Giant icebergs breaking off from ice sheets in Antarctica — once thought another sign of global warming — may in fact be helping hold climate change at bay as they melt, researchers say.

These giant icebergs, some as large as cities or even small countries — one that split off an Antarctic glacier in 2013 was the size of Singapore — can release nutrients into the ocean waters as they float and melt, triggering huge plankton blooms that can absorb a significant percentage of the carbon in the carbon cycle of the Southern Ocean, they say.

Satellite imagery has proved that as the giant slabs of floating ice melt, they leave behind great swathes of nutrients, in a process known as "ocean fertilization."

Those nutrients include things like iron, scraped up from land as glaciers slowly move across it and end up in the icebergs that split off as the glaciers reach the sea.

The finding came out of analyzing images of 17 examples of the giant icebergs often seen in the Southern Ocean, researchers report in the journal Nature Geoscience.

"It was a big surprise," says research team member Grant Bigg, an oceanography expert at the University of Sheffield in Britain.

"When you look at the giant icebergs, the influence of these meltwater nutrients is actually four to 10 times as large as we would have expected from looking at the ordinary sized icebergs," he says.

The nutrient-rich plume of the icebergs, some of which can be more than 10 miles across, can stretch for more than 600 miles, the researchers say.

The floating environments of tiny marine organisms created by the icebergs absorb an estimated 10 to 40 million tons of carbon annually, they report.

Those plankton blooms can last for more than a month following the passage through an area of a giant iceberg, they say.

The photosynthesis process, which plankton use in order to grow and reproduce, includes the absorption of carbon dioxide. When the plankton dies, it sinks to the floor of the ocean, effectively locking away all the carbon that was absorbed.

"We estimated that somewhere between five and 10 percent of all the carbon, which is exported from the surface waters of the Southern Ocean down to the deep ocean, ought to come from these iceberg plumes fertilizing the water and the phytoplankton growing and dying as a result," Bigg says.

Around 3,000 giant icebergs are floating around the Southern Ocean at any one time, the researchers point out, constantly calving off the ice sheets and glaciers of Antarctica.

"If giant iceberg calving increases this century as expected, this negative feedback on the carbon cycle may become more important than we previously thought," Bigg says.

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