Global diagnosis completed of ocean regions most sensitive to an iron-rich diet

December 14, 2000

Adding iron to the diet of marine plant life has been shown in shipboard experiments to boost the amount of carbon-absorbing phytoplankton in certain parts of the world's oceans.

A new study promises to give scientists their first global picture of the extent of these unique "iron-limited" ocean regions, an important step in understanding how the ocean's biology controls the flow of carbon between the atmosphere and the ocean.

The study by researchers at NASA's Goddard Space Flight Center (Greenbelt, Md.) and the Department of Energy's Oak Ridge National Laboratory, will be presented at the American Geophysical Union's annual meeting in San Francisco on December 15.

Oceanic phytoplankton remove nearly as much carbon from the atmosphere each year as all land-based plants. Identifying the location and size of nutrient-limited areas in the open ocean has challenged oceanographers for nearly a century.

"We know where the major iron-limited areas are, but we can't draw a line around the precise geographic extent of these areas," says Goddard's Michael J. Behrenfeld, a co-author of the new study. "This new result may help us do that."

The study pinpointed iron-limited regions by seeing which phytoplankton-rich areas of the world's oceans were also areas that received iron from wind-blown dust. Iron is one of the essential nutrients needed for microscopic marine plant life to flourish, along with nitrogen, phosphorus, and silicate. Where dust from arid regions around the world falls into the ocean depends on the location of the dust source and the rapidly shifting patterns of wind and weather.

The biologically productive ocean regions were identified by images from the SeaWiFS (Sea-viewing Wide Field-of-view Sensor) instrument on the OrbView-2 satellite, which maps global ocean biological activity. Because no similar satellite observations existed for dust-borne iron falling into the ocean, the researchers estimated the location of oceanic dust deposition with a newly improved global dust model. The model calculates where dust travels and falls by identifying the location of the major sources of wind-blown dust around the world and simulating atmospheric circulation patterns.

"The estimates of global dust deposition produced by this model are more accurate than previous ones," says Goddard scientist Paul Ginoux, "because we use a new assessment of dust sources based on TOMS (Total Ozone Mapping Spectrometer) satellite observations. The model also incorporates actual data on global weather patterns, rather than simulated circulation, produced by Goddard's Data Assimilation Office from satellite weather observations."

An annual cycle of average monthly dust deposition maps was produced using three years of simulations up to 1998. These maps were analyzed for a relationship with SeaWiFS 1998 monthly maps of ocean phytoplankton productivity. The areas where the correlation between iron deposition and ocean color were found to be high may indicate iron-limited regions.

To double check their results, the researchers compared their dust maps with ocean color maps from the Japanese ADEOS satellite and found that the regions of high correlation between dust deposition and ocean productivity were essentially the same.

"Global, satellite-based analyses such as this gives us insight into where iron deposition may be limiting ocean biological activity," says lead author David Erickson of Oak Ridge National Laboratory's Computer Science and Mathematics Division. "With this information we will be able to infer how the ocean productivity/iron deposition relationship might shift in response to climate change."

Behrenfeld is leading a series of NASA-supported research cruises to study in more detail iron-limited areas of the ocean. The first cruise was completed this September in the equatorial Pacific Ocean near Hawaii.
A map of the iron-limited regions is available at:

Editor's Note: "The Correlation Between Atmospheric Dust Deposition to the Surface Ocean and SeaWiFS Ocean Color: A Global Satellite-based Analysis," Erickson, D., et al. (Paper #A52G-07), to be presented Friday, Dec. 15 at 3:25 p.m., Moscone Convention Center, Room 123.

Rob Gutro/ Harvey Leifert
AGU Press Room
(Phone: 415/905-1007)

Lynn Chandler
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/614-5562)

Ron Walli
Oak Ridge National Laboratory, Oak Ridge, Tenn.
(Phone: 865/576-0226)

NASA/Goddard Space Flight Center

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