No idle drifter. Data-gathering robots like this one have found evidence that carbon taken up by phytoplankton at the surface gets transported to the depths of the ocean.

Carbon Sinks, But Does It Stay?

The debate over adding iron to the ocean to curb rising atmospheric carbon dioxide levels heated up this week, as an ambitious experiment in the Southern Ocean suggested that the ocean can take up more carbon dioxide than previously thought. But scientists caution that it's too soon to be sure.

The Southern Ocean contains vast areas of nutrient-rich water with few phytoplankton--tiny plants that gobble up carbon dioxide during photosynthesis. Previous experiments showed that a lack of iron was the likely limiting factor, and that adding iron to these areas caused the phytoplankton to bloom. Researchers proposed that "fertilizing" the areas with iron would trigger massive blooms that would suck carbon dioxide from the atmosphere, helping to brake global warming.

But there was a wrinkle. Scientists thought that only larger phytoplankton, known as diatoms, would sink to the deep ocean, taking the carbon with them and keeping it from returning to the atmosphere. Diatoms are scarce in large parts of the Southern Ocean, because the water is low in silica, a key ingredient they need to grow. That seemed to limit the areas that looked promising for iron fertilization.

The new experiments suggest this worry over silica was unfounded. Oceanographer Kenneth Coale of the Moss Landing Marine Laboratories in California and colleagues added iron to two 225-square-kilometer patches in the Southern Ocean, one with a lot of silica, and one without. Surprisingly, they found that both patches bloomed and took up carbon. In the low-silica area, smaller phytoplankton clumped together and sank, much as the diatoms in the high-silica area did. That suggests that much more of the ocean could take up carbon and bolsters the idea that increased iron levels in ancient oceans may have helped drive ice ages.

The experiment, known as the Southern Ocean Iron Experiment (SOFeX), also found that in both the high- and low-silica patches, fertilization boosted the concentration of organic matter 100 meters below the surface. That's an encouraging sign that the dying diatoms had locked up carbon at the surface and were carrying it with them to their watery grave.

Another surprising result was evidence that nitrogen, an essential ingredient for phytoplankton growth, escapes from the sinking particles and remains near the surface, where it can help spur the growth of more phytoplankton. “This could increase dramatically our estimation of the capacity of the Southern Ocean to store carbon,” says Coale. The research is reported in three papers published online 15 April by Science.

Although the results appear to be good news for proponents of iron fertilization, the ultimate fate of the carbon is still unknown, says biogeochemist Jorge Sarmiento of Princeton University. His computer models suggest that the effects of fertilization may be too short-lived to make much of a dent in the atmosphere's carbon dioxide concentration.

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