INDIA SUMMER MONSOON RAINFALL CHANGE OVER SOUTHEASTERN TIBETAN PLATEAU DURING THE LAST MILLENNIUM RECORDED BY LAMINAL STALAGMITE
The water vapor from the Indian Ocean has a significant impact on the dry/wet conditions in China and even East Asia.Here,we present a high-resolution oxygen isotope time series of stalagmite from Chituxianren cave in northwestern Yunnan,China to characterize regional Indian Summer Monsoon(ISM)monsoonal precipitation change over the past millennium.The Chituxianren cave(27°46′ N,99°38′E,with an elevation of about 3494 m at the entrance)is located in Shangri-La,northwestern Yunnan Province,southeastern Tibetan Plateau.The mean annual temperature and precipitation in the area are 6.3 ℃ and 650.8 mm,respectively.Influenced by the Indian Summer Monsoon,the rainy season(June to September)accounts for about 71％of the annual precipitation.The actively growing calcite stalagmite CTXRD1205 was collected from a corridor about 30 meters away from the entrance in the summer of 2012 A.D.The upper half,about 120 mm in length and 35～50 mm in diameter,is employed to build climate proxies in this study.The annually lamina with black carbon deposition can be clearly identified after halved and polished the sample and then thin section was made for lamina counting and thickness measurement(from the top to a depth of 60 mm).Total of 5 230Th dating were performed on MC-ICP-MS.A total of 237 subsamples were drilled along the vertical growth axis for stable oxygen isotope measurement,using a Kiel-Ⅳ Carbonate Device linked to Finnigan Mat-253 gas isotope ratio mass spectrometer.The δ18O time series records the evolution of the Indian summer monsoon with an average resolution of ca.9 years from 952 A.D.to 2012 A.D.There are 409 laminae identified from the upper 60 mm of CTXRD1205.The annual laminae thickness ranges from 33.0μm to 366.8 μm,with an average of 147 μm.The δ18O values of CTXRD1205 varies from-10.72‰ to-16.69‰,with an average of-14.31‰,and showing multi-decadal to centennial time scales change.The annual thickness sequence over the past 400 years has a similar variation trend with the δ18O sequence,with larger lamina thickness coincide to more negative δ18O values,both of which are interpreted to be index of the regional ISM precipitation.Our results exhibit that CTXRD1205 δ18O gradually increased from 1000 A.D.until ca.1430 A.D.,then it gradually decreased until ca.1600 A.D.,followed by a clearly increasing trend with a series of large amplitude muti-decadal oscillations to the present.The δ18O values in recent 200 years are significantly more positive in the last millennium.This long-term trend in-phase with that of stalagmite δ18O record from Northwestern India,but anti-phase with that from Central India,suggesting that the ISM rainfall over study region experience similar evolution history with that of Northeastern India.Superimposing on this long-term trend,there are a series of multi-decadal oscillations,especially,three drought events occur in the 14th century,early 15th century and 18th century,which are coincide with tree-ring based PDSI index from Bidoup Nui Ba National Park in Southern Vietnam.The 14～15th century drought events over the Indo-China Peninsula are broadly linked to demined of Angkor Empire centered in Cambodia.The CTXRD1205 δ18O sequence is negatively correlated with the total solar irradiance(TSI)on multi-centennial scale,suggesting that solar activity play a significant influence on monsoonal precipitation in the southeastern Tibetan Plateau.However,the impact of solar activity on ISM rainfall in the study region seems to be weakened during the last 200 years.Interestingly,correlation between CTXRD1205 δ18O sequence and the Pacific Decadal Oscillation(PDO)during the Current Warm Period(CWP)is inverse to that of in the Medieval Climate Anomaly(MCA)and the Little Ice Age(LIA).In CWP,wetter/dryer climate coincide to negative/positive phase of PDO,while wetter/dryer climate is linked to positive/negative phase of PDO during MCA and LIA.
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