Could dumping iron in the oceans slow global warming?

After day 24, however, the particulate matter — the remains of the algae that had sucked up the carbon — sank, traveling down from the surface layer, falling to depths between 328 feet (100 meters) to the seafloor, about 12,467 feet (3,800 m) below.

If this organic matter settles into the deep ocean, it may not reach the surface for centuries or millennia, depending on ocean circulation, Smetacek said. 

Much of the former phytoplankton bits are likely to have settled on the seafloor as "fluff" — "like a layer of fluff that you would find under your bed if you did not vacuum it for a long time," Smetacek told LiveScience in an email. "Eventually, this loose matter flattens into the sediments and a part gets buried; this stuff is sequestered for geological time scales." (Geologists measure time in terms of millennia to many millions, even billions, of years.)

His team estimated that for every iron atom they introduced into the eddy, at least 13,000 carbon atoms were taken up into the biomass of the algae, becoming available for export into deeper water. They also found that at least half of the organic matter associated with the bloom — nearly all of it made up of glass-walled diatoms — sank below, 3,280 feet (1,000 m).

Far from proven

In spite of the experiment's success, Smetacek is cautious about the implications for cleaning up human's greenhouse gas emissions.

"It's a very thorny subject," he said. "What we can say here at this stage is that we need to have more experiments (before) we can make any firms statements on that."

Many questions about the feasibility and safety of this approach remain. Buesseler points out that iron fertilization has the potential to stimulate toxic algae blooms; cause the production of nitrous oxide, a more potent greenhouse gas than carbon dioxide; or to suck oxygen out of water as the algae decompose, a phenomenon that is responsible for creating dead zones, like the one found in the Gulf of Mexico.

The approach also has limited potential, since even used on a large scale, it could only remove a fraction of the excess carbon dioxide humans are emitting.

Iron fertilization has another potentially important application, one unrelated to climate change, Smetacek said, suggesting that it may have the potential to restore an ecosystem in the Southern Ocean, where whales once fed on abundant swarms of krill.

In spite of the loss of whales to whaling, their prey, shrimplike krill, have declined dramatically. Smetacek believes this is because the whales played a crucial role in keeping the waters fertilized with iron, which prompted the blooms of phytoplankton, which feed the krill. He has proposed fertilizing a stretch of Antarctic sea ice with iron to see how it affects krill growth.

Follow Wynne Parry on Twitter @Wynne_Parry or LiveScience @livescience. We're also on Facebook & Google+.

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