Genggahai Lake Recorded Vegetation And Climate Change Of Northeastern Qinghai-Tibetan Plateau Since The Last Deglaciation

Paleoeclimate studies are essential for understanding the future climate changes and their impacts on our environment.A key area for investigating long-term environment changes is the Qinghai-Tibetan Plateau(QTP)because the environmental changes of QTP are highly sensitive and strong effect to climate changes.The Gonghe Basin is an intermontane basin,surrounded by desert steppe and steppe on the northeastern TP,where is influenced by Asian monsoon and westerly.Due to its unique topography,the past palaeoclimate studies in this region show a complicate humidity pattern since the last deglaciation.Hence,an in-depth analysis of the vegetation composition and climate change is necessary for understanding the environment change of the northeastern QTP responsed to global change.Genggahai Lake,a shallow grass-type lake located at the Gonghe Basin,consists of Upper Genggahai Lake and Lower Genggahai Lake.The Upper Genggahai Lake(36°11'N,100°06'E,2860 m a.s.l.)has a surface area of-2 km2.Maximum water depth is about 1.8 m.In February 2008 a 782-cm-long drilling core(GGHA)and a 750-cm-long drilling core(GGHB)were obtained from the center of Upper Genggahai Lake.By comparing the grain-size and magnetic susceptibility profiles of the two cores,missing sediments from core GGHA were supplement by corresponding sediments in core GGHB.The chronology for the lake sediments(GGHA)was established using accelerator mass spectrometry(AMS)14C dating of twelve aquatic plant macrofossil samples.For this study,a total of 156 samples from the well-described sediments of the core GGHA were selected for pollen analysis to detect vegetation compositions on Gonghe Basin and the climate changes since 15.3 ka.The results of the fossil pollen analysis showed that the study area was mainly covered by steppe-desert/steppe vegetation dominated by Artemisia,Amaranthaceae(=Chenopodiaceae)and Poaceae over the last 15.3 ka.In detail,six stages of regional vegetation change and climatic change were documented:(i)From 15.3 to 11.7 cal.ka B.P.,the vegetation was dominated by Artemisia and Poaceae,while the fluctuant A/C ratio indicated an unstable climatic conditions during this period,in particular at 12.9-11.7 cal.ka B.P.the expension of steppe desert responsed to rapid climate change(Younger Dry as event),(?)The percentage of tree pollen increased generally since 11.7 cal.ka B.P.;particularly,the vegetation,composed by conifer(Pinus),was dominated montane forest that showed a warm and moist between 10.2 and 6.3 cal.ka B.P..On the contrary,the low A/C ratio indicated a relative low humidity during this period,resulting in steppe-desert surrounding the study area.(?)From 6.3-5.6 cal.ka B.P.,montane coniferous forest declined and Artemisia-dominated desert-steppe vegetation that coversed the lake basin,showing that the relatively warm and the moist period came to an end.On the other hand,the A/C ratio and pollen concentrations increased with some fluctuations,which showed an increased effective moisture at the Genggahai lake.Additionally,a slightly high percentage of Cyperaceae pollen possibly reflected a small area of wetland around the lake,(?)During the 5.6-4.1 cal.ka B.P.,the montane forest continual declined and almost disappeared at 4.1 cal.ka B.P.,accompanied by expanding Artemisia-dominated steppe/desert steppe and shrinking Cyperaceae-dominated swamp meadow.On the contrary,the increasing value of A/C ratio indicated that the climate of Genggahai Lake area became relatively humid during this period.(?)From 4.1 to 3.1 cal.ka B.P.,local vegetation was steppe and steppe desert,while the swamp meadow became degraded because of limit moisture,and the high fluctuation of the A/C ratio indicated an unstable climatic condition in the study region.(?)After 3.1 cal.ka B.P.,the deceasing percentages of Cyperaceae and Artemisia and the increasing percentage of Ephedra and Nitraria(typical arid plants)illustrated that the climate was more arid than former stage.The percentage of tree pollen showed a consistent high-and-low variation with the intensity of Indian summer monsoon,reflecting strong coherence between the changes of montane forest and the summer monsoon.During intervals of strong summer monsoon montane forest expanded,and montane forest declined which could be linked to the weaker summer monsoon.The percentage of tree pollen was quite high during the early-mid Holocene,indicating that it was the warm and wet period.The peroid was consistent with recent paleovegetation records from the northeastern QTP such as Qinghai Lake and Dalianhai Lake,although these records show the different timing of maximum forest extension.The pollen source area,transport agent and depth-age model might be the reasons to lead to a discrepancy among the three lakes in terms of time of maximum tree pollen.It was notable that the low A/C ratio and the high percentage of tree pollen appeared simultaneously,in particular,during the early-mid Holocene.The low A/C ratio resulted most likely from a strong NH summer insolation that caused high evaporation at Gonghe Basin.Moreover,high insolation in the QTP could enhance the landscape temperature,which as a result could generate current downward flow that could reduce effective moisture at the north eastern border of Tibetan plateau and its surrounding areas.