A mega iceberg released 152 billion tons of fresh water
Three and a half years later, the main part of the iceberg, A-68A, drifted worryingly close to South Georgia. Concerns were that the iceberg would run aground in the shallow waters off the coast. Not only would this cause damage to the seafloor ecosystem, but it would also make it more difficult for wildlife on the island, such as penguins, to reach the sea to feed.
Using satellite measurements, scientists have traced how the A-68A shrunk towards the end of its journey, which thankfully prevented it from getting stuck. However, the downside is that released 152 billion tons of fresh water near the island, which could have a profound effect on the island’s marine life.
When A-68 was generated, it had a surface area more than twice the size of Luxemburg, one of the largest icebergs on record.
It lost a chunk of ice almost immediately after it broke off, resulting in the largest iceberg being renamed A-68A, and its offspring becoming A-68B. In April 2020, A-68A lost another chunk later named A-68C.
Antarctic icebergs are named after the Antarctic quadrant in which they were originally sighted, then a sequential number is assigned to them, and then if the iceberg breaks, a sequential letter is added to them.
During the first two years of its life, the A-68A remained in the cold waters of the Weddell Sea, near its main ice shelf. Here, he experienced little in the way of melting. But nevertheless, once the iceberg began its northward journey through Drake Passage, it traveled through increasingly warmer waters and began to melt.
In all, iceberg A-68A thinned 67 meters from its initial thickness of 235 meters, and the rate of melting increased sharply as the iceberg drifted into the Scotia Sea around South Georgia.
One Article published in Remote Sensing of Environment describes how researchers at the UK Center for Polar Observing and Modeling and the British Antarctic Survey they combined measurements from different satellites to chart how A-68A changed in area and thickness over its life cycle.
The A-68A’s journey was mapped using observations from five different satellite missions.
To track how the area of A-68A changed, used optical images from the mission Copernicus Sentinel-3 and from the MODIS instrument on the US Terra mission, along with radar data from the mission Copernicus Sentinel-1. While Sentinel-1 radar imagery offers all-weather capability and higher spatial resolution, MODIS and Sentinel-3 optical imagery have higher temporal resolution but cannot be used during the polar night and on cloudy days.
The A68 journey. Watch the video
To measure iceberg changes, 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 ice means you can calculate the thickness of the whole iceberg.
All of these measurements together allowed scientists to calculate how the iceberg’s volume changed, and thus how much freshwater it released.
Tommaso Parrinello, ESA CryoSat Mission Manager, said: “Our ability to study each 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 by measuring the height of surfaces, which is essential for calculating volume changes.”.
The new study reveals that the A-68A only briefly collided with the seabed and broke up shortly after, which makes it less risky in terms of blocking. By the time it reached the shallows around South Georgia, the iceberg’s keel had dropped to 450 feet below the ocean’s surface, shallow enough to avoid the seafloor that is about 500 feet deep.
If the keel of an iceberg is too deep, it can get stuck to the bottom of the sea.. This can be disruptive in many ways; scour marks can destroy wildlife, and the iceberg itself can block ocean currents and predator feeding paths.
But nevertheless, a side effect of the melting was the release of a colossal 152 billion tons of fresh water near the island, a disturbance that could have a profound impact on the island’s marine habitat.
When icebergs break off ice shelves, they move with ocean currents and wind, releasing cool, fresh meltwater and nutrients as they melt. This process influences local ocean circulation and encourages biological production around the iceberg..
Anne Braakmann-Folgmann, PhD candidate at the Center for Polar Observing and Modeling and lead author of the study, said: “This is a lot 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 A-68A took a common route through the Drake Passage, we hope to learn more about icebergs that take a similar path and how they influence the polar oceans.“.