Effect Of $p$CO$_2$ and temperature on the boron isotopic composition of the zooxanthellate coral $Acropora$ sp.

International audienceCulture experiments were carried out with $Acropora$ sp. (a branching scleractinian coral) in seawater at two $p$CO$_2$ conditions (438 and 725 $\mu$atm) and two temperatures (25 and 28 °C) in order to establish the $p$H and temperature dependence of the boron isotopic composition of the skeleton. A clear $p$CO$_2$ effect, but no temperature effect, on the coral boron isotope composition is seen. For corals cultured at ``normal $p$CO$_2$'' (438 $\mu$atm), the $\delta^{11}$B of the skeleton was 24.0 $\pm$ 0.2 parts per thousand at 25 °C, and 23.9 $\pm$ 0.3 parts per thousand at 28 °C. The values of $\delta^{11}$B measured for corals cultured at higher $p$CO$_2$ (725 $\mu$atm) were lower: 22.5 $\pm$ 0.1parts per thousand, and 22.8 $\pm$ 0.1parts per thousand at 25 and 28 °C, respectively. The $\delta^{11}$B of corals cultivated at both high and normal $p$CO$_2$ conditions are consistent with a dominant pH control, and are very close to that calculated from theoretical considerations. Thus, the corals do not seem to significantly alter ambient seawater for calcification with respect to pH. Co-variation between boron and carbon isotope values is explored