Peatland Sediments And Asian Summer Monsoon Evolution In Dajiuhu Basin Since The Last Glacial Maximum

Asian summer monsoon?ASM?,a central component of the global climate system,plays a significant role in regional and global land-air-sea interactions.A series of millennial-centennial-decadal scale abrupt oscillations in ASM variations occur since the last glacial maximum,which has profound influences on culture evolution in Asia.Thus,research in ASM variations and abrupt climate events is contributed not only to understanding global climate changes,but also to predicting global changes in future and achieving sustainable development.Dajiuhu is a rare alpine peatland in the subtropical region of China and has formed a thick continuous peat stratum since the last glacial maximum under the dominant control of the East Asian summer monsoon?EASM?.Therefore the Dajiuhu sediments can potentially improve our knowledge of the EASM evolution since the last glacial maximum in central China.In this dissertation,we measured several geochemical proxies,grain-size and pollen,to reconstruct climate changes since the last glacial maximum in Dajiuhu Basin.We also compared our Dajiuhu records with other available paleoclimate records,in order to provide an improved synthesis and understanding in the spatial-temporal characteristics and centennial-millennial scale abrupt oscillations of the ASM.Main conclusions are outlined as follows:1.Peatland development and its influence factors in Dajiuhu Basin.Based on the AMS¹⁴C dating and TOC,TN,Al and Ti records,we explore the peatland development history since the last deglaciation in Dajiuhu Basin.The results suggest that peat development presented significant responses to the large and rapid decreases in EASM precipitation,such as the Younger Dryas?YD?and 9.2 ka events.However,the obvious long-term decreasing trend of monsoon precipitation during the Holocene did not lead to the degradation of the Dajiuhu peat,probably resulting from relatively stable?summer?temperature.2.EASM variability recorded by the Dajiuhu peat during the Holocene.The geochemical records in Dajiuhu Basin,including Rb/Sr,?¹³C and C/N,were analyzed to explore the climate changes during the Holocene in Dajiuhu Basin.The Rb/Sr record is basically in agreement with the lake records from Gonghai and Huguangyan Lakes.It is indicated that the EASM intensity was relatively strong with several abrupt weak events during the early Holocene,which was probably associated with the changes of ice volume at northern hemisphere.The Holocene optimum period occurred at the mid-Holocene.Then the EASM intensity decreased obviously,probably responding to the changes in summer insolation at northern hemisphere.3.The 9.2 ka event:the strongest millennial scale collapse of the Asian summer monsoon during the Holocene.The geochemical proxies and pollen analysis show that the peat development was interrupted abruptly at around 9.2 cal ka BP,when the chemical weathering strength decreased and the tree-pollen declined.This suggests that a strong drier regional climatic event occurred at around 9.2 cal ka BP in central China,which was more obvious than the 8.2 ka event.In addition,based on the synthesis of our peat records and other Holocene hydrological records from ASM region,we further found that the9.2 ka event probably constituted the strongest abrupt collapse of the ASM during the full Holocene interval.We infer that the decline of the solar irradiance probably leads to the 9.2 ka event by driving the hydrological cycle in low latitudes directly?low latitude processes?,which is different from the 8.2 ka event,induced by the fresh water input in the North Atlantic?high latitude processes?.4.EASM variability recorded by the Dajiuhu sediments during the last glacial maximum and deglaciation.The records of Rb/Sr,Zr/Rb and grain-size in Dajiuhu Basin suggest that the EASM intensity remained weak without obvious oscillations,probably resulting from the weak insolation and the stable sea level and global temperature during the period of22.8-17.5 cal ka BP.During 17.5-15.5 cal ka BP,the precipitation decreased,which may be associated with the declined temperature gradient between northern and southern hemisphere.Meanwhile,the fresh water input in the North Atlantic also had influences on the weak EASM.However,precipitation began to increase since 15.5 cal ka BP,with no obvious decline during the YD event period,probably due to the retreat of ice sheet,the increased summer insolation at northern hemisphere and the climate conditions at the tropical Pacific.In addition,we also obtained a sediment core from Shuangchi Lake in northern Hainan Island,tropical China,of which geochemical proxies and grain-size were analyzed.The results show that precipitation was enhanced during the Little Ice age?LIA?in this region,and this was consistent with other hydrological records from tropical southern China.The synthesis of hydrological records from East Asia and the tropical Pacific suggest that the co-existence of a contracted Intertropical Convergence Zone and strengthened Pacific Walker circulation could contribute synchronously to the increased precipitation in the tropical region of southern China during the LIA.