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Climatology

Unraveling coral's secrets

What will happen if the climate continues to get warmer, glaciers melt and there is a sudden rise in sea level? The best way to find out is by taking a look at the past. With the help of core samples drilled off the Tahiti coral reefs, scientists are able to go back in time and unravel the last deglacial events.

mer climatology

© G. Camoin / CNRS Photothèque

The coral barrier reef off the north coast of the island of Tahiti.


 

Gilbert Camoin, CNRS senior research scientist at Cerege1 and co-leader of an international team of scientists is enthusiastic: “We've brought back a real treasure trove: a high-resolution paleoclimatic record!” In October 2005, a team of ten scientists boarded a drill ship and left Papeete, Tahiti, for a 42-day cruise. The “Tahiti Sea Level” expedition, which is part of the international Integrated Ocean Drilling Program (IODP),2 is the biggest drilling operation ever undertaken in a coral reef. The aims of the program are to reconstruct sea-level rise, to study climate changes and to evaluate the response of coral reefs to such changes,3 over approximately the last 23,000 years.4 “At the time of the Last Glacial Maximum (i.e., at the height of the last glaciation), sea level was 120-140 meters lower than it is today,” Camoin points out. “So we drilled down to that depth, about 1.5 kilometer offshore. In total, we drilled 37 cores from 22 sites.” But why chooseTahiti to collect reef cores? “The location of Tahiti in the Pacific Ocean is ideally suited to evaluate global climate changes,” Camoin explains. “There are strong atmosphere-ocean interactions in that area.”

 

Moreover, the coral reef surrounding Tahiti is a living entity made up of extremely sensitive organisms which react to any changes in the environment and leave a record of the changes when they're no longer living. But there remains the task of interpreting what the coral is telling us. This can be achieved through multidisciplinary studies. It is possible to reconstruct sea-level changes through the accurate dating of the corals. Furthermore, by analyzing the composition of the coral skeletons and, more specifically, by studying the ratio between two oxygen isotopes, 16O and 18O, as well as that of trace elements such as strontium or magnesium, it is possible to determine the salinity and temperature of the water at a precise moment in the past. Scientists have obtained long fossil coral cores thousands of years old, one of which is a priceless unbroken core of massive coral measuring 3.5 m in length. Its banding, composed of alternating light and dark growth bands (each of them averaging a centimeter in thickness), represents 350 years of coral growth. Though these massive corals only make up 5% of the total samples, they are of incomparable value. “It's been a real success,” says Camoin, who was worried that they would only find “cores broken up by cavities filled with water or sand. This would have meant that there were gaps in the record.”

 

In February and March 2006, scientists from the mission got together for a three-week meeting in Bremen, Germany. There the cores were indexed, classified and then sliced up into samples. Each scientist left with a share of the booty to carry out more advanced research in fields as varied as geochemistry, petrography (the study of the nature of rocks), microbiology, or magnetism.

 

At the same time, climatologists will integrate the information obtained from the core samples into their models and attempt to reconstruct the last deglaciation more accurately. “At the time, temperatures increased very abruptly, the ice melted and sea level rose very quickly.”  Similar changes could happen again if the climate continues to get warmer. “At present, we are living in an interglacial stage, and theoretically we should be heading towards a new glacial stage,” Camoin continues. “However, climate data over the last 150 years tells us that, paradoxically, the temperature of sea water has been steadily rising.” Is it possible that human-induced climate warming could counter the natural phenomenon of glaciation? “Analyzing our samples should help us get a clearer picture,” Camoin concludes.5

 

Fabrice Impériali

Notes :

1. Centre européen de recherche et d'enseignement des géosciences de l'environnement (European Center for Research and Teaching in Environmental Geosciences), CNRS / IRD / Université de Provence / Université Paul Cézanne, Aix-en-Provence joint research.
2. European division: European Consortium for Ocean Research Drilling (Ecord):
www.iodp.org and www.ecord.org
3. Another of the mission's goals is to be able to study for the first time microbial life inside a coral reef.
4. Between 23,000 and 13,000 years ago. In 1992 and 1995, drilling had already made it possible to get as far back as 13,800 years ago.
5. Next update in October 2007.


Contacts :

Gilbert Camoin
Cerege, Aix-en-Provence.
gcamoin@cerege.fr


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