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| Enlarge ImageA high-definition photograph taken by the ROV Hercules on Nashville Seamount (see map below) during a cruise in 2005. In the foreground you can see a live Desmophyllum dianthus coral with tentacles poking out, which is about 10 centimeters across. This solitary coral is living among another deep-water coral species, called Lophelia pertusa, a colonial coral with many polyps (bottom right), as well as many species of purple and orange octocoral (above D. dianthus). The pink arms of a brittle star stretch across the colonial coral toward the Desmophyllum and a spiny sea urchin (far right). (Photo courtesy of DASS05_URI_IFE_IAO_NOAA) |
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| Enlarge ImageDeep-sea corals form ecosystems on the seafloor, made up of many species of corals and associates. (Photo courtesy of DASS05_URI_IFE_IAO_NOAA) |
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| Enlarge ImageIndividual specimens of the coral Desmophyllum dianthus live for decades to hundreds of years and have visible banding patterns. The vertical growth axis of the coral runs from bottom (oldest) to the top (youngest) in this section cut through a coral specimen, and you can clearly see this banding.
Scientists are not yet sure whether the bands are deposited annually in D. dianthus, but they can still make multiple geochemical analyses along the growth axis to create a high-resolution record of conditions in the ocean in the past. (Photo by Jess Adkins, California Institute of Technology, and Tom Kleindinst, Woods Hole Oceanographic Institution) |
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| Enlarge ImageThe carbonate skeleton of a fossil D. dianthus coral is coated in a brown ferromanganese crust, which has to be removed before scientists can analyze the coral for uranium and thorium to determine its age. This coral is about 40,000 years old. The cut section is where a sample was removed for this analysis. (Photo courtesy of Laura Robinson) |
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| Enlarge ImageScientists collected thousands of samples of cold-water corals living on the seafloor during three expeditions between 2003 and 2005 to the New England Seamounts, a chain of extinct underwater volcanoes, starting with Bear, that stretches from the New England coast toward the mid-Atlantic. (Dan Sheirer, U.S. Geological Survey, and Jess Adkins, California Institute of Technology) |
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| Enlarge ImageA three-dimensional bathymetric map of Muir Seamount (looking from the southwest, with the topography vertically exaggerated by a factor of 10). Scientists collected cold-water coral samples on dives in the notch just to the left of the center part of the map. (Dan Sheirer, U.S. Geological Survey) |
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By Laura Robinson, Assistant Scientist
Marine Chemistry and Geochemistry Department
Woods Hole Oceanographic Institution
Are the climate changes we perceive today just part of the Earth system’s
natural variability, or are they new phenomena brought about by human
activities? One way to find out is to look back at the past to get a
long-term picture of how Earth’s climate has fluctuated. But it is a
major challenge to find records of past climate with sufficiently high
resolution to capture evidence of abrupt and brief climate shifts.
Scientists have used tree rings, whose width can reflect climate
conditions experienced during each growing season, and they have used
glaciers, which accumulate a new layer of ice with each year’s
snowfall. High-resolution ice cores from Greenland have shown multiple
events during the past 100,000 years in which air temperatures abruptly
rose and fell by as much as 10°C (18°F) within a decade.
We do not fully understand why these abrupt climate shifts happened,
but we do know that the ocean plays an integral role in the Earth’s
climate system. So another challenge is getting high-resolution records
of how conditions and circulation have changed over time in the shallow
and deep oceans.
Scientists have found clues to past oceanic conditions preserved in
sediment cores that accumulate in sequential layers on the seafloor.
Unfortunately, burrowing organisms can mix the sedimentary layers,
disrupting the chronology and obliterating traces of rapid climate
change.
One way to circumvent this problem is to use the carbonate skeletons of
corals. They can live for hundreds of years, accreting growth layers
that are not affected by mixing, and offer remarkable archives of past
climate.
Thriving in the dark depths
Much like tree rings or ice cores, many species of corals have growth
bands that we can count to make a timescale. The features of the bands
themselves may reflect the condition of the ocean at the time the coral
was alive and creating its skeleton. We can look at the thickness of
each band to gauge the corals’ growth rate during a given year, giving
us a general sense of ocean conditions. Or we can conduct complex
geochemical analyses to extract more precise information about the
ocean, such as temperature or nutrient levels, at the times when the
coral bands formed. Tropical coral reef structures have helped us
reconstruct important climate parameters such as the history of sea
level change, sea surface temperature, and even storm events.
The corals used in these studies are the ones people are most familiar
with: corals that have symbiotic relationships with photosynthetic
algae, which provide food in exchange for shelter, and therefore grow
in shallow, sunlit, tropical waters. As a result, we cannot use them to
look for changes in cold, dark places—at high latitudes, for example,
or in the deep ocean.
But some corals do not use algal symbionts and are able to live in cold
or dark waters. They filter-feed on plankton and organic particles that
fall from the surface waters to the corals living at depths as great as
5,000 meters (3 miles). Because of these organisms’ relative
inaccessibility, we know a lot less about their biology and ecology.
But they appear to live in all ocean basins and can live for hundreds
of years.
An underwater mountain chain
Like their shallow-water cousins, cold-water corals build carbonate
skeletons containing geochemical traces that reflect the chemical and
physical properties of the waters in which they grew. These corals do
not always accrete annual growth bands, but we know the direction in
which they grow, so we can use them to create records of the deep
ocean. Cold-water corals therefore offer great potential to reveal how
and when the deep oceans changed, at a level of detail that matches
what ice cores tell us about climate changes on land.
Jess Adkins, now an associate professor at the California Institute of
Technology, began this work as an MIT/WHOI Joint Program student using
samples that he picked out of the WHOI collection of materials dredged
from the seafloor. Over the past few years, I have been involved in a
research program with Adkins to find and collect live and fossil
cold-water corals from the New England Seamounts, a chain of extinct
underwater volcanoes in the North Atlantic. We have made detailed
photographic images of the seafloor to look for coral community
locations and then used submarines such as Alvin to collect many
thousands of fossil corals for climate reconstructions.
We use careful procedures to precisely date the corals using
radiometric techniques. Seawater contains naturally occurring
radioactive uranium that is incorporated into the coral skeletons; it
decays into thorium at a known rate so by measuring both uranium and
thorium, we can accurately determine the corals’ ages. We have found
that corals have been living on these seamounts for about a quarter of
a million years.
By dating many of the corals collected from the New England Seamounts,
we have found a surprising temporal relationship between the abundance
of corals and changes in the circulation of the deep sea. Deep-sea
coral populations on the New England Seamounts were abundant near the
times at which we know there were major reorganizations in the climate
and circulation of the North Atlantic. We do not know whether changes
in the sea surface environment increased the production of marine life,
leading to more food for corals, or whether ocean circulation changes
enhanced the dispersal of coral larvae.
New methods to date corals
To investigate changes in deep-ocean circulation, we also analyzed the
corals for carbon-14, a naturally occurring radioactive isotope of
carbon that forms in the atmosphere and dissolves into ocean surface
waters. Coral skeletal bands with high amounts of carbon-14 reflect
times when surface waters in the North Atlantic were sinking to the
depths of the ocean and flowing southward; bands with less carbon-14
reflect times when the deep-ocean circulation had shifted, and the
corals were bathed in waters that had probably flowed northward from
the Southern Ocean.
Because carbon-14 is nearly fully decayed after 45,000 years, we are
developing (with funding from the WHOI Ocean and Climate Change Institute)
new chemical techniques using other naturally occurring radioisotopes
that have longer half-lives. Our goal is to create a well-dated,
high-resolution record of North Atlantic deep-ocean circulation changes
over the past 80,000 years that can be compared to ice-core records of
climate.
As we discover cold-water corals over more and more parts of the
seafloor, we can realize their potential to provide unique records of
how the deep oceans and climate interact and to help us understand the
ocean’s role in climate change, past and future.
Posted: October 20, 2006 [top] |
