High Resolution Stalagmite δ～(18)O Records Over The Past 1200 Years In The Eastern Qinghai-Tibetan Plateau

Climate change During the Past 2000 years is one of the important research for "Past Global Changes" (PAGES) and the "Climate variability and predictability" (CLIVAR) two major international research project. Especially, Climatic Changes of the Past 1000 Years is an important hinge period which link up instrumental climatic data and reconstructed climatic records, during this period Human activities increasingly impact on climate and environmental impacts, and there were some major climatic events, such as the Medieval Warm Period (MWP. 900～1300AD) , the Little Ice Age (LIA. 1300～1900 AD) and the 20th century warming (1900AD to the present) , so more and more people research climatic and environmental Changes of the Past 1000 Years. Stalagmite record has a much longer time span and reserves integrated information, so it is a valuable supplement archive to records of ice core, loess, lake sediment and peat core, etc. Furthermore, stalagmite is an ideal material for precise U-series dating and yields independent time scales. Therefore, stalagmite has become a major data source for continent paleo-climate reconstruction. Among multi-proxy records in stalagmite including stable carbon/oxygen isotope, trace element and lamina, the stalagmiteδ¹⁸O is the most widely used proxies at present. There are multi-answers in explaining oxygen isotopic compositions of stalagmites from different regions because the shifts of the stalagmiteδ¹⁸O values are controlled by many factors, such asδ¹⁸O in precipitation, depositing process of calcite, cave temperature during depositing period, especially in the regions strongly influenced by summer monsoon. Therefore, it is necessary to study oxygen isotopic composition and its implications of modern stalagmite systematically. Although, great progress in history, amplitude and driving factors of the Asian monsoon variation has been made by the stalagmite records on glacial/interglacial and millennial time scales currently, much research work is still needed on modern stalagmite depositing process and the changes of oxygen isotope composition in responses to the climate change on short time scales. Based on ²³⁰Th/²¹⁰Pb dating and stable oxygen isotope analysis data, we have reconstructed precisely and high-resolution Asian monsoon records with an average 1-year-resolution monsoon record over the 1200 years by modern stalagmites from Huanglong Cave and Wanxiang Cave in the Eastern Qinghai-Tibetan Plateau, and drawn the following conclusions:1. The research on modern condition of stalagmite depositing and oxygen isotopic system in the depositing process suggests there is a remarkable consistency of oxygen isotopic composition between cave drip-water and meteoric precipitation around the cave, and the modern stalagmite is precipitating under isotopic equilibrium with the modern drip water.Therefore, we interpret stalagmiteδ¹⁸O values mainly depending on temperature andδ¹⁸O of meteoric precipitation.2. Based on the stalagmite oxygen isotope geochemistry theory and compared with local instrumental meteorological data, the stalagmiteδ¹⁸O records of Huanglong Cave andWanxiang Cave largely reflected the changes in theδ¹⁸O of precipitation conveyed through Southwest monsoon, while temperature effects are relatively small. The stalagmiteδ¹⁸O records in monsoon region of China reflect variations of Asian monsoon and represent the information of the oxygen isotopes of monsoonal precipitation not only on glacial/interglacial scales, but also on short time scales, and the stalagmiteδ¹⁸O more negative values, the greater the summer monsoon, the monsoon precipitation more, and vice versa.3. There is a similar changing trend betweenδ¹⁸O records of Huanglong Cave and Wanxiang Cave over the past 1200 years, indicating that the Asian monsoon is synchronous in the high-elevation areas and low-elevation areas during the past 1200 years : 900-1550AD, two stalagmiteδ¹⁸O values has generally increasing trend showing Southwest monsoon intensity was gradually weakening during this period; 1550～1800AD, stalagmiteδ¹⁸O values became step-wisely lighter (negative shift) indicating the Southwest monsoon intensity strengthened gradually and the summer monsoon precipitation gradually increased during this period; 1750～2002AD, Stalagmitesδ¹⁸O values begun to become gradually heavier demonstrating strong Southwest monsoon intensity has gradually weakened during this period. The two cave stalagmite records show that Climate Change and the Asian monsoon intensity is not stable and there were a number of short-time-scale (10a～100a) the monsoon events.4. There is a close relationship between the Asian monsoon variations constructed by stalagmiteδ¹⁸O records and reconstructed the Northern Hemisphere temperature record during the past 1200 years. Correlation analyses suggested that Asian monsoon intensity is positive correlation with temperature changes for the past 1200 years, i.e. the monsoon enhanced stronger, the temperature get warmer, and it is synchronous for Asian and temperature. Especially in the Medieval Warm Period, stalagmiteδ¹⁸O values became light, indicating relatively strong monsoon and more summer rainfall with a warmer climate; In the Little Ice Age, stalagmiteδ¹⁸O values got heavy suggesting the Asian monsoon weakened andthe summer rainfall decreased with a dry and cold weather. In 1900～2000AD, Temperature changes and stalagmite records take on negative correlation which suggesting the Asian monsoon became weakened, but climate is warming.5. There is a close link between stalagmite HL021δ¹⁸O record and drought-flood time series by using the historical climate data reconstruction period nearly 1000 within middle and lower reaches of Yangtze River. When stalagmiteδ¹⁸O values became heavy, indicating monsoon weakened and precipitation decreased, whereas, drought increased and flood reduced, and vice versa. Meanwhile, there is a similar relationship between stalagmite HL021δ¹⁸O record and dry-humid index time series by the use of historical documents and data inthe China monsoon region over the past 1200 years. Stalagmite HL021δ¹⁸O v'alues got light and the Asian monsoon got stronger with more precipitations, whereas, dry-humid index indicates rainfall increased and flood disaster increases. Monsoon weakening is phase with dry climate, drought increased and precipitation reduction correspondingly. Huanglong Cave stalagmiteδ¹⁸O record further reflects accurately the variations of Asian monsoon precipitation intensity over the past 1200 years.6.The cave stalagmiteδ¹⁸O values became gradually lower (lighter) with increasing latitude in the Asian monsoon region reflecting the effect latitude of Asian summer monsoon precipitation and stalagmite isotopic composition changes from Asian monsoon region has a broader climatic significance. In the recent 1200, the Asian monsoon regionδ¹⁸O stalagmite records show a similar trend which experienced similar three stage stalagmite: stalagmiteδ¹⁸O values got heavier, and then lighter, finally heavier again, which demonstrated Asian monsoon variations has three stage changes : 900～1550AD monsoon weakened andl 550～1800AD monsoon strengthened, at the beginning of 1800 AD the Asian monsoon weakened finally again.7. The power spectrum analysis result of stalagmitesδ¹⁸O records from Huanglong Cave and Wanxiang Cave showed that the Southwest monsoon variation has solar activities cycles and the El Nino-Southern Oscillation (ENSO) cycles, and the variations of stalagmite records is consistent with the vitiations of atmospheric ¹⁴C and cosmonuclide production changes, which indicating that the Southwest monsoon variations is solar activity and solar energy output is main forcing factor for the Asian monsoon changes on from the short time scale ( 10～100 years) . And there is close tie between the Southwest monsoon variations and ENSO cycles suggesting that an atmospheric-oceanic couple over the tropical Indian Ocean plays an important role in rapid changes of the Southwest monsoon variations over the 1200 years. The variability of Asian southwest monsoon, which is remotely relevant migration of inter-tropical convergence zone (ITCZ) , is synchronous with the North Hemisphere temperature change and the 5 0 records of Greenland ice core, which supports high-latitude forcing mechanism that high-latitude region is key regions for the Asian monsoon variations over the past 1200 years.