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

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 (δ¹⁸O and δ¹³C) 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 δ¹⁸O values during summer showed a negligible influence of the large intercolony variation in growth rate (2-10mmyr^-1) on δ¹⁸O variation, but δ¹⁸O 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 δ¹³C in slower growing corals, which we attributed to the kinetic behaviour of the calcification reaction. The seasonal fluctuation in δ¹³C corresponded not to changes in light intensity nor to δ¹³C 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 δ¹⁸O 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.