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| Enlarge ImageA slice through the center of a long-dead brain coral is a slice through human and ocean history. This 1,000-pound coral grew near Bermuda for 200 years. Scientists will analyze the coral's skeleton to decipher ocean temperatures during its lifespan. (Tom Kleindinst, Woods Hole Oceanographic Institution) |
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| A diver off Bermuda indicates the “big dead brain” that WHOI scientists recovered for analysis. This coral died 200 years ago, but remained in place to become prime real estate for other marine life to settle on. (Dr. Ross Jones and Alex Venn, Bermuda Biological Staton for Research) |
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| Enlarge ImageIt’s not easy to get to the center of a half-ton of coral. WHOI engineer Peter Landry (right) works with an employee of Fletcher Granite’s Chelmsford Quarry in North Chelmsford, Mass., to move, position, and slice the huge coral. (Dave Gray, Woods Hole Oceanographic Institution) |
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| Enlarge ImageWHOI Engineer Peter Landry (left) and WHOI Summer Student Fellow Nicholas Jachowski check the position of the massive coral between heavy braces before it is cut. (Dave Gray, Woods Hole Oceanographic Institution ) |
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| Enlarge ImageA granite quarry worker closely monitors the coral slicing operation. (Dave Gray, Woods Hole Oceanographic Institution) |
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| Enlarge ImageA heavy cross-section slice of coral skeleton is lifted away. (Dave Gray, Woods Hole Oceanographic Institution) |
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Anne Cohen
Research Specialist
Geology & Geophysics Department
and Kate Madin
Science writer Sometime around the beginning of the 17th century, a tiny drifting larva found the perfect piece
of real estate to settle down, on the shallow seafloor off the island
of Bermuda. It sprouted tentacles to catch prey, revealing itself as a
coral polyp, and a few days later, the small flower-shaped animal began
to build a hard exterior skeleton. The polyp grew and divided,
eventually multiplying into a colony of thousands, with the round shape
and convoluted surface of a brain coral. By the time it died around
1800, it looked like a boulder and weighed more than 1,000 pounds.
Two centuries later, Ross Jones (a biologist at the Bermuda
Biological Station for Research) and I found this massive,
well-preserved coral
while scuba diving off Bermuda. It was just what we were
looking for: a coral with a past. If it were big enough and dead for
long enough, it would have lived through a considerable part the Little
Ice Age, an era between 1350 and 1850 when Earth’s climate was very
different from today’s.
During this period, unforgiving cold gripped the North Atlantic region.
Europe and eastern North America endured cool summers and severe
winters. Rivers froze and glaciers advanced over land that is ice-free
today. The times were marked by persistent crop failures, famine,
disease, and mass migrations. The Norse, for example, abruptly
abandoned their settlements in Greenland.
Meanwhile off Bermuda, each individual polyp in our coral was accreting (growing)
skeleton, in daily increments that built up into seasonal and annual
layers, similar to tree rings. In corals, the chemistry of their skeletons
varies slightly but measurably as the temperature of the waters do. So
the skeleton of this long-lived coral offered a continuous record of
ocean temperatures and environmental conditions off Bermuda throughout
much of the Little Ice Age, a pre-industrial era for which no
instrumental records exist.
Records closeted in skeletons
The Bermuda brain coral, Diploria sp.,
was covered with marine organisms that had attached and burrowed into
its surface. When we cleared off the algae, sponges, burrowing worms,
starfish, and soft corals, we found that the coral’s surface was eroded—but
inside, the skeleton remained intact. The cover of this ancient “book”
was damaged, but its contents were unspoiled.
We recovered the dead coral from the seafloor and sent it back to Woods
Hole Oceanographic Institution. By using a new laser mass spectrometer
technique we developed for very fine-scale sampling of coral, we aim to
do the equivalent of reading the coral “book” page by page: to get a
week-to-week, perhaps even a day-by-day, account of ocean temperatures
over past centuries.
That would give climate scientists better records to
understand the ocean-atmosphere interactions that generate periodic,
short-term climate shifts such as El Niño and the North Atlantic
Oscillation, for example, or abrupt, longer ones, such as the Little
Ice Age. And understanding the oceans’ past behavior offers insights
into how corals and the climate might change as fossil fuel burning and
other human activities increase carbon dioxide levels in the atmosphere.
Until now, we have had few good records of ocean temperatures just before industrial activity
began.
Paleoclimatologists can infer past ocean temperatures over long,
thousand-year timescales by analyzing the chemical composition of
sediments that accumulate on the seafloor over millennia. But in
sediments, it
is hard to resolve a record of short-term and recent changes. Long-dead
corals can provide that record, and they also remain intact, unlike
sedimentary
layers, which are often disturbed by animal movements or currents.
Translating chemistry into temperatures
Corals’ skeletons are made of aragonite, a form of calcium
carbonate—the same substance marble, limestone, chalk, and clamshells
are made of. To grow their skeletons, corals accrete tiny “seed”
crystals at night underneath their tissue; during the day, long aragonite crystals
self-assemble on those seeds from the calcium and carbonate in seawater, just as snow crystals form around tiny ice crystals.
But seawater contains trace amounts of other elements such as strontium
(Sr), magnesium (Mg), and barium (Ba), which become incorporated into
the growing aragonite crystals. These give scientists a handy
thermometer.
Here’s how it works: The relative proportions of minor, or trace
elements (Sr, Mg, and Ba) versus the major element (calcium) that are
incorporated into aragonite as it grows depend on the temperature of
the seawater. Once coral skeletons form, their composition does not
change—not even over centuries. By sampling coral skeleton layers and
measuring their trace element-to-calcium ratios, we can derive a
chronology of ocean temperature, in locations where the corals lived.
Nanoscale sampling
For years, WHOI Senior Scientist Stan Hart had been using an instrument
called an ion microprobe, which measures small amounts of isotopes in
samples, to determine the composition of rocks. “Why not use this
instrument for paleoceanography in corals?” he suggested. And so, with
funding from the WHOI Ocean and Climate Change Institute, we
developred microscale analytical techniques for coral
skeletons.
The difficulty is in getting sufficiently small-scale samples.
Previously, we could only analyze samples in bulk. That was kind of
like putting the ancient book in a blender, chapter by chapter, and
getting an average account of the plot’s progress.
Another WHOI colleague, biologist Simon Thorrold, began to use a laser
to take nanoscale samples (called “laser ablation”) of calcium
carbonate in fish ear bones, whose isotopic composition he analyzed in
an ion mass spectrometer. With funding from the WHOI Ocean Life
Institute, Thorrold and I put the two techniques together and developed
a new laser mass spectrometer technique for sampling coral.
The technique lets us analyze samples at distances only 60 micrometers (0.002 inches)
apart within the skeleton, equivalent to just a week’s growth. That is
like reading the book line by line, or word by word. Already, by
analyzing daily growth layers, we have found that corals are sensitive
to tidal and lunar cycles. The chemical compositions of their skeletons
change abruptly with each new and full moon.
Analyzing chemical composition, we found that the seawater during our huge brain coral’s life was about
1.5o C (2.7o
F) colder than it is today, yet the coral grew faster than the corals
there now, thriving in the coldest period of
the Little Ice Age. We think water temperatures now are higher than
optimal for these corals.
What does that mean for this coral’s
progeny—the
brain corals off Bermuda today and tomorrow? If they see much warmer
temperatures, due to global warming, will they be able to thrive and
live long lives also?
Posted: March 9, 2007 [top] |
