An extraordinary iceberg is gone, but not forgotten

Maybe you remember the iceberg A68a, which enjoyed a few minutes of fame back in 2017 when it broke off an ice shelf on the Antarctic Peninsula. Hardly your daily iceberg, it was one of the largest ever seen, more than 100 miles long and 30 miles wide.

The iceberg drifted slowly through the icy Weddell Sea for some years before gathering steam as it entered the southern ocean. The last we heard from it, in 2020, it carried down the island of South Georgia in the South Atlantic, slightly constrained and hit by a voyage of more than a thousand miles.

Alas, the old A68a is no more. Last year, about 100 miles from South Georgia, it finally did what all icebergs eventually do: thinned out so much that it broke into small pieces that eventually drifted to nothing.

At its best, the A68a was nearly 800 feet thick, though everything except 120 feet was hidden below the waterline.

Ecologists and others had feared that the iceberg during its journey could be sunk near South Georgia. It could have kept the millions of penguins and seals that live and breed there from reaching their feeding areas in the ocean.

It did not happen. New research shows that the A68a drove more past and most likely only hit one element on the seabed briefly as it turned and continued until it broke up.

But the research also revealed another potential threat from the iceberg to the ecosystems around South Georgia. As it traveled through the relatively warm waters of the southern ocean into the southern Atlantic Ocean, it melted from below and eventually released a huge amount of fresh water into the ocean near the island. The influx of so much fresh water can affect plankton and other organisms in the marine food chain.

The researchers, led by Anne Braakmann-Folgmann, a doctoral student at the Center for Polar Observation and Modeling at the University of Leeds in the UK, used satellite images to monitor the shape and location of the iceberg during the journey. (Like other major Antarctic icebergs, it was named after a convention established by the US National Ice Center, which is slightly less flashy than that used for hurricanes.)

The images showed how the area of ​​the iceberg changed over time. The scientists also determined its thickness using data from satellites that measure ice height. When it broke, Mrs Braakmann-Folgmann said the A68a was more than 200 feet thinner overall.

The A68a left its mark. The researchers, whose results were published in the journal Remote Sensing of Environment, estimated that smelting near South Georgia resulted in the release of about 150 billion tons of fresh water. That’s enough to fill an Olympic-size swimming pool 61 million times, the researchers said, even though the ice was already floating, not contributing to rising sea levels.

Not only is the water fresh, not salty, but it also contains a large amount of iron and other nutrients. Mrs Braakmann-Folgmann is helping another group of researchers, from the British Antarctic Survey, who are trying to determine the ecological effects of the iceberg and meltwater.

When the iceberg was near South Georgia, researchers with the study were able to deploy autonomous submarines to take water samples. On the island, they used tracking equipment on some gentoo penguins and fur seals to see if the presence of the iceberg affected their foraging behavior.

Geraint Tarling, a biological oceanographer with the study, said preliminary results from the tracking data showed that the penguins and seals did not change foraging routes as they could have had the iceberg blocked their path or affected their prey.

“At least in the areas of the colonies that we saw, the impacts from the iceberg itself are not as devastating as we first feared,” said Dr. Tarling.

But there is still a lot of data to analyze, suggested Dr. Tarling, especially the water samples. A large influx of fresh water to the surface could affect the growth of phytoplankton at the lower end of the food chain, or it could alter the mix of available phytoplankton species, he said.

The complicated analysis is that 2020, when the iceberg approached South Georgia, also happened to be a bad year for krill, the small crustaceans that are just above phytoplankton in the food chain.

Dr. Tarling said that although the A68a was not grounded, a few other large icebergs have done so in recent decades. Grounding and towing an iceberg can wreak havoc on ecosystems on or near the ocean floor, he said.

And climate change could potentially lead to more grounding episodes. Warming causes parts of the huge Antarctic ice sheets to flow faster toward the ocean, leading to more calving of icebergs, which then travel north.

“What we’re looking at is a lot more movement of icebergs, which can actually erode these areas of the ocean floor,” said Dr. Tarling.

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