NSF PR 96-46 - September 3, 1996
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Ocean Sediments Contain Record of Past Vegitation
Fires in Africa
May Provide Link to 'Missing'
Carbon
Ocean sediments contain a record of past vegetation
fires, called biomass burning by scientists -- and
this record shows much more past burning, at least
in Africa during the Pleistocene era, than researchers
expected.
National Science Foundation (NSF)-funded marine geologists
David Verardo and William Ruddiman of the University
of Virginia in Charlottesville have found in their
research on ocean sediments the first detailed marine
record of late Pleistocene era fires on land. Their
paper, "Late Pleistocene charcoal in tropical Atlantic
deep-sea sediments: Climatic and geochemical significance,"
appears in the September issue of the journal GEOLOGY.
"Biomass burning, a result of fires started by lightning
strikes, is important for its relationship to climate
(drier climates have more burning) and its immense
effect on vegetation, and therefore land ecosystems,"
says Connie Sancetta, program director in NSF's marine
geology and geophysics program, which funded Verardo's
research along with NSF's climate dynamics program.
The burning of trees and grasses on land produces
charred particles, or charcoal. Charcoal may then
be transported long distances by winds and rivers
to coastal and ocean environments, where it's then
preserved in ocean sediments. Explains Verardo, "Charcoal
may be swept aloft in plumes rising from active fires,
and transported by prevailing winds from the source
area to the ocean within days to weeks of its initial
formation. It eventually sinks and becomes part of
the sediments at the bottom of the sea."
Verardo studied such sediments by analyzing a core
taken from the bottom of the eastern tropical Atlantic
Ocean. "It was full, much to my surprise, of a large
amount of charcoal," he says. "Given the great distance
to land and the regional slope of the sea floor, the
charcoal in this core must have been brought there
by winds." The charcoal is a mixture of particles
from both rainforest and savanna trees, as well as
grasslands. Research on modern-day air mass routes
for the eastern tropical Atlantic region indicates
that the sediment in the core lies below several large-scale
circulation pathways originating in Africa.
Says Sancetta, "Verardo's work is important because
he has uncovered a record of burning that goes beyond
historical records, so we can study natural burning
before human influence." This in turn will help scientists
determine whether human-induced burning has different
effects than natural burning, and how burning is related
to changes in ecology, and perhaps evolution, on longer
time scales. "Another implication of Verardo's work,"
adds Sancetta, "is that if a lot of the carbon [charcoal]
in ocean sediments is terrestrial in origin, then
it isn't marine in origin. And scientists have been
studying the marine record as a major place to find
out about past carbon dioxide changes. If they've
been making the wrong assumptions about how much marine
carbon is involved, then their estimates about carbon
dioxide in the oceans may be wrong."
Current models have not been able to determine exactly
how the total amount of carbon on earth is distributed.
According to Verardo, "This new data sheds much-needed
light on where the 'missing' carbon may lie: in ocean-bottom
sediments laden with charcoal from fires on land."
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