Growth-rate influences on coral climate proxies tested by a multiple colony culture experiment

Abstract: As application of coral-based climate reconstruction has become more frequent at tropical sites, increased attention is being paid to the potential ambiguities of coral thermometers that are intrinsic to the biomineralisation process, including the so-called vital effect, the growth-rate-related kinetic effect, and the [CO3 2−] effect. Here we studied how the growth rate influenced the skeletal oxygen and carbon isotope ratios (δ 18O and δ 13C) and the Sr/Ca ratio in a common-garden experiment involving the long-term culture of Porites australiensis clone colonies. Comparison of the seasonal minimum δ 18O values during summer showed a negligible influence of the large intercolony variation in growth rate (2–10mmyr−1) on δ 18O variation, but δ 18O was relatively sensitive to temporary intracolony growth-rate changes related to colony health. In contrast, the Sr/Ca ratio was robust against both inter- and intracolony growth-rate variation. We found a positive shift in δ 13C in slower growing corals, which we attributed to the kinetic behaviour of the calcification reaction. The seasonal fluctuation in δ 13C corresponded not to changes in light intensity nor to δ 13C of dissolved inorganic carbon in seawater, but to photosynthetic efficiency as measured by pulse-amplitude photometry. These findings support the inference that coral skeletal Sr/Ca and δ 18O in a long-lived colony can function as a palaeoclimate archive by recording signals of clonal growth. We also propose practical guidelines for the proper interpretation of coral records. [Copyright &y& Elsevier]