Field test stashes climate-warming carbon in deep ocean

Strategically dumping metal puts greenhouse gas away, possibly for good

Iron deliberately dumped into a patch of ocean has triggered a chain of events that pulls carbon dioxide out of the atmosphere and keeps it out. An explosion of microscopic life fertilized by the metal sank to the depths of the ocean after soaking up the greenhouse gas.

CARBON CAPTURER The Chaetoceros atlanticus diatom shown here plucked carbon from the atmosphere in a new experiment and carried the element down to the depths of the ocean. Marina Montresor, SZN, Alfred Wegener Institute

Geoengineering advocates who think iron could be useful for combating climate change will probably be heartened by the new finding, reported in the July 19 Nature. Carbon stuck on the seafloor tends to stay there a long time.

“Every one atom of iron removed 13,000 atoms of carbon” from the air, says Victor Smetacek, a biological oceanographer at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. That carbon probably settled on the seafloor, he says, “like dust collecting in the corner of the room.”

But critics of iron fertilization remain unconvinced.

“This new work will be the poster child for geoengineering,” says Philip Boyd, an ocean biogeochemist at the University of Otago in Dunedin, New Zealand. “But from a scientific standpoint, I don’t think the results are fundamentally a game changer.” In 1990, oceanographer John Martin of Moss Landing Marine Laboratories in California proposed that iron levels limit metabolic activity in today’s oceans. Photosynthetic algae need the metal to make chlorophyll. Iron dust blown from land during the dry conditions of the last Ice Age could account for the lower temperatures, Martin suspected, by spawning algae that sucked heat-trapping carbon dioxide from the atmosphere.

A dozen experiments conducted from research vessels have since used iron to stimulate algal growth in high-latitude waters. But demonstrating that the carbon absorbed by new growth makes it to long-term storage on the seafloor — instead of circulating back into the sky — has been difficult.

In February 2004, Smetacek and his colleagues traveled to a giant whirlpool in the Southern Ocean. Descending currents there tend to keep material trapped, providing a natural container for monitoring the consequences of iron fertilization.

Seeding the water with iron powder stimulated a huge growth of diatoms, algae encased in silica shells. After about 24 days, the bloom began to fade as the creatures died off. Water samples collected at different depths tracked the fate of the remains. Most of the carcasses sank rapidly to at least 1,000 meters below the surface. They carried more than half of the carbon they plucked from the atmosphere with them, the researchers estimate, and presumably deposited it on the ocean bottom.

“This is really the first experiment to see significant carbon removal from the atmosphere,” says Claudia Benitez-Nelson, a marine biogeochemist at the University of South Carolina in Columbia. The new findings contradict those from a 2009 fertilization of waters that lacked the silicon diatoms need to build their skeletons. There, other species of algae that flourished were devoured by tiny grazers before their remains could carry the carbon to the seafloor.

But Benitez-Nelson cautions that the “elegant” result doesn’t necessarily make iron fertilization a viable strategy for fending off climate change. The total carbon removed by the 14 metric tons of iron used in the experiment was minuscule compared with the amount pumped into the atmosphere every year. A surge in diatoms could also have unintended ecological side effects, she says, such as ramping up toxins in the water that poison other creatures. 

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