The Responses of Sr/Ca, δ18O, and δ13C in the Porites Coral Skeleton to Extreme Thermal Events in the Nansha Islands

In recent years, extreme thermal events have become one of the main reasons for coral reef degradation. Determining the variation of coral skeletal geochemical proxies in response to extreme thermal events would be vital evidence for reconstructing historical extreme thermal events using corals. In this study, cores from modern Porites corals YSL24 that lived in 1987-1999 and YSL2A that lived in 1971-1999 were collected from the Yongshu Reef, Nansha Islands, southern South China Sea (SCS) for high-resolution analysis of skeletal Sr/Ca, δ18O, and δ13C to determine the responses of these geochemical proxies to past extreme warming events. There is evidence that extreme warming events occurred in 1973, 1983 and 1998 in the southern SCS during the period 1971-1999. The Sr/Ca ratios in the Porites corals showed a significant negative correlation with the variation in local sea surface temperature (SST), and the values in the bands of these three years were much lower. This suggested a clear record of higher temperatures than other years in response to the warming events in these episodes. Compared with the Sr/Ca ratios, skeletal δ18O proxies in the two Porites corals also revealed a negative correlation with SST, but the δ18O values in these three bands were likely to be less sensitive to the thermal events. This is mainly because the variation in this proxy was controlled by both local SST and sea surface salinity, while the two factors had opposite effects on the seasonal variation of coral δ18O in the SCS. After the removal of the SST contributions, the ∆δ18O series of the Porites corals showed clear annual cycles and a negative correlation with seasonal precipitation, but the records in the bands of those warming years did not reveal clear anomalous shifts due to thermal stress, which might be an inappropriate proxy for identifying extreme warming episodes. In the skeletal δ13C series of Porites corals, an abnormal negative shift was detected during the summers of 1973, 1983, and 1998. This was likely caused by a substantial reduction in photosynthesis of symbiotic zooxanthellae in the coral host cells, a biological reaction of corals responding to thermal stress in the three years. In conclusion, the lower Sr/Ca ratios and negative δ13C shifts in the summer that occurred in the bands of Porites corals in this study, revealed significant coincidence with past extreme warming events. As the Porites corals had a stronger tolerance to thermal stress and much lower bleaching rates than other corals, the abnormal geochemical records in the Porites corals are potential signals indicating local past mass coral bleaching events in recent years