Space & Innovation

Iron Fertilization Develops a New Wrinkle

Iron fertilization may shift the ecology of the ocean from a plankton bloom that is better at removing carbon to a bloom dominated by diatoms that are best at removing iron. Continue reading →

Last year, American entrepreneur Russ George dumped a hundred tons of iron sulfate into the Pacific Ocean off Canada. A 3,861 square mile algal bloom formed.

That's geo-engineering. An iron-rich ocean is fertile, and allows colonies of photosynthetic plankton to thrive. George was hoping that the plankton would draw carbon dioxide from the atmosphere and lock it into the ocean as a way to address climate change. It's not the first time an iron fertilization experiment was carried out, but it was the first time it was done with so little supervision.

Independent experimentation is dangerous because we know very little about how iron gets used up in the oceans.

Now, a new study in Nature Communications suggests that the iron could be consumed by diatoms, which are photosynthetic plankton that have beautifully symmetric skeletons made of silica. The critters feed on the iron insatiably and add the element into their shells.

And when the diatoms die, they sink into the ocean, taking the iron with them.

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The loss of iron through diatoms is a natural process in the oceans off Antarctica, the study finds. The loss happens at four times the rate at which new iron gets added into the ocean by dust deposition or the melting of ice.

The implication of this study for iron fertilization experiments is this: the iron we add into the oceans will probably be removed quickly by the diatoms, said Ellery Ingall, the author of the study and a professor in Georgia Tech's College of Sciences, speaking to DNews while vacationing in France. How quickly the the diatoms remove the iron is unknown, but this could well be a new wrinkle in geo-engineering.

Ingall and his colleagues did their study along the coast of Western Antarctica, collecting samples of plankton and ice from an ice breaker in 2008 and 2009. They treated the samples chemically, and then used intense X-ray beams to study the critters. The X-rays were 400-nanometer spots that revealed amazing detail about the elemental chemistry of the diatoms - the presence of iron in this case.

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It is unclear why the diatoms incorporate iron in their shells. Ingall suggested it could be because the element is freely available, which makes the diatoms hog it in order to place other types of plankton at a disadvantage.

"Just like someone walking through a buffet line who takes the last two pieces of cake, even though they know they'll only eat one, they're hogging the food," said Ingall in a statement.

That would mean little iron is available for other types of plankton more efficient at capturing carbon dioxide from the atmosphere. The ecology of the ocean would shift, from a plankton bloom that is better at removing carbon to a bloom dominated by diatoms that are best at removing iron.

These are theories that need further testing, and the lack of knowledge is exactly why George's solo iron fertilization experiment in an important salmon feeding ground earned him a swift rebuke from environmentalists.

Other better monitored experiments have come out with unclear results about the benefits of fertilization. The experiments usually happen in the Southern Ocean, which is one of the largest sinks in the world for anthropogenic carbon dioxide. Every year in the fall, sea ice starts forming around the continent of Antarctica. The water immediately below the layer of ice becomes salty and dense, and it sinks even as waters from the ocean depths, high in nutrients and carbon dioxide, rise to the surface. And when the sea ice melts in the spring, plankton thrive in the nutrient-rich waters and the ocean becomes green-hued.

That's the theory at least. Scientists have been testing out ocean fertilization with varying success. An experiment in 2004 where scientists added iron into the Southern Ocean caused a significant diatom bloom over 24 days. Then the diatoms sank below the surface, suggesting that carbon sequestration is possible.

But in 2009, scientists dropped iron into the same ocean and found that the phytoplankton blooms were quickly eaten up by zooplankton. That limited the size of the bloom, and only a small amount of plankton sank into the ocean, sequestering carbon.

IMAGE: The Swedish ship Oden conducts experiments in the Southern Ocean off Antarctica. (Courtesy Georgia Tech)