Great excitement among scientists when a monstrous iceberg was born off the coast of Antarctica in 2017. The iceberg floats around for years and finally only finds its Waterloo in 2020. And now the demise of the mighty iceberg also turns out to be (or was) quite exciting.
Because in a new study – published in the magazine Geophysical Research Letters – scientists write that the iceberg – while it was sinking – had a huge impact on its immediate environment. For example, the iceberg appears to have single-handedly caused the surface temperature of the Southern Ocean to decrease locally by as much as 4.5 degrees. And the salinity also fell by more than 60 percent due to the melting iceberg. These are big changes; greater than any other change scientists have ever observed in response to a melting iceberg.
Like a estuary in the open sea
“The A68 iceberg was one of the largest and best-studied icebergs ever,” says researcher Roseanne Smith. “When it started to break up, it was a bit like a huge estuary had been installed in the open ocean, pumping cold, fresh water into the upper layer of the ocean.”
Over A68
The iceberg that is the focus of Smith and colleagues’ research was formed in 2017, when a huge chunk of ice broke off from the Larsen C ice shelf off the coast of Antarctica. At first, the resulting iceberg was about the size of the province of Gelderland. However, as time passed, more and more pieces of ice broke off from the iceberg. And in 2020 it was still about the size of Luxembourg. Around the same time, A68 had also covered a considerable distance – while floating – and was heading for the virtually uninhabited island of South Georgia. And there, that same year, the iceberg begins to break apart, releasing – as it melts – enormous amounts of cold, fresh water into a relatively small part of the ocean. Scientists are on top of it; Using satellites, they observe the melting iceberg and how it locally affects the upper centimeters of the Southern Ocean. It results in the discovery of unprecedented, local anomalies in ocean temperature and salinity, the paper said Geophysical Research Letters to read.
The journey of A68, from Antarctica to South Georgia. Image: ESA.
Fytoplankton
It naturally raises the question to what extent these changes have also affected the residents of those waters. That’s difficult to say at the moment, Smith says Scientias.nl. “A68 has probably had the greatest direct impact on the lower reaches of the food chain, i.e. on microalgae or phytoplankton – which live at the surface of the ocean. Each species is adapted to specific conditions, including a certain water temperature and a certain salinity. So a change in those conditions – such as occurs when a huge iceberg dumps cold, fresh water nearby – is likely to affect a wide range of different phytoplankton species in this area.” If you look at it this way, the melting iceberg seems like bad news for phytoplankton – and possibly also the organisms higher up in the food chain that (largely) depend on phytoplankton for their survival.
Complex story
But it’s a lot more complicated than that, Smith says. Because such a melting iceberg can indeed make life difficult for phytoplankton by temporarily changing the temperature and salinity of their old familiar ocean. But at the same time, the iceberg can also promote the growth of (other) phytoplankton species. “Iceberg meltwater is rich in dissolved iron, and iron is something the Southern Ocean generally lacks. So icebergs can ‘fertilize’ the ocean, so to speak.” And thus stimulate the growth of phytoplankton. “So looking at the giant iceberg A68 and its melting and collapse off the coast of South Georgia in the summer of 2020/2021, was that event good news for algae? That’s difficult to say. A lot of phytoplankton already grows naturally in the waters around South Georgia in the summer. For now, however, satellite data tentatively suggests that phytoplankton increased in some areas and decreased slightly in others.”
At the same time, it cannot currently be ruled out that the melting iceberg has treated the surrounding ocean not only with iron, but also with phytoplankton. “It is possible that the iceberg itself also harbored phytoplankton species that are not normally found this far north,” says Smith. “Those species could have escaped during the iceberg melting and started growing in the waters around South Georgia because they like the cold, sweet meltwater that was released around them.” It is something that researchers have seen happen in previous studies of other (smaller) melting icebergs. But it has not yet been proven whether A68 also carried phytoplankton. “We simply don’t have enough evidence to say now that that is the case. We also cannot yet say whether the net effect of the iceberg on the South Georgia ecosystem was positive or negative. Studies on this will be published in the coming months and years,” Smith expects. “Because during that time (when the iceberg melted, ed.) many water samples were collected and they are still being examined.” So to be continued.
Huge distance
What we do know for sure now is that the impact of the iceberg also extended beyond the immediate vicinity of South Georgia. The cold, fresh meltwater was carried away from the island by currents in an easterly direction. And not such a short distance either. “What surprised me most was the enormous distance over which the meltwater plume from A68 was spread,” says Smith. “Before A68, we had already determined that temperature drops and changes in the salinity of the water at the ocean’s surface could still be detected even tens of kilometers away from the edge of the iceberg. This is because the meltwater is carried away by local currents. But in the case of A68 it was a completely different story: the plume consisting of cool, fresh meltwater was carried away over an enormous distance. At one point this plume was even more than 1000 kilometers long! That’s about the distance between Paris and Vienna!”
Hero role for satellites
The only reason researchers were even able to determine the enormous distance over which the meltwater was spread is because they used satellite data. “Most previous studies of meltwater plumes from icebergs used sensors on ships or buoys, but you would need a lot of ships and a lot of time (and therefore a lot of money) to measure the size of the meltwater plume from icebergs in this way. A68 to measure. Only thanks to satellite data have we managed to determine its full length and distribution over a period of several months.”
The fact that the meltwater was carried so far also means that any impact the meltwater may have had on phytoplankton near South Georgia was at least temporary. “It is unlikely that the impact of the meltwater was felt by phytoplankton after the winter of 2021,” says Smith. “The iceberg had completely melted away in April 2021 (i.e. during the autumn in the Southern Hemisphere, ed.) and the remaining cold, fresh meltwater had already been carried away from South Georgia by currents. By spring, everything around South Georgia was probably back to normal.”
Grooves in the seabed
Is there nothing left in 2023 that reminds us of the origins and demise of the mighty iceberg A68? Certainly! “To do that, we have to dive beneath the rough, stormy surface of the ocean, toward the bottom around South Georgia,” says Smith. “It is common for large icebergs to scrape the bottom of the seabed as they move through relatively shallow coastal waters.” It results in permanent grooves in the seabed. It also affects everything that lives on the seabed. “When icebergs scrape the seabed, they bulldoze the fragile ecosystem on that seabed and can result in the death of more than 98 percent of local animals and plants. It can then also take a long time (of the order of years) for that ecosystem to recover.” South Georgia has seen many icebergs travel through its coastal waters, so the seabed is home to many grooves that provide lasting evidence of this. “A68 is believed to have struck the seabed south of South Georgia in mid-December,” Smith said. And this means that life on the seabed may be so affected that it will take some time to recover. And in that scenario, the seabed itself will in any case be permanently a few grooves richer.
The research into the impact that monstrous icebergs have on the local ocean and ecosystems is important. Because it is expected that we will see many more of these large icebergs forming in the near future. “Climate models show that further warming of the atmosphere and ocean due to greenhouse gases affects the stability of the many ice shelves around Antarctica. If those ice shelves break up, they could release many large icebergs into the Southern Ocean in a short period of time,” Smith explains. “More icebergs also means more iron-enriched freshwater reaches the Southern Ocean. And that results in decreasing salinity and more stratification in the upper parts of the ocean, in other words: those upper layers become more difficult to mix with deeper water layers.” The latter has everything to do with the fact that fresh water has a lower density than salt water and therefore sinks less easily. “It can have different effects,” Smith continues. For example, the stratification of the water makes it more difficult for surface water to sink to great depths. “And that’s important, because the sinking of that surface water helps store carbon and heat in the deeper parts of the ocean.” In addition, the meltwater can also influence the growth of sea ice, the researchers expect. “And that’s important because sea ice helps to reflect the sun’s energy, so less sea ice means more solar energy (and therefore heat) is absorbed (and vice versa).” And finally, icebergs can also leave their mark on local ecosystems in various ways. “Which of these effects will be the most dominant is difficult to say,” Smith acknowledges. “But if the frequency of formation of large icebergs increases substantially, the temporary influences of each individual iceberg may stack up and result in larger, longer-lasting and more widespread changes in the Southern Ocean, with implications for marine ice, the growth of phytoplankton and the sinking of surface water.”
2023-10-04 06:34:45
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