Iron Tonic Greens Up the Ocean

The far southern Pacific is a biological desert with a paucity of microscopic life. But all this wasteland needs to transform into an oceanic Garden of Eden is a bit of iron, oceanographers claim. Their latest venture in marine gardening confirms once again that a dearth of iron limits biological productivity in some parts of the ocean and suggests that a natural iron source may have helped pace the comings and goings of the ice ages. Dosing with iron, however, still looks like a bad way to try to tame greenhouse warming.

The late oceanographer John Martin first contended in the 1980s that places like the Southern Ocean lack a nutrient other than nitrogen and phosphorus that microscopic plants need. The limiting factor, he claimed, was iron. Two field experiments have shown that salting the seas with iron solutions causes ocean plant growth to bloom (Science, 11 August 1995, p. 759). Martin further hypothesized that at least some of the new growth would sink. Such a transfer of carbon from the atmosphere down into the deep sea is the key to one scheme for dealing with carbon dioxide from fossil fuels.

In the latest test of iron's ocean-fertilizing powers, researchers directed by Philip Boyd of the University of Otago in Dunedin, New Zealand, dribbled a solution of iron sulfate across an 8-kilometer-diameter patch of ocean 2000 kilometers south-southwest of Hobart, Tasmania. As had happened in the previous experiments, microscopic plant growth took off, tripling the amount of chlorophyll in the water by the time the oceanographers left 13 days later.

The research project--called the Southern Ocean Iron Release Experiment (SOIREE)--didn't turn up any evidence that the new plant growth had sequestered any organic matter below the top 100 meters of the ocean. But SOIREE oceanographer Andrew Watson of the University of East Anglia in Norwich, United Kingdom, believes they would have seen such carbon dioxide sequestration if they had followed the fertilized water long enough. Even so, any attempt to suck down massive amounts of the greenhouse gas into the ocean would be "stupid and irresponsible," Watson says. "There's no way to sequester as much carbon as we're producing."

Watson and his colleagues do see a role for marine plants in influencing climate over the millennia, however. In a computer model, also described in the 12 October issue of Nature, pulses of dust known to deliver iron to iron-poor waters during glacial periods can increase oceanic productivity and sequester additional atmospheric carbon dioxide in the deep sea. This would tend to deepen the glacial chill of ice ages.

This modeling is not liable to settle the question of what controls the millennial waxing and waning of the greenhouse. Oceanographer Robert Toggweiler of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, has his own model that raises and lowers carbon dioxide concentrations with changes in deep-ocean circulation rather than plant life. "It's just hard to do it with biology," he says.

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