Evolution of Asian monsoon variability revealed by oxygen isotopic record of middle Holocene massive coral in the northern South China Sea

A 40-year δ18O record of the ∼5.4 ka BP Porites lutea from the east coast of Hainan Island in the northern South China Sea (SCS) was analyzed to investigate evolution of Asian monsoon variability during the middle Holocene. The mean, maximum and minimum of the coral skeletal δ18O were 0.49‰, 0.69‰ and 0.14‰ higher than those of modern corals respectively. Thus, the sea surface temperature (SST) for mean annual, winter and summer at ∼5.4 ka BP was 2.2 °C, 3.1 °C and 0.6 °C lower, and accordingly the surface salinity (SSS) was 1.5‰, 2‰ and 1‰ higher than today.The δ18O amplitude of the ∼5.4 ka coral for winter–summer (∼26% higher than modern values), winter (∼23% higher) and summer (∼18% lower than that of the modern corals) implied enhanced seasonality in winter, but decreased seasonality in summer for SST, SSS and Asian monsoon conditions at ∼5.4 ka BP relative to today. Thus, seasonal increment of coral δ18O mainly resulted from decrease of winter SST and increase of winter seawater δ18O during the middle Holocene, which possibly correlated with seasonal changes of insolation.The ∼5.4 ka coral δ18O values indicate that interannual variability of the Asian monsoon was much stronger in winter–summer and winter, although much weaker in summer during the middle Holocene relative to today. Spectral analysis revealed a significant variance with a period of 18.7 years and sub-significant variances at interannual periods of 2.3–2.6 years, which implies that ENSO variance and a teleconnection between the atmospheric ENSO and monsoonal rainfall during the middle Holocene existed, although they were much weaker than today. The long-term decrease trends of the coral δ18O minima, mean and maxima are controlled by the long-term trends of SST increase, and SSS and local insolation decreases from the middle Holocene to the present.