Holocene intensification of the East Asian summer monsoon:timing and mechanism

The Holocene history of the East Asian summer monsoon is of special importance to constraining the predictions of the precipitation pattern and hydrologic cycle in eastern Asia under the background of global warming in the near future.The timing of intensification of the summer monsoon during the Holocene,however,is still debated due partly to uncertainties of radiocarbon dating of different materials and partly to discontinuities of the sedimentary sequences resulting from changes in the facies.Daihai and Hulun Lakes are located in the modern northern limit of the summer monsoon and particularly sensitive to changes in the monsoon precipitation.Highresolution,multi-proxy records of the sediment cores recovered in the central part of the lakes document changes in the hydrology and ecology of the lake regions and reveal a detailed process of East Asian summer monsoon variations during the Holocene.During the early Holocene before 8000 cal a BP,and herbs and shrubs dominated the Daihai Lake basin in company with patches of mixed pine and broadleaved forests,indicating a dry climate.The input of terrestrial organic matter to the lake was decreased,denoting weak surface runoff in the lake catchment.In Hulun Lake,dry steppe prevailed over the lake region,and the pollen-derived mean annual precipitation was low.Low-salinity D.stevensoni was abundant,warm-adapted Ilyocypris spp.was scarce,and cold-adapted C.neglecta and C.of.houae appeared frequently,denoting fresh but cold waters of the lake.Coming to the middle Holocene after 8000 cal a BP,largescale covers of mixed coniferous and broadleaved forests were developed in the Daihai Lake basin,marking a humid climate.A greater amount of terrestrial organic matter was transported to the lake,implying intensified surface runoff in the lake catchment.In Hulun Lake,grasses and birch forests expanded,and the mean annual precipitation increased markedly.Ilyocypris spp.flourished,and Mg/Ca,Sr/Ca and δ;O of ostracode-shell carbonates were as low as those during the early Holocene,implying that the lake water was still fresh but became warmer.These data indicate that the East Asian summer monsoon was not intensified until 8000 cal a BP,showing a lag of 3000 cal a behind the maximum summer insolation in the Northern Hemisphere.We suggest that the time lag between the summer monsoon intensification and the maximum summer insolation might result from a stagnant northward retreat of the polar front in the North Pacific Ocean due to the existence of remnant ice sheets in the Northern Hemisphere,which would hamper the northward penetration of the summer monsoon front,thereby suppressing the monsoon precipitation over northern China.