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Africa exports bumper crop of dust
By Sid Perkins
Northwestern Africa has always been a
prodigious source of airborne dust. But
dust supplies really took off when com
mercial agriculture came to the region in
the 19th century, a new analysis suggests.
Climate exerts a strong influence on
African dust emissions: When precipita
tion is lower than normal, atmospheric
concentrations of dust increase. But new
data gleaned from sediments deposited
on the North Atlantic seafloor suggest
that since the 19th century, human
activity — especially agriculture along
the southern fringe of the Sahara, a
region known as the Sahel — has played a
big role too, says Stefan Mulitza, a paleo
climatologist at the University of Bremen
in Germany. He and his colleagues report
their findings in the July 8 Nature.
The researchers analyzed two sedi
ment cores taken from the seafloor
about 30 kilometers from the mouth
of the Senegal River. That material
contains both finegrained, ironrich
sediment carried there by the river and
siliconrich dust blown to the site by pre
vailing winds, Mulitza says.
One core, which measures
more than 5 meters long,
covers the last 3,200 years,
the team’s radiocarbon dat
ing suggests. The other core,
only 43 centimeters long,
provides a highresolution
look at sediment that has
accumulated more recently.
The researchers found
that for most of the last
3,200 years, changes in
Airborne dust has increased since agriculture for
export took off in Africa’s Sahel in the early 1800s.
precipitation were strongly correlated
with dust emissions. But since the early
1800s — when commercial agricul
ture greatly expanded in the western
Sahel— dust emissions have increased
substantially. As early as the 1700s,
Portuguese settlers in Africa began exten
sive farming of maize, which was soon
replaced by millet and sorghum. But
dust emissions really skyrocketed when
farmers began growing groundnuts such
as peanuts during the region’s “cash crop
revolution,” which dates to the 1840s.
The uppermost layers of seafloor
sediment also chronicle an increase
in windblown dust reaching the site
after 1968, the first year of an extended
drought in the Sahel, Mulitza says.
“This is potentially a very important
study,” says Natalie Mahowald, an atmo
spheric scientist at Cornell University.
The new findings back up her team’s
recent models, which show a doubling
of dust emissions from northern Africa
during the 20th century. Those analy
ses suggest that increased dust flux has
substantially affected global climate,
she notes. Determining the modernday
balance of dust emission sources could
help scientists better understand past
climate as well.
Of methane and
microorganisms
Hydrates in northern ocean
basins an ecological concern
By Sid Perkins
Methane released from arctic seafloors
by global warming could cause a cascade
of ecological ills, scientists propose in the
June 28 Geophysical Research Letters.
Worldwide, particularly in deeply
buried permafrost and highlatitude
ocean sediments where pressures are
high and temperatures are below freez
ing, icy deposits called gas hydrates hold
immense amounts of methane (SN:
6/25/05, p. 410). Studies indicate that
sediments beneath parts of the Arctic
Ocean and North Pacific have large res
ervoirs of the planetwarming green
house gas, says Scott M. Elliott, a marine
biogeochemist at Los Alamos National
Laboratory in New Mexico. The climate
warming expected in coming decades
will likely melt or destabilize some of
those hydrates, releasing their trapped
methane, he explains.
Elliot and his colleagues’ new model
shows that the freed methane would
be an unexpected bounty for microbes
that eat the compound, consuming
dissolved oxygen and producing carbon
dioxide in the process. And the waters
downcurrent of a large methane plume
could lose as much as 95 percent of their
oxygen. Ocean acidification result
ing from the increased carbon dioxide
could stifle the growth of phytoplank
ton and other marine organisms. The
added microbial activity would also
deplete key nutrients including nitrate,
copper and iron.
