The water produced by the melting of the iceberg A23a is not flowing into the sea: this could spell the end of the “Antarctic monster”.
We have already told you about A23a, but now an even stranger phenomenon is happening on the most observed iceberg in the world: the water produced by melting ice is not flowing into the sea, but is accumulating to form a kind of enormous pool. A signal that could anticipate the end of the iceberg.
Special guard. The giant in question is one of the largest tabular icebergs ever observed. Satellite images show a surprising feature: along the entire perimeter of the iceberg runs a sort of raised edge of ice, like a continuous barrier. The result is a giant natural “tub” that traps melt water. The dimensions are impressive: the surface area occupied by the pool reaches approximately 800 square kilometers, approximately two thirds of the surface area of Rome.
The story of the iceberg. In some areas the water appears intensely blue, indicating depths that could reach several meters. Overall, the volume of accumulated water is probably in the billions of liters: enough to fill thousands of Olympic-sized swimming pools. An enormous load, which weighs on an already weakened structure.
A23a is not a young iceberg. It separated from the Filchner-Ronne Ice Shelf in 1986 and was then more than five times larger than it is today. For years it held the record for the largest iceberg on the planet. But its slow journey north, into progressively warmer waters and atmospheres, accelerated its deterioration. Fragmentation now appears inevitable, and the meltwater that stagnates on its surface could represent the final blow.
Could it “explode”? The accumulation of melt water on the surface of icebergs is one of the key mechanisms of instability of floating ice. Liquid water is denser than ice and tends to seep into any fracture. When it penetrates cracks, it applies strong downward pressure, widening them. If the temperatures then drop and the water refreezes, the volume increases and acts like a real explosive wedge. “If this water penetrates the fractures and subsequently refreezes, it can act as a wedge and cause the iceberg to explode from the inside,” explains Mike Meredith, an oceanographer at the British Antarctic Survey.
Frequent phenomenon. A process known as hydrofracturingalready observed in the collapse of ice shelves, such as the Larsen B ice shelf in 2002. According to Douglas MacAyeal, a glaciologist at the University of Chicago, the presence of this “edge effect” is not entirely anomalous for exceptionally large icebergs.
“My hypothesis is that the edges of the iceberg curved downwards, forming a kind of arch dike on the upper surface,” he explains. “This deformation holds the melt water inside.”
The curvature would be the combined result of wave erosion, differential melting, and the natural tendency of tall ice cliffs to bend, even when they appear perfectly vertical. A fragile balance, destined to break sooner or later. Finally, the streaks of water visible from above tell an even older story: they are the traces of the ice streams that flowed when A23a was still part of the Antarctic coastal ice sheet.
Under control. A23a is kept under control by optical and radar satellites (Sentinel, Landsat), because it represents an almost natural laboratory case study: a gigantic, old, structurally weakened iceberg, which enters an advanced phase of surface melting. Any sudden fractures or breaks will provide valuable data to understand how the large Antarctic ice shelves might behave in the future.
Furthermore, the melting of an iceberg of this size releases enormous quantities of cold fresh water, which locally alters the salinity and stratification of the ocean. This can influence regional-scale ocean circulation, nutrient distribution and the productivity of phytoplankton, the basis of the Antarctic food chain. In some cases, the passage and fragmentation of icebergs create real “biological oases”, but the long-term effects remain poorly understood.