A Way to Trap Carbon Deep in the Ocean
2012-07-19 06:56 by Anja Reitz
By adding iron to the ocean, the researchers induced a massive algal bloom. A significant portion died and sank to the ocean floor, carrying its sequestered carbon along with it to the depths. The finding contributes to science’s understanding of the global carbon cycle and has implications for potential ways of mitigating rising levels of carbon dioxide, which contributes to climate change.
“Phytoplankton blooms are all over, but their fate was not previously known,” said Victor Smetacek,
a biological oceanographer at the Alfred-Wegener Institute for Polar
and Marine Research in Bremerhaven, Germany. “While the experiment was
going on, we saw the stocks start to sink — they went down very fast,”
he said. “I was very excited to see this happening.”
Analyses of ice cores have shown correlations between lower levels of
atmospheric carbon dioxide and higher concentrations of iron. And past
experiments have shown that adding iron, a limiting but essential
nutrient for ocean life, to areas of the ocean otherwise rich in
nutrients generates phytoplankton blooms.
But ocean currents and hungry phytoplankton predators had confounded previous efforts to study whether any of the algae sinks to the ocean floor.
To work around those challenges, the researchers identified a 40-mile-wide eddy, or swirling column of self-enclosed water that excludes surrounding ocean currents, for carrying out their experiment. They added several tons of iron – 14 tons of iron, or ferrous sulfate — to the water.
Because the eddy acts like a “giant test tube” or tornado, Dr. Smetacek and his colleagues were able to compare the iron-rich waters with surrounding areas that had an iron deficit.
The team followed the phytoplankton’s development for five weeks. Diatoms, a type of phytoplankton that builds hard external silica shells, dominated the bloom, peaking around the fourth week of the experiment.
“Diatoms are like the thistles of the ocean — they’re nobody’s idea of a good meal,” Dr. Smetacek said. (Diatom does have a natural predator, krill, but it was largely absent in the area.) Thanks to the diatoms’ unpalatable shells, the researchers did not have to worry about predators’ gobbling them up before the organisms died naturally.
As the scientists recount in an article in Nature, the algal bloom extended to 100 meters (328 feet) deep in the ocean, showing that even in very deep layers, intermittent light allows the organisms to flourish. “We could see the bloom developing and increasing in size like a big cloud,” Dr. Smetacek said.
The diatoms grouped together to form slimy masses, which soon died and began sinking. The researchers showed that more than 50 percent of the algae sank below 1,000 meters.
There, the organisms’ carbon content could probably be stored in seafloor sediments and in deep-water columns for centuries, the authors concluded. Dr. Smetacek suggests that the new results would apply anywhere in the ocean where high levels of silica are present to allow diatoms to bloom.
Of course, the ocean’s capacity for carbon sequestration would mitigate only a fraction of the world’s current annual carbon dioxide emissions, Dr. Smetacek said. Still, it could eventually be a useful geoengineering technique for alleviating climate change, he suggested.
“Even if we stop all emissions tomorrow, we still have to remove the excess CO2 from the atmosphere,” he said.
But Dr. Smetacek said he did not favor large-scale ocean iron dumping without further testing. “We need to keep our options open, but first we must have more research to see if this is viable, and what the side effects there are,” he said.
Source: The New York Times, by Rachel Nuwer,