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International Polar Year

Reading Seashells

Scallops could make remarkable indicators of Southern Ocean temperatures. Yet much investigation is still needed before using them effectively. This is why a team of divers and researchers from Brittany have undertaken the MACARBI mission last winter. Their objective is to study the growth patterns and life cycles of these scallops, which will eventually make it possible to use their shells as ocean temperature records.


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© E. Amice/CNRS Photothèque

The MACARBI campaigns require 150 dives in all, often in below-freezing waters. They are organized by LEMAR's diving technician, Erwan Amice, a former bomb disposal diver in the French Navy.



Having to fly for nearly 50 hours, then boarding a ship for a 6-day long 2700 km journey over harsh seas constantly battered by powerful crosswinds is definitely an ordeal. And it’s not over once you reach your destination; you then need to put on a thick wetsuit, a double hood, and wear large gloves to survive the freezing waters.
No one said scallop fishing in the Antarctic was easy, yet this is all part of the third campaign of the MACARBI mission, a French acronym for “Antarctic environmental monitoring based on the study of bivalve carbonate structures.” What Laurent Chauvaud and his colleagues at LEMAR1 in Brest intend to do is use the Antarctic scallop–a relative of the well-known gastronomical treat, the great scallop–as an instrument to monitor the marine environment. Among other things, this small mollusk has the amazing ability to record in its shell the temperature of the water.
“It all began quite fortuitously in the early 1990s when I was doing my PhD at the University of Western Brittany,” recalls Chauvaud. “I had to study the life cycle of the great scallop as part of a broad environmental study of the Brest harbor. I was interested in ecosystem disturbances that were causing mollusk stocks to crash in the region.” The Pectinidae, a large family to which scallops (including the great scallop) belong, have a special feature: The shell surface has grooves that form as it grows, rather like tree rings. With clockwork precision, the great scallop makes one groove a day. Chauvaud decided to build on this observation. He compared the space between the grooves–in other words, the rate of growth (fast if the grooves are far apart, slow if the grooves are close together)–with the water temperature, recorded at regular intervals, and the appearance of phytoplankton. This experiment paid off. Every disturbance in the scallop’s environment, such as very low water temperature, or a phytoplankton bloom, was recorded in its shell.
Wherever he repeated the experiment, in Morocco, Norway or California, Chauvaud observed the same phenomenon. Then, working with isotope measurement specialists at both Stanford University and the US Geological Survey in San Francisco, he went one better and succeeded in using the great scallop as a thermometer that, amazingly, is accurate to the nearest 0.5°C. “The ratio of the two stable isotopes of oxygen found in the shell, oxygen-16 and oxygen-18, is closely dependent on the temperature of the water where it forms,” he adds.

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© D. Amouroux/IPREM

To analyze trace elements, like metals, that the scallop assimilates as it grows, the researchers use laser ablation together with inductively coupled plasma mass spectrometry. The laser is used to pulverize the surface of the shell. This matter is then directly transferred to the spectrometer.



But the great scallop and the other members of the scallop family could also be useful for monitoring other environmental tracers. “We were able to measure the salinity of the water or look for the presence of trace elements such as certain metals–which are indicators of the various influxes at the interface of water and the seabed sediments. This was done using chemical analysis obtained by laser ablation and inductively coupled plasma mass spectrometry, which was carried out by David Amouroux at IPREM2 in Pau,” Chauvaud explains. Archaeologists are also interested in scallops, which could help date neighboring objects in sediment layers. “In short, scallops form a record par excellence of the marine environment,” Chauvaud reckons. “They’ve existed for nearly 30 million years and have covered practically the entire planet.”

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© E. Amice/CNRS Photothèque

The scientists took advantage of the MACARBI campaigns to study the marine ecosystem, especially the migration of biofilms. These clumps of microorganisms, which carpet the seafloor, move up and down through the sediment following a day/night cycle.



And this is true even of the Antarctic and its icy waters. In a context of global warming, to be able to monitor how the temperature of the oceans changes over time is of utmost importance. “But before we can use the Antarctic scallop as a marine thermometer, we must calibrate it first,” Chauvaud points out. This is why the scientists at LEMAR launched the MACARBI mission, in partnership with IPEV3 and IUEM.4 “We need to know how it grows and breathes and what it eats, so that we can correlate its growth rate with water temperatures.” And this growth rate is considerably slowed down by the cold environment; whereas the great scallop makes a new groove every day, the Antarctic scallop makes just one a year. In order to successfully carry out this calibration, several tons of scientific equipment will be transported ahead of the researchers to the Dumont d’Urville base–where the scientists have been going every southern summer since 2005. It will then be up to these intrepid diver-researchers from Brest to decipher the scallop’s way of life and turn this humble mollusk into a precision instrument for measuring the temperatures of the Southern ocean.

Fabrice Demarthon

Notes :

1. Laboratoire des sciences pour l'environnement marin (CNRS / Université de Brest).
2. Institut pluridisciplinaire de recherche sur l'environnement et les matériaux (CNRS / Université de Pau).
3. Institut Paul-Émile Victor.
4. Institut universitaire européen de la mer (www.univ-brest.fr/IUEM/).

Contacts :

Laurent Chauvaud,
LEMAR, Brest.
laurent.chauvaud@univ-brest.fr


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