Study Challenges Idea of Seeding Oceans With Iron to Curb Global Warming

By Hillary Mayell
for National Geographic News
January 8, 2001
Two researchers may have put the final stake in the heart of the so-
called Geritol solution to global warming, a proposal that has intrigued
the scientific community for more than a decade.

The idea, which
got its name from a tonic touted to treat the effects of iron-poor
blood, asserts that seeding the oceans with iron will dramatically
increase levels of phytoplankton and therefore draw more carbon dioxide
from the atmosphere.

Evidence from ice cores shows that huge blooms of phytoplankton—the microscopic algae that are the basis of the marine food chain—occurred in the waters of the Southern Ocean during peaks in ice ages.

A decade ago, a scientist named John Martin proposed a theory known as the "Iron Hypothesis," which attributed the sudden growth spurts of phytoplankton to an increase in the amount of iron in the sea. Iron acts as a fertilizer for plants.

The theory was borne out by experiments. When scientists seeded small areas of the ocean with iron, big phytoplankton blooms occurred.

But where did the additional iron that fueled the historic pattern of growth spurts in phytoplankton [production] come from? Martin suggested the iron came mainly from wind-swept dust from land that was carried out to sea and deposited into the oceans.

Now, in a study published in the December issue of Paleoceanography, Gabriel Filippelli and Jennifer Latimer challenge that idea. They suggest that the increased amounts of iron were delivered predominantly from deep ocean waters that rose from below—a scenario they call the "Upwelled Iron Hypothesis."

"We looked at Antarctic ice cores and also in ocean cores to test the idea of higher productivity during ice ages," said Filippelli, a paleo-oceanographer and associate director of the Center for Earth and Environmental Sciences at Indiana University–Purdue University, Indianapolis (IUPUI). "We found that yes, phytoplankton productivity is higher, and yes, iron content is higher. But the iron content is far in excess—about ten times higher—than what could be delivered by dust."

The "Geritol Solution"

"Phytoplankton grows on a seasonal basis in the oceans around the world," said Filippelli. "Over longer time scales, we know that ocean plant life was at its most robust in the Southern Ocean during glacial intervals—for example, about 150,000 years ago, and then again 20,000 years ago. The question has been, what caused the increases in productivity?"

Like terrestrial plants, phytoplankton engage in photosynthesis, using sunlight as an energy source to combine water molecules and carbon dioxide and convert them to plant food.

The phytoplankton use carbon dioxide from the atmosphere; therefore, the larger the amount of phytoplankton in the world's oceans, the more carbon dioxide is being drawn from the atmosphere.

Carbon dioxide levels in the atmosphere have been climbing steadily, from 200 parts per million (ppm) 18,000 years ago to a pre-industrial level of 280 ppm. Today the figure is around 370 ppm.

The rapid increase in atmospheric carbon dioxide is almost entirely attributable to human activities. The increase concerns scientists because of the likelihood that the trapped gases in the atmosphere will create a "greenhouse" effect, increasing average global temperatures.

There are basically only two ways to decrease carbon dioxide levels: reduce emissions or create more carbon sinks.

The "Geritol solution" suggests that actively fertilizing the Southern Ocean with massive amounts of iron would spur the growth of phytoplankton, thus creating a giant carbon sink.

Upwelled Iron Hypothesis

Filippelli and Latimer, a doctoral student at IUPUI, argue that the excessive iron that spurred major phytoplankton blooms in the past is not the result of an external factor—dust—but comes from inside the ocean itself.

During ice age intervals, glaciers recede and much more land is exposed. Rivers end up dumping the silt, mud, and clay they carry directly into the ocean rather than it being filtered by deltas. As a result, the "dirt load" in the ocean increases significantly, the two researchers say.

"About 80 percent of the sediment that would be deposited on the continental margins during a glacial period goes right into the sea," said Filippelli. This sediment is swept along in ocean currents that begin in Greenland and continue down through the North Atlantic, staying in the deep sea for hundreds of years.

A problem with the Geritol solution, said Filippelli, is that "the ocean strips away iron really quickly."

Experiments in which dissolved iron was dumped on the surface of the ocean showed that the iron was concentrated in the region only a week or two before circulation caused it to disburse through dilution or downwelling (sinking from the light-filled surface ocean), Filippelli explained. "It would take massive amounts of iron to sustain bloom productivity," he said.

He noted, however, that several patent applications are pending for delivery systems that would keep iron added to the ocean from being disbursed so widely that it would fall to concentrations too low to spur phytoplankton growth.

The Geritol solution worries environmentalists. "We just don't know the long-term implications of deliberately altering" such a massive ecosystem, Filippelli argued.

"There's very little regulation right now of what you can do in the open ocean," he said, "and as long as you have people hoping to make a profit off of creating a carbon sink, you have the potential for a problem."

If the source of the iron is silt carried in the huge hemipelagic load created by the ending of an ice age, and it is being delivered by waters upwelling from the deep ocean, "the Geritol solution is just not going to work," Filippelli said.

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