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A Journey to the Ocean's Twilight ZoneA conversation with marine biogeochemist Ken Buesseler |
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| Enlarge ImageTo stay awake analyzing as many thorium isotope samples as possible, Ken Buesseler and colleagues brought espresso makers and good coffee out to sea, earning his laboratory the nickname “Café Thorium.” (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution) |
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| Enlarge ImageParticles sinking from sunlit surface waters through the ocean’s dimly lit twilight zone are swept sideways by currents. Conventional moored or tethered traps designed to catch the particles are like “rain gauges in hurricanes,” said WHOI biogeochemist Ken Buesseler. He and engineer Jim Valdes are designing a new-generation neutrally buoyant untethered vehicle called the Twilight Zone Explorer, which will be swept along with the currents. It will surface periodically to relay data via satellite. (Illustration by Jack Cook, Woods Hole Oceanographic Institution) |
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| Enlarge ImageThis "quilt" of microscopic ocean organisms collected during the 2005 VERTIGO cruise to the North Pacific was created by Mary Wilcox Silver, professor of oceanography at the University of California, Santa Cruz. (Mary Wilcox Silver, University of California, Santa Cruz) |
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You are about to enter another dimension. You’re moving into a place of
both shadow and substance, of things and ideas; a journey into a
wondrous part of the ocean, whose boundaries are 300 to 1,600 feet (100
to 500 meters) below the surface, where sunlight fades into blackness.
There’s no signpost up ahead, but your next stop is the ocean’s
twilight zone.
No, your host won’t be Rod Serling; he’s unavailable, and besides, he
was no expert on how the oceans work. For this twilight zone, your best
guide is Ken Buesseler, chair of the Marine Chemistry and Geochemistry
Department at Woods Hole Oceanographic Institution. As chief scientist
of a research project called VERTIGO, he mustered an arsenal of
instruments and a small army of ocean scientists from many institutions
and disciplines— biologists, chemists, physical oceanographers, and
engineers—and led research cruises in 2004 and 2005 to explore this dim
and dimly understood region of our planet.
What’s lurking in the ocean’s twilight zone?
We now know that many organisms call the twilight zone their
home. But how do they make a living in this zone where no plants grow?
What do they eat? What adaptations do deep-living zooplankton and fish
have that enable them to survive, reproduce, and flourish in the
twilight zone?
There’s a thin skin on the top of the ocean, where light penetrates,
photosynthesis happens, and phytoplankton grow. Some zooplankton and
fish in the twilight zone migrate daily to surface waters to feed on
phytoplankton, or they eat each other. But many critters in the deep
ocean or on the bottom eat detritus—dead phytoplankton or the feces of
zooplankton, for example—that sinks from the surface like manna from
heaven. It’s called “marine snow.” It’s the food supply for the deep
sea.
Fascinating for biologists, but what’s in it for a chemist?
We think of the surface waters as a factory that produces
sinking particles, and these particles are the primary means by which
materials—carbon, for example—get from the surface to the deep ocean.
There are really only two ways for materials to get there—either by
currents that sink to the deep ocean (but that is a slow process and
occurs only in a few regions, near the Arctic and Antarctic), or by
hitching a ride on sinking particles.
Personally, I’ve never been that interested in theoretical chemistry
per se—reaction rates, kinetics, thermodynamics—but in measuring and
tracking how chemicals move through the oceans, because that can tell
you how the Earth, the ocean, and atmosphere work.
My specialty is measuring radioactive isotopes in the ocean, especially
thorium-234, which is “sticky”—it tends to bind to sinking particles
and marine snow. It also acts as a clock because every 24.1 days, half
of it decays. So by tracking thorium-234, we can measure how fast
elements—particularly carbon—move through the ocean and how they are
transformed by phytoplankton and zooplankton along the way.
Is that how your lab got the nickname “Café Thorium”?
Once we discovered the usefulness of thorium-234, the problem
was that with existing methods, we could only get maybe 10 to 20
measurements on a research cruise. I figured out better ways to collect
and process samples for thorium analyses and get hundreds of
measurements quickly.
That jumped things into high gear, and we began collecting samples on
many cruises all over the world. We streamlined the process so that one
person could physically do the job—if he or she could stay awake enough
hours.
Caffeine was helpful to make that happen. So along with our scientific
gear, we packed our own espresso makers and good coffee, so that at 3
a.m. when we were still working, we did not have to drink hideous
coffee that had been on the stove since dinnertime. We designed a logo
for the lab with a thorium atom, a coffee cup, and a bolt of lightning.
We had a little fun and made T-shirts with our logo. And we kept that
tradition going for...well, we still do.
Why have you focused on the twilight zone?
It’s a critical link between the surface and the deep ocean. We’re
interested in what happens there, what sinks into it and what actually
sinks out of it. It’s an important component of the oceanic food web,
and we’re also interested in how much carbon is exported into the deep
ocean. The oceans are taking up about half the carbon dioxide released
due to fossil fuel burning. Carbon dioxide is a greenhouse gas, so the
more that is in the atmosphere, the warmer our climate becomes. Unless
the carbon that gets into the ocean goes all the way down into the deep
ocean and is stored there, the oceans will have little impact on the
atmosphere and the climate.
How does that work?
Most of the surface ocean is like your lawn. When it grows,
photosynthesis transforms carbon dioxide from the atmosphere into
organic carbon in the grass. When you mow your lawn or winter comes
along, that organic carbon decomposes back into inorganic carbon and
carbon dioxide. It recycles itself efficiently and produces no net
changes in carbon dioxide levels in the atmosphere.
Most of the marine plants, or phytoplankton, that grow and die in the
surface ocean just decay away, and carbon is exchanged back and forth
with the atmosphere. But any carbon in these plants that sinks through
the twilight zone will end up in a large reservoir of deep-ocean waters
that don’t come back to the surface and can store carbon dioxide for
hundreds of years. That has an impact on climate. And it all goes back
to how particles are cycled and transported through the twilight zone.
That explains VERTIGO, which—in the
oceanographic tradition of constructing tortured acronyms—stands for
VERtical Transport In the Global Ocean. Tell us about it.
The key questions we are studying are: Where do marine particles
come from? How do marine plants and animals create and destroy
particles in the ocean? How quickly do particles sink? How deep do they
go? Are all marine particles the same? Will climate changes change the
forecast for marine “snow”?
As on land, the ocean has different ecosystems with very different
conditions and fauna. So we mounted research cruises to places with
different characteristics: off Hawaii in 2004 and in the northwest
Pacific near Japan in 2005.
The scientists onboard included chemists, biologists, and physical
oceanographers, because the phenomena we’re studying require all of
those to understand it. And we brought out all sorts of equipment,
including specially designed instruments, to make measurements of
what’s happening in the twilight zone. On the last cruise, we had
something going in or out of the water almost every hour, 24 hours a
day, over 20 days.
What kinds of instruments?
We had instruments called CTD rosettes that collect water samples and
use sensors to measure water temperature, salinity, oxygen and the
number and types of marine particles. We deployed our unique new
vehicles, the Neutrally Buoyant Sediment Traps, which work like no
other trap to accurately capture sinking material in the ocean. (See
"Swimming in the Rain.") We used other traps from drifting surface buoys, including
one that separates particles by sinking speed and one that collects
microscopic-scale photos of sinking marine particles. We collected
plankton samples with specialized net systems and chemical samples with
large-volume filtration systems.
We’ve collected an amazing amount of new and unique data and samples.
So, more than from any prior study, we should get a three-dimensional
view of changes over time in the twilight zone.
Any preliminary findings?
We found a big contrast in the two locations we studied. Off
Hawaii, we found that 80 percent of the particle carbon was recycled
back into dissolved carbon by the time it sank to 500 meters (1,600 feet), and so
most of the carbon didn’t make it down to the deep ocean. In the
northwest Pacific, 50 percent actually made it down, which is much
higher than previous estimates.
We really don’t know yet why there is this big difference. It may have
to do with lower temperatures in the northern Pacific, which slow down
the bacterial breakdown of organic carbon. It may also have to do with
the northwest Pacific having lots of silica, which plankton incorporate
into their shells. Those shells are heavier than those made by critters
off Hawaii, so they may sink faster. Off Hawaii, particles may sink
more slowly, giving bacteria more time to break them down. This was
something that we scientists kind of knew in our hearts, but we hadn’t
been able to measure it well before.
One of the amazing things about ocean sciences is that we are still in
a “discovery” stage. Every time we drop our water bottles in and look
at the data, there are still surprises.
—Lonny Lippsett
Posted: August 16, 2006 [top] |
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