Impact of a Supergiant Iceberg on the Surface of the Surrounding Ocean of Antarctica

The melting of the supergiant iceberg A-68 had a huge impact on the ocean around South Georgia in the sub-Antarctic, significantly changing the temperature and salinity of the Southern Ocean, with potentially important consequences for this ecologically important region.

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The melting of the supergiant iceberg A-68 had a huge impact on the ocean around South Georgia in sub-Antarctica. Credit: Povl Abrahamsen, BAS


In 2020, A-68, a Luxembourg-sized supergiant iceberg that broke off from the Larsen C ice shelf in 2017, drifted very close to South Georgia before starting to fragment, releasing huge amounts of fresh and cold meltwater in a relatively small amount of region.

Researchers from the British Antarctica Survey and the University of Sheffield used satellite data to observe how the melting of the iceberg affected the temperature and salinity, from the top first centimeters of the ocean surface. The results have been published in the journal Geophysical Research Letters.

Environmental changes in the melting of the megaiceberg

They observed that the meltwater caused extreme anomalies in the temperature and salinity of the waters on the surface of the ocean, demagnitudes that have not been reported so far in any previous disintegration of icebergs.

The researchers recorded temperatures up to 4.5°C cooler than the average.The salinity was reduced by more than 10 psu (a way of measuring the amount of salt in the water), which means that the ocean surface reached about two-thirds of its normal salinity.

This "signal" of the melted iceberg finally spread far beyond South Georgia; it was transported by ocean currents to form a long column that stretched more than 1,000 kilometers across the South Atlantic, approximately the distance from Land's End to John O'Groats.

The sign of the meltwater also took a long time to disappear: it was still visible more than two months after the iceberg disintegrated.

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Image of iceberg A68 near South Georgia on 14.12.20. Credit: NASA MODIS

Impacts of melting icebergs

These changes in the physical conditions of the ocean surface have important links with biological conditions. For example, meltwater contains dissolved iron that stimulates the growth of microscopic plants called phytoplankton that form the basis of the ocean's food web.

However, this melting water can also have negative impacts, altering the temperature, salinity and nutrient conditions to which many of the inhabitants of the Southern Ocean are adapted to thrive.

The detachment of this huge iceberg created a unique opportunity to evaluate the impact of the melting on the conditions of the ocean surface.

Climate scientist Roseanne Smith of the British Antarctica Survey and co-author of the study said: "The A-68 iceberg was one of the largest and most studied icebergs. When it began to break, it was like installing a large iceberg in the estuary of the river towards the open ocean, pumping fresh and cold water into the surface layer.

"Our results show that each giant iceberg that melts can have widespread and lasting impacts on the surface conditions of the Southern Ocean, and this has consequences for the plant and animal life that lives there."

Professor Emeritus Grant Bigg, of the University of Sheffield, said: "This is the largest iceberg whose impact on the Southern Ocean has been tracked throughout its life, this impact being widespread and lasting. We know that it collided with the continental shelf in front of South Georgia, so it will have left long-term scars on the seabed there, as well as the significant cooling and cooling of the ocean that will probably change the surface flows of the local ocean.

"Climate change is likely to cause more landslides of giant icebergs in the future; it is important to monitor them so that future impacts on ocean circulation, biology and geology of the seabed can be evaluated and predicted."

Almost half of the lost mass of the Antarctic ice sheet comes from detached icebergs: they release enormous amounts of fresh and cold water as they move and break, which affects the temperature and salinity of the Southern Ocean. These changes have far-reaching impacts, such as affecting the growth of sea ice and local currents, and they also release nutrients and iron into the water, stimulating primary production.

Supergiant icebergs, such as the A-68, may become more common in the future as the climate warms up, causing the collapse of the Antarctic ice shelves.

Reference

R. M. Smith et al, Impact of Giant Iceberg A68A on the Physical Conditions of the Surface South Atlantic, Derived Using Remote Sensing, Geophysical Research Letters (2023). DOI: 10.1029/2023GL104028