The launch of an underwater vehicle offers a precise view of the underside of the world’s largest floating ice shelf. One of the least observed places, it has revealed itself in part to ocean physicists, for example stalagtites of underwater ice or evidence of the retreat of the glacier grounding line.
“The ice took on superb attitudes” wrote Jules Verne in Twenty Thousand Leagues Under the Sea. Here it was no Nautilus but Icefin, an underwater vehicle in the shape of a long tube filled with sensors, a propulsion system and ballasts. No Captain Nemo, but an American-New Zealand team piloting Icefin under the floating platform of the Ross Sea in West Antarctica, downstream of the Kamb Ice Stream. As for the Thwaites glacier, surprising results were published on March 2 in Nature Geoscience.
The “ice pump”
Researchers have observed for the first time in a submerged crevasse of about fifty meters high, ice concretions appearing under the effect of an absorption phenomenon. “We were looking at ice that had just melted less than 100 feet below, flowed up into the crevasse and then refroze,” said Justin Lawrence, a physicist from Cornell University, in a statement.
Crevasse C, downstream of the ice flow, is a fully submerged crevasse. When Icefin went up one of the walls, “the thermal driving steadily decreased”, write the authors. More than 400 meters below the surface of the ice shelf, the color and texture of the ice changed from “smooth, melting and cloudy,” according to the authors, to green-tinged ice dotted with gullies. “And then it just got weirder as we went higher up,” the physicist said.
A sensor from Icefin recorded the lowest thermal conduction: -0.006 °C in the ceiling of the crevasse: “We found a trench of 2 to 3 m wide, filled with rounded globular formations of several tens of centimeters in diameter with linear channels and protrusions in the form of brinicles”, a clever name for an ice stalactite, used by the authors.
This phenomenon of sea ice formation is the “ice pump” and can stabilize submerged crevasses. This is the origin of the green icebergs called “jade”.
Along the most inclined wall, upstream of the crevasse, the “ice pump” seems to be on its way too. “We observed (…) a similar transition (…) between accretion ice and white ice, with larger vertical gullies,” the authors describe. It is likely that when ice forms again, the excreted salt flows down the walls and shapes gullies.
Withdrawal
Five crevasses were explored along the 5 kilometers of underwater exploration, going under the floating platform to the foundations of the West Antarctic Ice Sheet that rests on the continental shelf, downstream of the Kamb Ice Stream. The researchers observed the signs of its withdrawal.
The ice-free sedimentary bed form bore traces in the form of parallel striations several dozen meters long. In the past, the ice ridges around the crevasses carved ruts still visible in the substrate. These ridges were 20 to 40 m wide and at least hundreds of meters long.
Other shapes testify to the thinning of the cap and its progressive detachment from the land base during the retreat of the submarine front. “We can look at those sea floor features and directly connect them to what we saw on the ice base,” said Justin Lawrence. “We can, in a way, rewind the process.”
The Ross Barrier constitutes one third of the world’s floating platforms (the largest). Streams feed the Ross Sea shelf with ice flow at an average of about 100 meters per year. Among them, the Kamb glacial current, known for its century-long cycle of acceleration and deceleration, has been stagnant since 1865.
This stagnation leads to an accumulation of ice upstream which compensates 25% of the ice loss in West Antarctica. But if it were to start up again, it would be involved in 12% of Antarctica’s contribution to sea level rise. In short, Icefin went to the bedside of a sleeping giant.
Camille Lin, PolarJournal
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