Controls on stable isotope fractionation of alkaline earth metals in cultured corals
Final Report Abstract
The chemical and isotopic compositions of coral skeletons are well considered archives for paleo-environmental reconstructions, since important knowledge about dynamics of past climate change is derived from coral-based records. However, there are still open questions regarding biomineralisation and element incorporation into the coral hard tissues. We studied the stable isotope composition of Ca and Sr in combination with trace element concentrations to gain further insights into biomineralisation and potential applications as paleoenvironmental proxies. Porites sp. cultured at variable temperatures revealed a small but significant temperature dependent Ca isotope fractionation of about 0.02‰/°C and an invariant Sr isotope fractionation. The growth experiments at different pH-values showed no significant impact of pH on the fractionation of Ca and Sr isotopes. Acropora sp. cultured at different salinities with and without symbionts, revealed constant δ44Ca values. Also the Mg/Ca ratio of symbiotic and aposymbiotic specimens did not systematically differ. In contrast, the U/Ca strongly deviated between symbiotic and aposymbiotic specimens and is also affected by temperature induced bleaching. These results provide evidence that the enhanced coral calcification due to symbionts is caused by the increase in pH by CO 2 consumption and not by the supply of organic substances as matrix for biomineralisation. Porites corals grown in outdoor tanks under seasonal variations confirm the geochemical signatures of the controlled culture experiments. Down-core records of fossil corals from the Great Barrier Reef revealed consistent results, but also point to additional factors besides temperature, salinity and water carbon chemistry that need to be considered.
Publications
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(2015) Controlling factors of Ca isotope fractionation in scleractinian corals evaluated by temperature, pH and light controlled culture experiments. Geochimica et Cosmochimica Acta. 167,80-92
Inoue, M., Gussone, N., Koga, Y., Iwase, A., Suzuki, A., Sakai, K., Kawahata, H.
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(2018) A simple role of coral-algal symbiosis in coral calcification, Geochimica et Cosmochimica Acta 235,76-76-88
Inoue, M., Nakamura, T., Tanaka, Y., Shinzato, C., Suzuki, A., Yokoyama, Y., Kawahata, H., Sakai, K., Gussone, N.
