In July 2017, a giant iceberg, named A-68, broke away from Antarctica’s Larsen-C Ice Shelf and began an epic journey across the Southern Ocean. Three and a half years later, the main part of the iceberg, A-68A, drifted ominously near South Georgia. Concerns were that the iceberg would run aground in the shallow waters offshore. This would not only damage the seabed ecosystem, but also make it difficult for island wildlife, such as penguins, to get to the sea to feed. Using satellite measurements, scientists mapped how the A-68A narrowed towards the end of its journey, which fortunately prevented it from getting stuck. However, the downside is that it released a colossal 152 billion tons of fresh water near the island, potentially having a profound effect on the island’s marine life.
When A-68 was created, it had an area more than twice the size of Luxembourg – one of the largest icebergs ever recorded.
He lost a chunk of ice almost immediately after calving, causing the largest iceberg to be renamed A-68A, and his offspring to become A-68B. In April 2020, A-68A lost another piece subsequently called A-68C.
Antarctic icebergs are named after the Antarctic quadrant in which they were originally sighted, then a sequential number, then if the iceberg breaks up, a sequential letter is added.
For the first two years of its life, A-68A remained in the cold waters of the Weddell Sea near its mother ice shelf. Here it experienced little melting. However, once the iceberg began its northward journey through the Drake Passage, it passed through increasingly warm waters and began to melt.
In total, the A-68A iceberg thinned 67 meters from its original thickness of 235 meters, with the rate of melting increasing sharply as the iceberg drifted in the Scotia Sea around South Georgia .
An article published in Environmental remote sensing describes how researchers from the Center for Polar Observation and Modeling in the UK and the British Antarctic Survey combined measurements from different satellites to determine how the A-68A changed in surface area and thickness throughout its life cycle. The journey of the A68
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The A-68A’s journey was mapped using observations from five different satellite missions.
To follow the evolution of the area of the A-68A, they used optical images from the Copernicus Sentinel-3 mission and the MODIS instrument from the US Terra mission, as well as radar data from the Copernicus Sentinel mission. -1. While Sentinel-1 radar imagery offers all-weather capability and higher spatial resolution, MODIS and Sentinel-3 optical imagery has higher temporal resolution but cannot be used during polar night and cloudy days.
To measure changes in the iceberg’s freeboard, or the height of the ice above the sea surface, they used data from ESA’s CryoSat mission and the US ICESat-2 mission. . Knowing the freeboard of the ice makes it possible to calculate the thickness of the whole iceberg.
All of these measurements combined allowed scientists to calculate how the iceberg’s volume changed and therefore how much fresh water it released.
Tommaso Parrinello, ESA’s CryoSat mission manager, said: “Our ability to study every movement of the iceberg in such detail is due to advances in satellite techniques and the use of a variety of measurements. Imaging satellites record the shape of the iceberg and data from altimetry missions like CryoSat add another important dimension as they measure the height of surfaces – which is essential for calculating changes in volume.
The new study reveals that the A-68A only briefly collided with the seabed and broke up soon after, reducing the risk in terms of jamming. By the time it reached the shallow waters around South Georgia, the iceberg’s keel had shrunk to 141 meters below the ocean surface, shallow enough to just avoid the seabed that is d about 150 meters deep.
If an iceberg’s keel is too deep, it can get stuck on the sea floor. This can be disruptive in many ways; scour marks can destroy wildlife, and the iceberg itself can block ocean currents and predator feeding routes.
However, a side effect of the melt was the release of a colossal 152 billion tonnes of fresh water near the island – a disturbance that could have a profound impact on the island’s marine habitat.
As icebergs break away from ice shelves, they drift with ocean currents and wind, releasing cool, cold meltwater and nutrients as they melt. This process influences local ocean circulation and promotes biological production around the iceberg.
Anne Braakmann-Folgmann, a PhD student at the Center for Polar Observation and Modeling and lead author of the study, said: “This is a huge amount of meltwater, and the next thing we want to know is whether it had a positive or negative impact on the ecosystem around South Georgia.
“Because the A-68A took a common route through the Drake Passage, we hope to learn more about icebergs taking a similar trajectory, and how they influence the polar oceans.”