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April
27, 2007 A major study sheds
new light on the role of carbon dioxide
once it's transported to the oceans' depths. The research indicates
that instead
of sinking, carbon dioxide is often consumed by animals and bacteria
and
recycled in the "twilight zone," a dimly lit area 100 to 1,000 meters
below the surface. Because the carbon often never reaches the deep
ocean, where
it can be stored and prevented from re-entering the atmosphere as a
green-house
gas, the oceans may have little impact on changes in the atmosphere or
climate. The research is the
result of two international expeditions
to the "These results are
particularly important to our
efforts today to improve the predictive capacity of numerical models
that
relate ocean carbon to global climate change on different time scales,"
said Don Rice, director of NSF's chemical oceanography program. It also adds a new
wrinkle to proposals to mitigate climate
change by fertilizing the oceans with iron--to promote blooms of
photosynthetic
marine plants and transfer more carbon dioxide from the air to the deep
ocean. "The twilight zone
is a critical link between the
surface and the deep ocean," said Ken Buesseler, a biogeochemist at
Woods
Hole Oceanographic Institution and lead author of the new study, which
is
co-authored by 17 other scientists. "We're interested in what happens
in the
twilight zone, what sinks into it and what actually sinks out of it.
Unless the
carbon goes all the way down into the deep ocean and is stored there,
the
oceans will have little impact on climate change." Buesseler was the
leader of a project funded by the National
Science Foundation (NSF) called VERTIGO (Vertical Transport In the
Global
Ocean). The twilight zone
acts as a gate that allows more sinking
particles through in some regions and fewer in others, complicating
scientists'
ability to predict the ocean's role in offsetting the impacts of
greenhouse
gases. Using new
technology, the researchers found that only 20
percent of the total carbon in the ocean surface made it through the
twilight
zone off These sinking
particles, often called "marine
snow," supply food to organisms deeper down, including bacteria that
decompose the particles. In the process, carbon is converted back into
dissolved organic and inorganic forms that are re-circulated and reused
in the
twilight zone and that can make their way to the surface and back into
the
atmosphere. The problem, say
scientists, is that particles sink slowly,
perhaps 10 to a few hundred meters per day, but they are swept sideways
by
ocean currents traveling many thousands of meters per day. To collect
sinking
particles, oceanographers use cones or tubes that hang beneath buoys or
float
up from sea floor. That, Buesseler said, "is like putting out a rain
gauge
in a hurricane." While many studies
have investigated the surface of the
ocean, little research has been conducted on the carbon cycle below.
The
VERTIGO team examined a variety of processes to open a new window into
the
difficult-to-explore twilight zone. They successfully used a wide array
of new
tools, including an experimental device that overcame a longstanding
problem of
how to collect marine snow falling into the twilight zone.
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