GEOENGINEER?: The strand of Chaetoceros atlanticus pictured here can help pull CO2 from the atmosphere and bury it in the sea, given enough iron to bloom. Image: Courtesy of Marina Montresor, SZN / Alfred Wegener Institute
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."