December 18, 2000

 

GLOBAL DIAGNOSIS COMPLETED OF OCEAN REGIONS

MOST SENSITIVE TO AN IRON-RICH DIET

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.

Images to support this release can be found at:
ftp://www.gsfc.nasa.gov/earthpix/agu/