A 4700-year record of hydroclimate variability over the Asian monsoon intersection zone inferred from multi-proxy analysis of lake sediments

The sedimentology, geochemistry, and plant microfossils of volcanic lake sediments from Cambodia were analysed to reconstruct a 4700 year record of change in moisture availability. This site sits at the heart of the intersection zone of the East Asian and Indian summer monsoons – a climatic system that is socio-ecologically important but poorly understood. Proxies for lake deepening and shallowing indicate a stepwise weakening of the summer monsoon from c. 4700 to 1700 cal. years BP, after which it appears to gradually strengthen. This trend is punctuated by seven dry events between 4300 and 1540 cal. years BP. The results present one of the longest, continuous records of palaeo-hydrological change in the Asian monsoon intersection zone. In doing so, they suggest that the climate of the region was broadly comparable to the East Asian summer monsoon (EASM) province to approximately 2240 cal. years BP, and the Indian summer monsoon (ISM) province between 2240 and 1700 cal. years BP. The subsequent, late Holocene hydroclimate appears to reflect a combination of both subsystems. These findings highlight dynamism in EASM and ISM dominance over mainland southeast Asia over the past five thousand years, but overall demonstrate regional vs. local scale influence on the hydroclimate of the region. The multi-decadal periods of strong drying that punctuate longer-term monsoon patterns demonstrate the natural variability of the mid- to late- Holocene climate of the intersection zone. This is of consequence given that apparently less severe drought events of the last millennium have had dramatic socio-economic consequences. Resultantly, planning for the resilience of socio-ecological systems within the region needs to account for the real potential of similar scale events occurring in the future, particularly given the increased extremes predicted for Asian summer monsoon region over the 21st century.