New evidence for the periodic bleaching and recovery of Porites corals during the mid-late Holocene in the northern South China Sea

Abstract

Scattered evidence indicates the occurrence of coral thermal bleaching events during the mid-Holocene. However, information on such occurrences is still very rare. Ten Porites coral cores were drilled from the Wenchang fringing reef of Hainan Island, northern South China Sea. These cores contain mortality surfaces and/or growth discontinuities dating from the mid-late Holocene between 3.4 and 5.7 thousand years ago (ka, before 1950 AD). Monthly Sr/Ca, δ18O and δ13C were analyzed to reconstruct sea-surface temperature (SST), sea-surface salinity (SSS), and photosynthesis intensity during the events. Results show that the mortality surfaces and growth discontinuities of five corals are associated with high SST of 30.3 °C–33.6 °C and high SSS of 34.3–38.3. The abrupt concurrent negative shifts of 1.8‰ or more in δ13C indicate a decrease in the photosynthesis intensity, most likely attributable to the loss of zooxanthellae. These lines of evidence imply that the mortality surfaces and growth discontinuities were the consequence of coral bleaching under abnormally high temperature. The observed recovery directly after bleaching events indicates that corals in the South China Sea are capable of rehabilitation after experiencing severe environmental stresses.