| The Holocene, the most recent geological period, has long been regarded as an analog to the future of natural environment process. Thus, studying climate change during the Holocene is of great importance for predicting the future, particularly under the context of global warming. The southwest monsoon is an important component of global climate that transports heat and moisture from the warmest part of the tropical ocean to higher latitudes. The seasonal reversal of monsoon winds and associated monsoon rainfall over the Asia has a direct effect on the socio-economic development in the densely populated Asian region. Hence, reliable forecasting of monsoon rainfall is an important issue in the Asian. To increase the accuracy of monsoon forecasting, one needs to understand the variability of monsoon rainfall at different time scales from decadal, centennial and millennial time scales. The nature of the monsoon climate during the Holocene is essential both to understand the present climatic conditions and to predict the future climatic processes. Therefore detailed and precise reconstruction of the Holocene history of the southwest monsoon climate is becoming increasingly important. As the Yunnan Province is closed to the Qinghai-Tibet Plateau and controlled by southwest monsoon, it becomes the key area of studying the paleoclimate change about southwest monsoon during the Holocene.Tiancai Lake is an alpine glacial erosion lake which locates in the northern part of the mountains and canyons of the southeast edge of the Qinghai-Tibet Plateau. The altitude is3898m and the largest water depth is7m. The area of lake is relatively small and only covers0.02km. Also, its drainage area is relatively small, so that the source of lake sediment is relatively simple and stable. Tiancai Lake is a highland far away from human habitation and the climate type is cold temperate humid monsoon. The mean annual temperature is2.5℃and the average annual precipitation is about910mm.A926cm-long sediment core (TCK1) was collected at a water depth of6.8m in the central part of Tiancai Lake (26°38’03.80" N,99°43’00.00"E). The sedimentary core was sectioned at1cm interval and926samples were obtained. Thus the proxies of grain size, magnetic susceptibility, loss on ignition(LOI), total organic carbon(TOC), total nitrogen (TN), stable organic carbon isotope composition, brightness and redness were analyzed.The sedimentary record of Tiancai Lake is continuous, sensitive and high resolution. Based on the precise chronology through the210Pb,137Cs and AMS14C dating and analyses of grain size, magnetic susceptibility, loss on ignition, total organic carbon, total nitrogen, stable organic carbon isotope composition, brightness and redness, it was considered that Tiancai Lake went through six major stages in the paleoclimate and paleoenvironment history since about12cal. ka B.P.. In contrast to other records in the southwest monsoon area, it was used to restore the history of climate and environmental evolution in the southwest monsoon area since about12cal. ka B.P.. During the late Younger Dryas, the southwest monsoon got weak, the climate was cold and dry. In the early Holocene, the temperature was starting to rise gradually, the southwest monsoon got stronger on the whole, the climate was relatively more humid except for a relatively drier period. During the middle Holocene, the temperature was higher, the southwest monsoon got weaker, the climate was relatively drier. In the late Holocene, the fluctuation of the southwest monsoon intensity was relatively frequent with four obvious alternations in this period. When the southwest monsoon intensity was relatively stronger, precipitation was more abundant and climate was more humid, and vice versa. |
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