A piece of Antarctic iceberg A-68, shown here in 2017, calved from the Larsen C ice shelf. On Wednesday, NASA released a photograph of it, showing it shaped in a nearly perfect rectangle. (Mario Tama/Getty Images)
A college-campus-size iceberg spotted on the Antarctic peninsula margins is upending public expectations of classic Titatnic-esque icebergs, with sharp spires jutting from the ocean surface.
An aerial photo shared by NASA on Wednesday captured a rectangular slab of ice sliced so smoothly that it appears unnatural. Experts believe that the iceberg fractured from Larsen C, a large ice shelf fed by several glaciers on the east side of the peninsula, in May. They’re still unsure, however, whether it will cause the rest of the shelf to destabilize.
According to Christopher Shuman, a research scientist at the University of Maryland Baltimore County’s Joint Center for Earth Systems Technology at NASA: “The takeaway message is that ice shelves release large icebergs from time to time. They do this naturally.”
It means that the ice shelf is changing, which is not surprising given the changes they’ve seen nearby, he told The Washington Post.
Larsen A was another ice shelf farther north on the peninsula that broke up in 1995. Larsen B broke up in 2002. Larsen C itself calved an even larger iceberg in 1986, Shuman said. It was recorded at 46,000 square kilometers — larger than Maryland — until last July, when it calved an iceberg the size of Delaware, known as A-68.
[Watch 10 billion tons of ice fall into the ocean]
The rectangular iceberg, photographed as part of a topographic mapping project tracking changes in polar ice shelves called Operation Ice Bridge, fractured in May, after A-68 crashed into Bawden, a rocky ice-covered island in the northwest, and created several small berg fragments.
“This is not good news for the Larsen C in a general sense,” Shuman said. The iceberg could continue to be resupplied with ice from land-based glaciers, as it was after the bigger iceberg broke up in 1986.
A larger problem, Shuman said, is that the ice front is as far west as it’s ever been, based on satellite imagery. “The fact that the edge is so far west is not a good sign; Larsen A and Larsen B broke up just up the peninsula,” he said.
[Listen to the eerie song of Antarctica — melting]
According to National Snow and Ice Data Center research scientist Twila Moon, it’s also not terribly surprising that the iceberg fractured in straight lines. That’s consistent with an iceberg calved from floating ice in that region, she said. Ice is a mineral; it has a crystal-like structure and breaks the way a shard of glass would.
“When a piece of glacial ice is in contact with the ocean floor, the interference at the base causes it to calve off differently, creating misshaped icebergs,” Moon explained. But when an iceberg breaks off from a shelf of floating ice, like Larsen C, there’s no friction controlling how it breaks up.
“That’s when large tabular icebergs form,” she said, which differ in shape from bulky and pointed non-tabular icebergs.
Like Moon, Shuman said the shape is not surprising, though it’s unusual to see such sharp angles.
“It’s all about the force of the ice striking the iceberg. When ice is fracturing, it tends to break on the line of force,” he said. Moreover, there are other pieces nearby that are relatively rectangular.
“It’s a part of the life cycle for ice shelves: They push out far enough that they break off in big chunks,” he said. “We do not know yet for sure that this was climate-change-related or that we’re looking at a dramatic change.”
Still, Shuman added, there are other areas with very definite signs that ice fronts are retreating inland dramatically and at a full-blown pace.
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https://www.washingtonpost.com/science/2018/10/24/theres-perfectly-rectangular-iceberg-floating-antarctica-heres-how-it-got-that-way/Bagikan Berita Ini
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