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

The climate and environmental significance of the stable strontium isotope composition (delta Sr-88) in coral skeletons deserves further investigation. To improve our understanding of stable strontium isotope fractionation during the biomineralization process, correlations between monthly coral skeletal delta Sr-88 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 delta O-18), pH and its proxy delta B-11, and precipitation (representing river runoff) show clear annual cycles, but their correlations with coral delta Sr-88 were not significant during 2008 and 2009. However, both total solar irradiance (TSI) and coral delta Sr-88 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 delta Sr-88 is not temperature-dependent, but TSI may play a major role in controlling coral delta Sr-88 values at our study site. The underlying mechanism of TSI-induced Sr-88/Sr-86 fractionation in coral skeletons may be similar to that associated with coral delta C-13. That is, symbiotic zooxanthellae preferentially uptakes Sr-86 during photosynthesis leaving the internal strontium reservoirs enriched in Sr-88; consequently, increased uptake of Sr-88 by the coral skeleton occurs when photosynthetic rates increase. However, the correlation between coral delta Sr-88 and delta C-13 values may be masked by the carbon input from terrestrial river runoff and the kinetic isotope fractionation effect during the calcification process.

Automated summary

This study investigated the climate and environmental significance of stable strontium isotope composition (delta Sr-88) in coral skeletons. The results showed that delta Sr-88 values were not significantly correlated with factors like sea surface temperature, but showed a significant correlation with total solar irradiance (TSI). TSI may play a major role in controlling coral delta Sr-88 values at the study site.