An Extraordinary Iceberg Is Gone, but Not Forgotten

You may remember the A68a iceberg, which enjoyed a few moments of glory in 2017 when it broke away from an ice floe on the Antarctic Peninsula. Barely your everyday iceberg, it was one of the largest ever seen, over 100 miles long and 30 miles wide.

The iceberg drifted slowly through the icy Weddell Sea for a few years, before gaining momentum as it entered the Southern Ocean. Last we heard of it, in 2020, it was heading for the island of South Georgia in the South Atlantic, a bit shrunken and battered by a journey of over a thousand miles.

Alas, the old A68a is no more. Last year, about 100 miles from South Georgia, it finally did what all icebergs do: thinned so much it broke into small pieces that eventually drifted into nothingness.

At its peak, A68a was nearly 800 feet thick, though all but 120 feet was hidden below the waterline.

Conservationists and others had feared that during its journey the iceberg would run aground near South Georgia. This could have prevented the millions of penguins and seals that live and breed there from reaching their feeding grounds in the ocean.

This does not happen. New research shows that the A68a made more than one drive-through and most likely only briefly hit a seafloor feature as it spun and continued until it broke.

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

The scientists, led by Anne Braakmann-Folgmann, a PhD student at the Center for Polar Observation and Modeling at the University of Leeds in Britain, used satellite imagery to monitor the shape and location of the iceberg in the course of his journey. (Like other large Antarctic icebergs, it was named according to a convention established by the US National Ice Center, which is a bit less flashy than that used for hurricanes.)

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

A68a left its mark. The researchers, whose findings were published in the journal Remote Sensing of Environment, estimated that melting around South Georgia had released around 150 billion tonnes of fresh water. That’s enough to fill an Olympic-sized swimming pool 61 million times, the researchers said, although because the ice was already floating, its melting didn’t contribute to sea level rise.

Not only is the water fresh and unsalted, but it also contains a high amount of iron and other nutrients. Ms 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 the melting waters.

When the iceberg was near South Georgia, survey scientists were able to deploy autonomous underwater gliders to collect water samples. On the island, they used tracking devices on some gentoo penguins and fur seals, to see if the presence of the iceberg affected their foraging behavior.

Geraint Tarling, a biological oceanographer on the survey, said preliminary results from tracking data showed penguins and seals were not altering feeding routes because they could have been blocked off the iceberg or affected their prey.

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

But there’s still plenty of data to analyze, Dr Tarling suggested, especially water samples. A large influx of fresh water to the surface could affect phytoplankton growth, at the low end of dietary change, or it could alter the mix of phytoplankton species available, he said.

To complicate the analysis, 2020, when the iceberg was approaching South Georgia, also turned out to be a bad year for krill, the small crustaceans that sit just above phytoplankton in the food chain.

Dr Tarling said that although A68a has not grounded, a few other large icebergs have in recent decades. The grounding and dragging of an iceberg can wreak havoc on ecosystems on or near the seafloor, he said.

And climate change could potentially lead to more stranding episodes. Warming is causing parts of Antarctica’s huge ice sheets to flow faster towards the ocean, leading to more iceberg calvings which then move north.

“What we’re looking for is a lot more iceberg movement that could actually dig into these areas of the seabed,” Dr Tarling said.