Ice melange is a mixture of ice shelf fragments and windblown snow that acts as a glue to bind large rifts in Antarctica's floating ice. NASA JPL and UCI researchers discovered that large icebergs can be formed when rifts are opened by thinning the substance. Credit: Beck/NASA Operation IceBridge
NASA's Jet Propulsion Laboratory and the University of California Irvine have studied the dynamics behind the calving of Delaware-sized icebergs A68 and C68, both from Antarctica's Larsen C Ice Shelf in July 2017. They found that the most likely cause was a thinning of an ice melange. This is a mixture of windblown snow, seawater and iceberg debris, which normally heals rifts.
The researchers published a paper today in Proceedings of the National Academy of Sciences. They report that melange thinning was a key driver of ice shelf collapsing. Radiative warming from above and the circulation of ocean water below ice shelves can gradually degrade ice melange over many decades, they claim.
This new information about rift dynamics reveals a link previously overlooked between climate change, ice shelf stability and land-borne glaciers' faster flow into the ocean.
Eric Rignot (UCI professor of Earth system sciences), said that the melting of the ice melange, which glues large sections of floating ice shelves together, is one more way climate change could cause Antarctica's rapid retreat. With this in mind, we might need to reconsider our estimates of the timing and magnitude of sea level rise due to polar ice loss. It could happen sooner than expected and with a greater impact.
The researchers used NASA's Ice-sheet and Sea-level System Models, observations from Operation IceBridge, and data from NASA and European spacecrafts to assess hundreds of Larsen C rifts to determine their vulnerability to breaking. The researchers selected 11 cracks from top to bottom for an in-depth analysis. They then modelled the three possible scenarios to determine which one made them break the most: If the ice shelves thinned due to melting, if the melange became thinner, or if both the melange and the ice shelf thinned.
"Many people intuitively thought, "If the ice shelf is thinned, you're going make it more fragile and it's going be broken," said Eric Larour (NASA JPL research scientist, and group supervisor.
The model instead showed that the average annual widening rate dropped from 79 to 22 metres (259 to 72 feet) by thinning the ice shelf. The melange and ice shelf were also thinned to slow rift widening, but only to a lesser degree. However, when modeling melange thinning only, scientists discovered a widening in rifts at an annual average rate of 76-112 meters (249-367 feet).
Larour explained that the difference is due to the nature of the substances.
He said that the melange is thinner then ice. "When the melange thickness is less than 10 to 15 meters, it's similar to water. The ice shelf rifts release and crack the melange.
Warmer ocean water can reach the melange even in winter because rifts run through the entire thickness of the ice shelf.
Hydrofracturing is the dominant theory behind large iceberg-calving events in Antarctic Peninsula. This involves melt pools at the surface allowing water to seep through cracks in ice shelf. The water expands when it freezes again, Rignot said. This theory does not explain how Larsen C ice shelves could be broken in Antarctic winter, when there were no melt pools.
He stated that he and other members of the cryosphere research community witnessed the Antarctic Peninsula's ice shelf collapsing as a result of a retreat that started decades ago.
Rignot stated that "we have finally started to search for an explanation as to how these ice shelves began retreating and becoming instabile configurations that became unstable decades prior to hydrofracturing could take effect on them." Although the thinning of ice melange may not be the only explanation, it is sufficient to explain the deterioration we have observed.
Rignot, Larour, and Mattia poinelli, both UCI associate scientists in Earth system science, were part of this NASA-funded project.
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Additional information: Physical processes controlling the movement of Larsen C Ice Shelf in Antarctica prior to the calving of A68, Proceedings of National Academy of Sciences (2021). www.pnas.org/content/118/40/e2105080118 Journal information: Proceedings of the National Academy of Sciences Physical processes controlling the rifting of Larsen C Ice Shelf, Antarctica, prior to the calving of iceberg A68,(2021). DOI: 10.1073/pnas.2105080118