, a type of glacier that consists of a channel of faster-moving ice surrounded by slower-moving ice.
Kamb, like much of the rest of the West Antarctic Ice Sheet, rests on a bed hundreds of meters below sea level. The New Zealand team used hot water to make melt a narrow hole in the ice just downstream of the “grounding zone,” where the glacier heaves from its muddy bed and floats on the ocean.
The US team then lowered a remote-controlled vehicle called Icefin through 580 meters of ice and into the seawater below. The researchers piloted Icefin up to a kilometer from the borehole, navigating via cable-transmitted video. At the time of shipment, the Icefin team was affiliated with Georgia Tech in Atlanta, but has since moved to Cornell University, except for Lawrence. He now works for Honeybee Robotics, a private company in Altadena, California.
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Icefin found that much of the seawater below Kamb is about 0.3 degrees Celsius above freezing. But directly below the ice is a cooler layer, a mix of seawater and glacial meltwater just 0.02 to 0.08 degrees Celsius above freezing. Based on these measurements, Lawrence and his colleagues estimate that the exposed underside of Kamb is melting about 26 centimeters per year.
On the other hand, recent measurements on the increasingly unstable Thwaites Glacierabout 1,400 kilometers to the northeast, found the seawater in the glacier’s anchorage zone 1 to 2 degrees Celsius warmer than at Kamb – and the ice melting 5 to 40 meters per year.
The new discovery at Kamb makes sense, says Christina Hulbe, a member of the New Zealand team from the University of Otago, because the seabed at Kamb is relatively shallow. It is therefore not exposed to the deep, warm ocean currents that hit Thwaites.
Much of Antarctica is bordered by cold ocean environments similar to Kamb, she says. “So it’s important to understand this system.”
Greenish globes of re-frozen ice fill the cracks at Kamb
As Icefin slid, its sonars detected huge basal fissures up to 55 meters across in the ice above. These cracks likely formed when the floating part of the glacier, the ice shelf, flexed up and down with the ocean tides.
Lawrence and his colleagues guided the ROV into one of these cracks and found its icy white sidewalls carved into narrow vertical grooves. Icefin climbed 40 meters high, until the rilles suddenly disappeared – replaced by a jumble of ice globes, which seemed to fill the top half of the crevasse.
The globes were greenish – a tint often seen in the winter ice that forms on the surface of the ocean. This color makes Lawrence and his colleagues think that the globes form from the ultracold mixture of seawater and meltwater flowing through a crack and refreezing, gradually filling the crack, from top to bottom, over several decades. They think this is happening in all the crevices they have observed. “These cracks effectively heal on their own,” he says.
This refreezing process could also explain the strange vertical grooves in the walls of the crevasse, Lawrence speculates. As the water freezes, salt is forced out of the newly formed ice crystals, creating tiny pockets of highly concentrated brine. This dense brine runs down walls, melting grooves in the ice – the same way salt melts ice when sprinkled on a sidewalk in winter.
Observing the refreezing of crevices beneath Kamb “is quite exceptional,” says Ginny Catania, a glaciologist at the University of Texas at Austin who was not part of the project. These cracks “can propagate all the way to the surface and cause icebergs to calve,” she says, which can shrink the pack ice if it happens too quickly, destabilizing the glacier and raising sea levels.
But if the crevices can actually heal, it could make these ice shelves more resistant to calving — and more stable — than scientists thought, at least as long as the ice continues to bathe in cold water below. .
A series of instruments installed in the hole continued to measure the temperature and salinity of the water under the ice, transmitting this data by cable to the surface of the ice, and home by satellite until that the batteries run out two years later. This data shows that the conditions below remained cool and comfortable for Kamb.