At various points in Earth's history, dust fell into the ocean and fed algae, which gobbled up carbon dioxide and sank to the bottom of the sea, taking greenhouse gas with them and cooling the world. That's a key conclusion scientists are drawing from an unusual 2004 experiment in which they grew a massive algae bloom in the Southern Ocean. Data from the experiment may also tell researchers whether seeding the seas with iron is a good way to curb global warming.
Before the 2004 study, known as EIFEX, the European Iron Fertilization Experiment, scientists had conducted 11 experiments at sea to explore how trace quantities of iron may encourage the growth of algae. Those projects had proven the first half of the so-called iron hypothesis: namely that windblown dust from land provided the trace nutrient of iron to catalyze the growth of massive algae blooms in the ancient ocean.
But no one had effectively confirmed the second half of the hypothesis that through photosynthesis, the carbon dioxide from the ancient atmosphere was absorbed into the cells of the algae in those blooms, and when they died or were eaten, that carbon sank deep into the ocean. The resulting lower atmospheric CO2, the argument goes, would mean lower temperatures, suggesting that the mechanism was at least partially responsible for triggering past ice ages.
"The source and sink of carbon from glacial to interglacial periods is the holy grail of oceanography," says oceanographer Victor Smetacek of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, who led the EIFEX expedition and was the lead author on a paper about it published online today in Nature. "It still hasn't been found, [but] with this paper we are showing that this is probably the place to look."
Experiments in the open ocean are by nature logistically difficult to perform, but EIFEX was particularly grueling. To grow the massive bloom, which swelled to 800 square kilometers, Smetacek and his team used satellite imagery to identify a 100-kilometer-wide whirlpool, known as an eddy. Within this feature, amounting to a natural beaker, the scientists released 14 tons of iron sulfate dissolved in seawater. The nutrient catalyzed the growth of a bloom that within 2 weeks was visible by satellite. Over the course of the 37-day experiment, aboard the German research vessel Polarstern, the scientists continually steamed in and out of the bloom to take measurements, weathering storms and rolling seas at 49° southward of Antarctica—just between the famed latitudes known as the Roaring Forties and the Screaming Fifties.
As the bloom died and zooplankton devoured it, the researchers were able to track the sinking of waste particles beneath the surface all the way to the ocean floor. Known as "marine snow," the particles were roughly 80% slime or mucus—what remains after algae cells die—15% living algae, and 5% fecal pellets from zooplankton which had eaten the algae. In all, at least half of the total biomass of the bloom sank below a depth of 1000 meters, presumably sequestering that carbon from the atmosphere for centuries.
Haggling over that crucial amount of flux is why the paper took so long to appear, says Smetacek, but oceanographer Ken Buesseler of the Woods Hole Oceanographic Institute in Massachusetts lauds the detailed calculations in an accompanying commentary in Nature, adding that the study "was similar to natural" algal blooms.
The EIFEX paper is "a careful scientific study" that has "refined our understanding of biogeochemical processes that influence climate," adds John Cullen, an oceanographer with Dalhousie University in Halifax, Canada. But its confinement to an eddy and the use of iron sulfate instead of natural iron-bearing dust make it hard to know "how this experimentally induced bloom reflects natural processes." To find that out, he says, future longer, larger-scale experiments, perhaps using natural dust, are required.
Some scientists have proposed seeding the ocean with iron to grow algae, which would capture carbon dioxide and thus help curb global warming—part of a suite of ideas known as geoengineering. Smetacek and Buesseler say experiments like EIFEX, performed on a larger scale could reveal whether this is a valid strategy. Cullen, however, has warned that such projects cannot resolve key objections to mass scale fertilization for geoengineering.
Still, says Smetacek, "We have to get our act together and propose such experiments." But he acknowledges that governments, wary of controversy, have shied away from funding further ocean fertilization projects, and he's skeptical of corporate efforts to support them, fearing a lack of scientific objectivity.