European Geosciences Union, Vienna, Austria, April 24-29.

Reynaud S., Hemming N. G., Juillet-Leclerc A., Gattuso J.-P. 2005.
Effect of pCO2 and temperature on the Boron isotopic composition of the zooxanthellate coral Acropora sp.

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Résumé :
We have used a unique coral culture technique, which allows the manipulation of seawater temperature and pCO2, in order to assess the effects of these variables on boron isotopic composition in corals. Corals are important archives for geochemical proxies such as oxygen, carbon isotopes, trace elements, and more recently, boron isotopes. Boron isotope uptake in carbonates is controlled primarily by pH. Since a decrease in ocean pH can be interpreted as an increase in pCO2, several studies sought to apply the boron isotope paleo-pH proxy to determinations of past atmospheric CO2. Corals should provide an ideal material for recording the paleo-pH of surface water.

We cultured nubbins of a branching scleractinian coral Acropora sp. and 4 conditions have been simulated: 430 µatm-25.3°C (referred as “normal pCO2, normal temperature”), 446 µatm-28.2°C (“normal pCO2, high temperature”), 712 µatm-25.1°C (“high pCO2, normal temperature”) and 738 µatm-28.3°C (“high pCO2, high temperature”). Since the seawater used in this experiment had a known and constant TA (2.604 ± 0.004 meq kg-1), we only changed the seawater pH to get a fixed value of pCO2. The regulation obtained during this experiment mimicked the shift in pCO2 that would occur within one century.

Our results indicate that d11B in corals is primarily driven by changes in seawater pH and is not affected by temperature. For corals cultured at “normal pCO2”, the d11B of the skeleton is 24.0 ± 0.2 ‰ at 25°C and 23.9 ± 0.3 ‰ at 28°C. The values of d11B measured for corals cultured at higher pCO2 were lower: 22.5 ± 0.1 ‰ and 22.8 ± 0.1 ‰ at 25 and 28°C, respectively. The d13C values of the skeleton are also a function of the pCO2 treatment. Corals exposed to normal pCO2 exhibit higher d13C values (–2.81 ± 0.13 and –2.75 ± 0.16 ‰, respectively for normal and high temperature) than corals exposed to high pCO2 (-4.21 ± 0.17 and -4.14 ± 0.14 ‰, respectively for normal and high temperature). We observed a positive d13C -d11B correlation in the samples.

Our major conclusion indicates a lack of buffering by the coral on this range of pCO2. Although competing effects of respiration, photosynthesis, and carbonate ion effect could make the interpretation of the d13C-d11B co-variation difficult, this experiment should allow the use of corals for studying past changes in ocean chemistry. Changes in pH of surface ocean water that result from changes in atmospheric pCO2 can be recorded in corals.