Can Iron-Enriched Oceans Thwart Global Warming?

John Roach
for National Geographic News
June 9, 2004
Virtually all life in the world's oceans is directly or indirectly dependent on one-celled plants called phytoplankton. These plants, which live at the ocean surface, feed on ocean nutrients to survive.

But about 20 to 30 percent of these crucial nutrients sink out of reach of the phytoplankton each year, according to Jorge Sarmiento, a professor of atmospheric and ocean sciences at Princeton University in New Jersey.

Through his research, Sarmiento is trying to tease out the details of how the ocean supplies the nutrients that phytoplankton need to survive and, in turn, support everything else that makes a living in or off the ocean.

Sinking Nutrients

Sarmiento equates the sinking nutrients in the ocean to leaves falling off trees. Bacteria break down the organic matter of fallen leaves, releasing their nutrients for reuse by plants in the future.

"The big difference in the ocean is that much of the organic matter sinks out of reach of the surface ocean, where there is enough light for photosynthesis," Sarmiento said. "Thus when bacteria break this organic matter down back into dissolved nutrients, the phytoplankton cannot get at the nutrients."

Ken Buesseler is a marine chemist at the Woods Hole Oceanographic Institution in Massachusetts who studies the flow of sinking particles. Buesseler notes that once particles like nutrients sink to the deep ocean, they remain isolated for hundreds to thousands of years.

"Ocean currents determine the mixing rates and how long certain deep waters remain isolated," Buesseler said.

According to Sarmiento's research, ocean currents eventually transport a large fraction of these deep-water nutrients to ocean around Antarctica. There, the nutrients return to the surface and are redistributed throughout the world's oceans.

Iron Hypothesis

Buesseler recently applied his study of the nutrient flow to the so-called iron hypothesis. Some scientists argue that by adding iron to areas of the ocean that are iron deficient, populations of iron-starved phytoplankton would blossom.

In turn, these robust phytoplankton populations would help fight global warming by removing carbon dioxide from the atmosphere via increased photosynthesis. (The process entails plants using energy from the sun to convert carbon dioxide and nutrients into complex organic compounds to form new plant material.)

Some of this carbon sinks to the deep ocean along with other nutrients as phytoplankton die. As a result, some scientists hypothesize that increased phytoplankton would isolate additional carbon in the deep ocean for hundreds or thousands of years.

"These particles carry carbon and other associated elements from the surface to the deep sea," Buesseler said. "If the newly formed carbon—essentially organic matter—were to remain in the surface ocean, marine bacteria would simply consume this organic matter and convert it back to carbon dioxide."

To test the hypothesis that iron-enriched oceans result in more carbon sinking to the deep ocean, international teams of scientists have traveled to the waters around Antarctica and actively added iron to encourage phytoplankton blooms.

Buesseler and his colleagues participating in the experiment studied how much carbon sank to the deep ocean in these iron-enriched areas versus non-iron-enriched areas.

"We measured an increase in carbon flux in the iron-fertilized patch and did not see an equivalent increase in flux outside in the control stations," he said. "However, this increase was not particularly large."

According to Buesseler's calculations, about 900 more tons (816 metric tons) of carbon sank to the deep sea as a result of the experiment. Humans emit about ten million times that amount of carbon into the atmosphere each year through the burning of fossil fuels.

Sink Importance

Buesseler said it is possible to enrich larger areas of the oceans around Antarctica with iron, but that the net result would likely be the removal of only a few percent of the extra carbon in the atmosphere.

However, he has not given up on the idea of using carbon sinks to combat climate change.

"We need to explore ways to decrease inputs of carbon dioxide to the atmosphere and enhance carbon sinks," Buesseler said. "The ocean is one of the sinks and the link between the surface ocean and deep ocean with respect to these sinking particles is one of the most poorly understood areas of ocean science."

A paper by Buesseler and his colleagues describing their research appeared in the April 16 issue of the journal Science.

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