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8/18/13, "Newly discovered ocean plume could be major source of iron," Woods Hole Oceanographic Institute News Release
"“We’ve assumed that low helium means low iron, and our study finds
that that’s not true,” (Mak) Saito (a WHOI associate scientist and lead author of the study) says. “There’s actually quite a lot of iron
coming out of these slow-spreading regions in the Atlantic, where
people thought there would be little to none.”
And that has profound implications, because iron is a critical
element for ocean life. Iron is known to spur the growth of
phytoplankton in many marine habitats, especially those important in the
ocean’s carbon cycle, which, in turn, impacts atmospheric carbon
dioxide levels and Earth’s climate.
Because more than half the world’s
seafloor ridges are slow-spreading, the team’s discovery suggests there
may be far more iron from these locations than previously estimated.
“We need to understand where iron is in the ocean and where it’s
coming from to understand the role of iron in the marine carbon cycle
with any confidence,” Saito says.
Saito and his colleagues hope future studies will reveal the exact
shape and extent of the plume, and just how much of its iron and other
micronutrients persist and rise to the surface. Answering these
lingering questions will help researchers truly understand how
hydrothermal vents affect the ocean as a whole, Saito says.
The research was supported by the U.S. NSF-Chemical Oceanography program
and the Gordon and Betty Moore Foundation (grant GBMF2724)....
Scientists have discovered a vast plume of iron and other
micronutrients more than 1,000 km long billowing from hydrothermal vents
in the South Atlantic Ocean. The finding, published online Aug. 18 in
the journal Nature Geoscience, calls past estimates of iron
abundances into question, and may challenge researchers’ assumptions
about iron sources in the world’s seas.
“This study and other studies like it are going to force the
scientific community to reevaluate how much iron is really being
contributed by hydrothermal vents and to increase those estimates, and
that has implications for not only iron geochemistry but a number of
other disciplines as well,” says Mak Saito, a WHOI associate scientist
and lead author of the study.
Saito and his team of collaborators—which includes WHOI researchers
and a colleague affiliated with the University of Liverpool
(U.K.)—didn’t set out to find iron plumes in the South Atlantic.
They
set sail aboard the R/V Knorr in 2007 as part of the Cobalt,
Iron and Micro-organisms from the Upwelling zone to the Gyre (or
CoFeMUG, pronounced “coffee mug”) expedition, which intended to map
chemical composition and microbial life along the ship’s route between Brazil and Namibia. As the scientists traveled
the route, they sampled the seawater at frequent intervals and multiple
depths along the way, and then stored the samples for in-depth analysis
back on land.
Their route passed over the Mid-Atlantic Ridge, a band of mountains
and valleys running along the Atlantic Ocean floor from the Arctic to
the Antarctic where several of the Earth’s major tectonic plates are
slowly spreading apart. Hydrothermal vents, or fissures in the Earth’s
crust, are found along the ridge, but they haven’t been extensively
studied because slow-spreading ridges are thought to be less active than
fast-spreading ones. Past studies using helium, which is released from
the Earth’s mantle through hydrothermal vents and is routinely used as
an indicator of vent activity, have found little coming from
mid-Atlantic vents, and researchers have assumed that means the vents
spew little iron as well.
So Saito and his colleagues were surprised by what their samples
revealed when later studied in the lab. Once filtered and analyzed, some
of the seawater showed unexpectedly high levels of iron and manganese. When
Abigail Noble, then a WHOI graduate student, and Saito plotted the sites
where the iron-rich samples were taken, they realized the samples
formed a distinct plume—a cloud of nutrients ranging in depth from 1,500
to 3,500 meters that spanned more than 1,000 km of the South Atlantic
Ocean.
“We had never seen anything like it,” Saito says. “We were sort of
shocked—there’s this huge bull’s-eye right in the middle of the South
Atlantic Ocean. We didn’t quite know what to do with it, because it went
contrary to a lot of our expectations.”
The plume’s ratio of iron to helium was 80-fold higher than ratios
reported for faster-spreading ridges in the southeastern Pacific Ocean.
The serendipitous discovery casts doubt on the assumption that
slow-spreading ridges are iron-poor, and it raises questions about the
use of helium as an indicator for iron flux in hydrothermal vents, Saito
says."...
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