Seasonal and interannual variability of the Mid-Holocene East Asian monsoon in coral δ¹⁸O records from the South China Sea

Understanding the full range of past monsoon variability, with reference to specific monsoon seasons, is essential to test coupled climate models and improve their predictive capabilities. We present a 54-year long, high-resolution skeletal oxygen isotope (δ¹⁸O) record extracted from a well-preserved, massive Porites sp. coral at Hainan Island, South China Sea, to investigate East Asian monsoon variability during summer and winter ∼4400 calendar yr ago. Analysis of modern coral δ¹⁸O confirms that Porites from Hainan Island are well positioned to record winter monsoon forcing of sea surface temperature (SST), as well as the influence of summer monsoon rainfall on sea surface salinity (SSS). The coral record for ∼4400 yr ago shows ∼9% amplification of the annual cycle of δ¹⁸O, in good agreement with coupled ocean-atmosphere models showing higher summer rainfall (lower coral δ¹⁸O) and cooler winter SSTs (higher coral δ¹⁸O) in response to greater Northern Hemisphere insolation seasonality during the Middle Holocene. Mean SSTs in the South China Sea during the Mid-Holocene were within 0.5 °C of modern values, yet the mean δ¹⁸O for the fossil coral is ∼0.6‰ higher than that for the modern coral, suggesting that the δ¹⁸O of surface seawater was higher by at least ∼0.5‰, relative to modern values. The ¹⁸O- enrichment is likely to be driven by greater advection of moisture towards the Asian landmass, enhanced monsoon wind-induced evaporation and vertical mixing, and/or invigorated advection of saltier ¹⁸O-enriched Pacific water into the relatively fresh South China Sea. The ¹⁸O-enrichment of the northern South China Sea ∼4400 yr ago contributes to mounting evidence for recent freshening of the tropical Western Pacific. Today, winter SST and summer SSS variability in the South China Sea reflect the interannual influence of ENSO and the biennial variability inherent to monsoon precipitation. Spectral analysis of winter SSTs ∼4400 yr ago reveals a strong ENSO cycle at 6.7 y, which is significantly longer than the average 3.6 y cycle observed since 1970. The results suggest that the influence of ENSO on winter SSTs in the South China Sea was well established by ∼4400 yr ago. However, spectral analysis of summer SSS ∼4400 yr ago shows no significant ENSO cycle, suggesting that teleconnections between ENSO and summer monsoon rainfall were restricted. Taken together, the results indicate marked differences in ENSO monsoon interactions during the winter and summer monsoon seasons in the past. The fossil coral δ¹⁸O record also shows that the amplitude of interannual SST and SSS variability was stronger ∼4400 yr ago, despite ENSO variabilitybeing significantly weaker in the Pacific region. Thus it appears that the strengthened Mid-Holocene monsoon was sensitive to forces, other than ENSO, that acted as alternative drivers of interannual monsoon variability. If this is the case, greater interannual climate variability could accompany the strengthening of the Asian monsoon predicted to occur during the 21st century as transient greenhouse warming preferentially warms Eurasia, even if ENSO perturbations remain relatively stable.