Evaluation of coral skeletal δ88Sr as a paleoclimate proxy in the northern South China Sea

Abstract

The climate and environmental significance of the stable strontium isotope composition (δ88Sr) in coral skeletons deserves further investigation. To improve our understanding of stable strontium isotope fractionation during the biomineralization process, correlations between monthly coral skeletal δ88Sr values and climatic and environmental variables (along with their corresponding geochemical proxies) between 2008 and 2009 were studied using a massive zooxanthellae Porites coral head from Sanya Bay on the south coast of Hainan Island in the northern South China Sea. Sea surface temperature and its corresponding geochemical proxies (Sr/Ca and δ18O), pH and its proxy δ11B, and precipitation (representing river runoff) show clear annual cycles, but their correlations with coral δ88Sr were not significant during 2008 and 2009. However, both total solar irradiance (TSI) and coral δ88Sr values followed a gradually increasing trend over the study period and showed a statistically significant correlation. These results indicate that the fractionation associated with coral skeleton δ88Sr is not temperature-dependent, but TSI may play a major role in controlling coral δ88Sr values at our study site. The underlying mechanism of TSI-induced 88Sr/86Sr fractionation in coral skeletons may be similar to that associated with coral δ13C. That is, symbiotic zooxanthellae preferentially uptakes 86Sr during photosynthesis leaving the internal strontium reservoirs enriched in 88Sr; consequently, increased uptake of 88Sr by the coral skeleton occurs when photosynthetic rates increase. However, the correlation between coral δ88Sr and δ13C values may be masked by the carbon input from terrestrial river runoff and the kinetic isotope fractionation effect during the calcification process.