Perhaps you remember iceberg A68a, which experienced a few minutes of fame back in 2017 when it broke off an ice shelf on the Antarctic Peninsula. Hardly your ordinary iceberg, it was one of the biggest ever observed, more than 100 miles long and 30 miles wide.
The iceberg floated slowly through the frigid Weddell Sea for a few years, before gathering up steam as it entered the Southern Ocean. When last we heard from it, in 2020, it was bearing down on the island of South Georgia in the South Atlantic, a bit shrunken and bruised from a voyage of more than a thousand miles.
Alas, ol’ A68a is no more. Last year, some 100 miles off South Georgia, it ultimately did what all icebergs inevitably do: thinned so much that it broke up into small fragments and eventually drifted off to nothingness.
In its prime, A68a was nearly 800 feet thick, however all but 120 feet of that was concealed below the waterline.
Ecologists and others had feared that along its journey the iceberg would become grounded near South Georgia. That might have blocked the millions of penguins and seals that live and breed there from reaching their feeding grounds in the ocean.
That didn’t happen. New analysis suggests that A68a performed more of a drive-by and most likely just struck a feature on the seafloor briefly before it pivoted and kept traveling till it broke apart.
But the investigation also identified another potential threat from the iceberg to ecosystems around South Georgia. As it went through the relatively warm waters of the Southern Ocean into the South Atlantic, it melted from underneath, finally releasing a massive quantity of fresh water into the sea near the island. The influx of so much fresh water could impact plankton and other organisms in the marine food chain.
The scientists, led by Anne Braakmann-Folgmann, a doctorate 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 during the length of its journey. (Like other huge Antarctic icebergs, it was called following to a convention set by the U.S. National Ice Center, which is a bit less spectacular than the one used for hurricanes.)
The images showed how the area of the iceberg changed over time. The researchers also measured its thickness using data from satellites that detect ice height. By the time it split up, Ms. Braakmann-Folgmann said, A68a was more than 200 feet thinner overall.
A68a left its imprint. The researchers, whose findings were published in the journal Remote Sensing of Environment, calculated that melting in the region of South Georgia resulted in the discharge of around 150 billion tons of fresh water. That’s enough to fill an Olympic-size swimming pool 61 million times over, the researchers added, although because the ice was already floating its melting did not contribute to sea-level rise.
Not only is the water fresh, not salty, but it also includes a considerable amount of iron and other nutrients. Ms. Braakmann-Folgmann is aiding another group of researchers, from the British Antarctic Survey, who are trying to determine the ecological implications of the iceberg and the meltwater.
When the iceberg was approaching South Georgia, scientists accompanying the survey were able to deploy autonomous underwater gliders to obtain water samples. On the island, they used monitoring devices on some gentoo penguins and fur seals, to investigate whether the presence of the iceberg influenced their foraging behavior.
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Geraint Tarling, a biological oceanographer with the project, stated that preliminary findings from the tracking data suggested that the penguins and seals did not modify foraging routes, as they might have had the iceberg blocked their way or affected their prey.
“At least in the sections of the colonies that we witnessed, the repercussions from the iceberg itself are not as terrible as we first feared,” Dr. Tarling added.
But there is still much data to study, Dr. Tarling noted, especially the water samples. A big influx of fresh water on the surface might disrupt the growth of phytoplankton, at the lower end of the food change, or it could alter the mix of phytoplankton species accessible, he said.
Complicating the analysis is that 2020, when the iceberg was nearing South Georgia, also occurred to be a horrible year for krill, the little crustaceans that sit immediately above phytoplankton in the food chain.
Dr. Tarling added that although A68a did not become grounded, a few other huge icebergs had in recent decades. Grounding and dragging of an iceberg can wreak havoc on ecosystems on or near the seafloor, he said.
And climate change might potentially bring to more grounding events. Warming is driving parts of the massive Antarctic ice sheets to flow faster into the ocean, resulting to more calving of icebergs that then migrate north.
“What we’re looking at is a lot more movement of icebergs that might actually gouge these parts of the sea floor,” Dr. Tarling added.
