A23a: Journey of the World’s Largest Iceberg

by YOUxTalks | Dec 16, 2024 | Nature

A23a, a colossal iceberg with an area of 3,800 square kilometers (1,500 square miles), and a thickness of 400 meters (1,312 feet), first broke free from the Antarctic coastline in 1986. Its size makes it more than twice the size of Greater London, and its thickness surpasses the height of Europe’s tallest skyscraper, The London Shard. Initially, A23a became grounded in the Weddell Sea after it calved from the Filchner Ice Shelf, remaining stuck for over three decades due to its deep keel that anchored it to the seafloor.

A Shift in Movement: The Early Struggles and Vortex Trap

The iceberg started moving northward in 2020, but it became trapped again in the spring of 2023 by a rotating water column near the South Orkney Islands. This vortex, known as a Taylor Column, had been first described by physicist Geoffrey Ingram Taylor in the 1920s. It occurs when a current encounters an underwater obstacle, in this case, the Pirie Bank, a 100-kilometer-wide bump on the ocean floor. This created a full-depth rotating mass of water that kept A23a spinning on the spot.

A23a’s behavior during this period highlighted the importance of understanding the ocean floor’s topography. Although the area just north of the South Orkney Islands is relatively well-mapped, much of the world’s seafloor remains unexplored. Only a quarter of Earth’s seafloor has been surveyed to modern standards, leaving large gaps in understanding ocean currents and their interactions with icebergs like A23a.

Breaking Free: A23a’s New Journey North

After several months of remaining stuck in the vortex, A23a has now broken free and is drifting further north. The iceberg’s movement was initially stalled by the vortex, but now it is resuming its journey through the Antarctic Circumpolar Current (ACC), a powerful current that circulates around Antarctica and carries icebergs into the South Atlantic.

This is a common trajectory for icebergs from the Weddell Sea, a region often referred to as “iceberg alley.” Once in the South Atlantic, A23a will encounter warmer waters that could eventually cause it to break apart and melt. This is a typical fate for large icebergs that exit the Southern Ocean.

The Environmental Impact of A23a’s Journey

As A23a continues to drift, scientists are closely monitoring its journey. The iceberg’s movement through the Southern Ocean and into the Atlantic Ocean is important not only for its potential to break up and melt but also for its environmental impact. Icebergs like A23a provide vital nutrients to the surrounding waters as they melt, supporting marine life by fertilizing the ocean with the mineral dust that was trapped in the ice during its formation.

The massive iceberg’s melting could spark ecosystems in otherwise less productive waters, supporting a range of marine organisms. Researchers from the British Antarctic Survey (BAS) and the Woods Hole Oceanographic Institution are studying how these nutrients affect local ecosystems, particularly after A23a’s passage.

A Potential Threat to South Georgia’s Ecosystem

One concern regarding A23a’s movement is its potential to ground at South Georgia Island. If the iceberg’s massive bulk becomes lodged on the island’s shallow continental shelf, it could disrupt the foraging habits of millions of seals, penguins, and other seabirds that rely on the surrounding waters. A23a’s presence could interfere with the animals’ feeding patterns, especially for those breeding on the island, potentially affecting the ecosystem and the food supply for young animals.

Inevitable Melting

While A23a’s journey may still hold surprises, its ultimate fate is clear: it will eventually melt. Over time, all icebergs, no matter their size, eventually decay as they encounter warmer waters. The eventual break-up and melting of A23a is a natural process that contributes to the ocean’s role in distributing heat energy globally.

As it moves northward, A23a’s gradual demise will not only affect marine ecosystems but also serve as a reminder of the dynamic interactions between ice, ocean currents, and the climate system.