Paleoclimatic Changes Of Typical Periods Recorded In Lake Sediments On The Tibetan Plateau

The unique geomorphic features and thermal conditions of the Qinghai Tibet Plateau play an important role on climate change of Asia,even the global climate.Lakes on the Qinghai Tibet plateau are good carriers for the paleoclimate research.The last deglaciation climate has experienced dramatic fluctuations,which is an important period of studying gradual and mutation processes.Here we researched the sediments of Salt Lake Dangxiong Co terrace and Salt Lake Zabuye by biomarker indices?alkenones and n-alkanes?during the last deglaciation.The last deglacial sediments of Salt Lake Dangxiong Co are rich in alkenones,which are characterized by a predominance of C₃7 Methyl alkenones?C₃7:Me?and C₃8 alkenones,a small proportion of C₃9 alkenones,and trace amounts of C₄0 alkenones.Sources of long chain alkenones during 20.2¹2.0 ka BP might belong to Greenland species and / or G.oceanica;sources during 12.0¹1.4 ka BP might belong to C.lamellosa.Based on the unsaturation index of long chain alkenones,the reconstructed mean annual air temperature during the late Last Glacial Maximum was about 0.2 °C,while the B?lling/Aller?d period was characterized by a rapid increase in temperature from 0.8 °C to 2.4 °C,?2.9?followed by an abrupt decrease in temperature to-0.9 °C in the early Younger Dryas.The alkenones and n-alkanes record indicates that Dangxiong Co experienced a cold and wet climate in the early part of the last deglaciation after the Last Glacial Maximum,a warm-dry B?lling/Aller?d,and a cold and dry early Younger Dryas.Based on the study of distribution characteristics and compound-specific carbon isotopes of n-alkanes from Lake Zabuye,we found that the paleoclimatic significance of the ?¹³C values among different carbon chain lengths might be different.The ?¹³C values of mid chain n-alkanes are mainly controlled by the carbon source of lake water and lake antigenic environment,which probably reflects the change of aquatic environment and lake water temperature.Long chain n-alkanes mainly come from the lake surrounding vegetation,the compound specific ?¹³C values of which may be the coupling of regional temperature and humidity,indicating the variations of regional effective moisture.According to the distribution characteristics of n-alkanes and compound specific carbon isotopes,change of algae,bacteria,aquatic plants,herbs and woody plants during the last deglaciation are reconstructed.The proportion of aquatic plants in lake sediments was higher than the surrounding terrigenous plants.During the early part of the last deglaciation?23.1¹6.8 ka BP?,the climate changed from cold-moist to cold-dry condition,algae,bacteria and aquatic plants was prosperous,proportion of herbaceous plants was higher than the woody plants;during B?lling/Aller?d period?16.8¹2.6 ka BP?,all proxies implied warm climatic conditions,woody plants were gradually prosperous;and the Younger Dryas?12.6¹1.8 Ka BP?was characterized by cold and wet climate,with algae and bacteria dropped,submerged and floating plants flourished,proportion of woody plants was higher than the herbaceous plants.Salt Lake Dangxiong Co and Zabuye have experienced approximately the same temperature change,which was coherent with the classical climate pattern observed in the Northeastern Atlantic,implying the existence of a teleconnection between Asian monsoon area and the North Atlantic,and high latitude climate variations.However,the humidity changes of two lakes were slightly different,which might be related to the sampling resolution,the difference of the lake environment and uncertainties of these indices.Past Global Changes?PAGES?and Asia paleoclimate change during last two millennia?PAGES-Asia 2K?focus on the high-resolution paleoclimatic reconstruction of the last 2000 year.In order to obtain high-resoltion data,Synchrotron Radiation X-ray Fluorescence was used to measure elements in the sediment from Lake Toson.Based on Principal component analysis?PCA?,the analyzed elements have been interpreted as: Factor-1?K,Ti mainly coming from lithogenic materials?may be mainly regulated by precipitation and aeolian processes;Factor-2?Ca?Sr?could be controlled by evaporation-precipitation,which is related to aridity;Factor-2?Br?is a biologically-related element,warmer temperature would increase biologic production.The data show a warm-dry climate during the Medieval Warm Period?MWP?and a cold-wet condition during the Little Ice Age?LIA?.Additionally,spectral analysis of the element record suggests that climate variability during the last millennium at Lake Toson was dominated by cyclical fluctuations with periodicities similar to those of ENSO and solar variability.