Fertilizing the ocean with iron could help reduce atmospheric carbon dioxide levels, according to newly released findings of a research cruise. Why? In a word, diatoms.

A hunger for iron rules the microscopic sea life of the Southern Ocean surrounding ice-covered Antarctica. Cut off from most continental dirt and dust, the plankton, diatoms and other life that make up the broad bottom of the food chain there can't get enough iron to grow. And that's why some scientists think that artificially fertilizing such waters with the metal could promote blooms that suck CO2 out of the air. Then, when these microscopic creatures die, they would sink to the bottom of the ocean and take the carbon with them.

Such blooms occur naturally, of course, so the first part of the hypothesis is not controversial. What remained questionable until now is whether such blooms in fact sequestered much carbon or if it was being quickly recycled back into the atmosphere. The problem for scientists is that oceanic waters tend to mix, which makes monitoring and delineating an experiment in the ocean challenging.

The solution, devised by biological oceanographer Victor Smetacek of the Alfred Wegener Institute for Polar and Marine Research in Germany and his colleagues, was to use an eddy. Such swirling currents can be remarkably self-contained. In fact, the new research to be published in Nature on July 19 shows that less than 10 percent of the eddy's waters mixed with the surrounding ocean. (Scientific American is part of Nature Publishing Group.)

With such ideal conditions, the group dissolved seven metric tons of iron sulfate in acidic seawater and spewed the solution into the ship's propeller wash starting on February 13, 2004, covering a circular patch in the eddy of some 167 square kilometers. That's the equivalent of adding 0.01 gram of iron per square meter, levels similar to those found in the wake of a melting iceberg. They then monitored the fate of the patch off and on for five weeks, while also adding supplemental iron fertilizer after two weeks to keep concentrations high enough to promote growth.

As expected, microscopic sea life bloomed. Chaetoceros atlanticus, Corethron pennatum, Thalassiothrix antarcticus and nine other species of diatoms grew in abundance, boosting the amounts of chlorophyll, organic carbon and other signs of life in the waters to depths of as much as 100 meters beneath the surface.

By the middle of the third week after the researchers stopped adding iron, the bloom began to die. So many diatoms died, in fact, that they overwhelmed any natural systems for decay and fell in large numbers below 500 meters in depth. At least half of the total bloom biomass sank below 3,000 meters, according to the scientists' calculations. Fresh diatom cell corpses littered the seafloor as well, and the research team believes that much of the bloom ended at the bottom as a layer of fluff. "Since the aggregates sank so rapidly and the water column was more or less 'empty' on day 50, they must have settled out," Smetacek argues. "Layers of fluff have been reported from various regions, including the Southern Ocean."

The results offer fresh hope to would-be geoengineers hoping to draw down ever-increasing concentrations of industrial CO2 in the atmosphere, such as the ill-fated company Planktos and its failed bid to fertilize the ocean off Ecuador with iron. This new experiment induced carbon to fall 34 times as fast as natural rates for nearly two weeks—the highest such rate ever observed outside the laboratory. As the deceased oceanographer John Martin of Moss Landing Marine Observatories in California famously said in 1988: "Give me half a tanker of iron, and I'll give you the next ice age."

But such fallen carbon only resides in the deep for a few centuries at best. Eventually, it makes its way back to the surface as the ocean's bottom water circulates and rises anew near the equator (although carbon buried in sediment might stay buried longer). And such techniques might be capable, at best, of sequestering one billion metric tons of carbon dioxide per year (based on the extent of iron-deficient waters around the globe), compared with annual human emissions of more than eight billion metric tons and rising. "There is massive uncertainty in this figure, and until much more research is done no serious scientist should express any confidence in such estimates," of iron fertilization's geoengineering potential, cautions oceanographer Richard Lampitt of the National Oceanography Center in England, who also argues that more research into such potential geoengineering techniques is needed due to the failure of global efforts to curb greenhouse gas emissions.

One key to the whole experiment's success turns out to be the specific diatoms involved, which use silicon to make their shells and tend to form long strands of cellular slime after their demise that falls quickly to the seafloor. A similar cruise and experiment in 2009 failed despite dumping even more iron fertilizer over an even larger area of the Southern Ocean. The eddy chosen for that experiment lacked enough silicon to prompt these particular diatoms to grow. Instead, the experiment yielded bloom of algae, which was readily and rapidly eaten by microscopic grazers. As a result, the CO2 in the algal bloom returned to the atmosphere.

In fact, these iron-seeding experiments could backfire by producing toxic algal blooms or oxygen-depleted "dead zones," such as the one created in the over-fertilized waters at the mouth of the Mississippi River. At present, scientists have no way to ensure that the desired species of silica-shelled diatoms bloom. In short, Smetacek says, the type of bloom—and therefore the ability to sequester CO2—"cannot be controlled at this stage."