Vegetation Evolution And Climatic Change Documented By Hala Lake Sediments In The Northeasten Qinghai-Tibetan Plateau Since The Last Glacial Maximum

A series of dramatic climatic changes,such as Younger Dryas?YD?event,have occurred on the global scale since Last Glacial Maximum?LGM?.In China,more precise and fine dated geological records are needed to provide reference for wide effect of YD event.Alao,there are great regional differences of moisture condition change in China during the Holocene,for controlled by different atmospheric circulation systems.The northeastern Qinghai-Tibetan Plateau,influenced by Asian monsoon system and the westerlies,shows more complicate character of climate change.Researching the climate change in this key region helps to understand the regional response to global climate and interaction of Asian monsoon and the westerlies.Hala Lake?97°24?-97°47?E,38°12?-38°25?N,4078m a.s.l.?,located in the northeastern Qinghai-Tibetan Plateau,was selected as the study site.A core HL3B?576-cm-long?was restrived in the center of Hala Lake in 2013.In this study,we had built a age-depth sequence for HL13B through comparison with HL13A,which was dated by AMS ¹⁴C,and finished 84 pollen samples to reconstruct the history of vegetation evolution in Hala Lake area since the LGM.Some other proxies,like chironomid,grain size,loss on ignition and magnetic susceptibility,were also analyzed and combined with the results of pollen analysis to reconstruct the climatic change of this area since the LGM.Main conclusions are outlined as follows:1.The result of pollen analysis in core HL13B showed that during the LGM?24.3-15.2 cal ka BP?the vegetation in Hala Lake basin was extremely sparse and mainly covered by alpine desert steppe dominated by herbs like Artemisia,Amaranthaceae?=Chenopodiaceae?and Poaceae.Between 15.2 and 12.9 cal ka BP,Hala Lake basin was mainly covered by alpine desert steppe vegetation dominated by Artemisia,Amaranthaceae,Poaceae,Cyperaceae and Asteraceae.Between 12.9 and 11.6 cal ka BP,alpine desert vegetation dominated by Amaranthaceae,Artemisia,Poaceae,Poaceae,Cyperaceae and Asteraceae.During the early and middle Holocene?11.6-5.1 cal ka BP?,vegetation condition became better,a wet alpine desert steppe and alpine meadow vegetations was dominated by Artemisia,Poaceae,Amaranthaceae,Cyperaceae.During the late Holocene?after 5.1 cal ka BP?,vegetation condition became bad and Hala Lake basin was covered by alpine desert steppe and alpine meadow vegetations with the same vegetation types as middle Holocene,with the expansion of desert vegetation dominated by Amaranthaceae and the degradation of alpine meadow dominated by Cyperaceae.2.In combination with sediment physical and geochemical variables(grain size,magnetic susceptibility,LOI₅₀₀00 and LOI₉₅₀),pollen and chironomid assemblages were employed to reconstruct regional climate change.During the LGM?24.3-15.2 cal ka BP?,this region experienced a cold and dry climate and there were two phases?24.3-22.7 cal ka BP and 20.0-17.0cal ka BP?,in which the grain size of sediment was distinctly fine,probably indicating lake being frozen during extremely cold periods.Between 15.2 and 12.9 cal ka BP,the climate became warmer and wetter,this time may be equivalent of Bolling-Allerod event.Between 12.9 and 11.6cal ka BP,the temperature was relatively low and the moisture was down to the minimum of the past 24.3 ka.This period was likely to be equivalent of Younger Dryas event.During the early Holocene?11.6-8.2 cal ka BP?,the temperature increased constantly,the moisture reached the maximum of the past 24.3 ka.Around 8.2 cal ka BP,there was an obvious cold and dry climate event,which was equivalent of the famous 8.2 ka cold event.During the middle Holocene?8.2-5.1cal ka BP?,effective moisture declined significantly and the climate became drier.During the late Holocene?since 5.1 cal ka BP?,the effective moisture continuously declined.3.The result of core HL13B from Hala Lake corresponded with the results from other cores of the same lake and other archives of Qinghai Lake,Genggahai Lake,Chaka Salt Lake and had the similar tendency with India summer monsoon?ISM?and the solar radiation intensity in June at35?N,indicating that pollen and other proxies in HL13B could recorded the regional climate in Hala Lake and northeastern Qinghai-Tibetan Plateau.During the LGM,the climate in Hala Lake was primarily controlled by the westerlies.In the early Holocene,with the intensity of ISM,effective humidity increased and reached the maximum.In the middle and late Holocene,with the recession of ISM,effective humidity gradually declined.The climate evolutionary process in Hala Lake since the LGM reflected by multi-proxies of core HL13B was primarily driven by the change of summer insolation of Northern Hemisphere.As to the rapid changes of millennial and centennial scale?Younger Dryas?,North Atlantic Thermohaline Circulation?THC?might be the main driving force.