The Sedimentary And Ecological Environment Research Of Jilantai Basin During The MIS 3

There are numerous lakes in the arid and semi-arid area of western China,which are very sensitive to the paleoclimate and paleoenvironment,such as the regional drought-wet changes and the ecological evolution.Therefore,by researching the sedimentary and geomorphic evidence,we can rebuild the history and process of the lake level fluctuation during the lake evolution,and investigate the evolution of the regional paleoenvironment.In recent years,the OSL dating of some paleo-lakeshore terrace seems to deny former researches,which insisted that the paleolake in western China were generally occur high lake level during the marine isotope stage 3(hereinafter referred to as MIS 3)by 14C dating results;and some dating results had been explained that the paleoclimate and paleoenvironment of western China were different to the rest of the world during MIS 3.Obviously,whether or not the paleolake develop widely in western China is still at variance in academic circle at present.Previous researches suggested that there was once a mega lake covering most area of the Jilantai Basin and Hetao Plain sometime prior to 60?50 ka.The research of the "Jilantai-Hetao"Megalake is significant for understanding either evolution of the Yellow River in Hetao region in late Quaternary,character of the regional climate,or evolution of the Ulan Buh Desert and Hobq Desert.If there was a mega lake,the Jilantai Basin studied in this paper was located in the western lake basin.The previous researches suggested that the Jilantai Basin once appeared the paleolake during the MIS 3 by 14C dating results,the highest lakeshore terrace of the paleolake was formed before 60?50 ka,and a series of lakeshore terrace with the altitude of 1060-1050 m and 1044-1035 m,respectively,were formed until to the Holocene.However there were no evidences showing that the sediments formed in the phase of the MIS 3 came from the borehole drilled in the present Ulan Buh Desert approach the present Jilantai Salt Lake.So the paleoenvironment of the present Ulan Buh Desert during the MIS 3 had been construed as desert by previous researches.The research situation indicates that it is necessary to study the natural environment of the Jilantai Basin during the phase of the MIS 3.The paper aims at providing some suggestions about this question.Therefore,this paper excavated the new sections(1060-1050 m a.s.l.),located in the western Jilantai Salt Lake of which the lakeshore terrace was formed in the MIS 3 according to previous researches,and measured the OSL age of eolian sand from the sections by the quartz grains.In addition,this paper drilled sediments from 81 m depth in Dengkou subuplift within the Jilantai Basin,trying to reveal whether or not the Jilantai Basin was a paleolake during the MIS 3 by contrastive analysis of the sedimentary sequences and the OSL dating results of the sections and borehole.Besides,this paper preliminarily studied the ecological environment of the Jilantai Basin during the MIS 3 by pollens analysis results.This paper's preliminary conclusions are as follows:(1)The Jilantai Basin once was covered by a deep lake before 76 ka,and the results of this paper furtherly support that if there was the so-called Megalake Jilantai-Hetao covering the most area of the Jilantai Basin and the Hetao Basin,the highest lake level should be sometime prior to 76 ka and likely before 90 ka if OSL ages of k-feldspar were widely accepted.Between the phase of 76?52 ka the lake level of the Jilantai Basin was reduced,and there was accumulated aeolian sediments within the basin.During the time span of 52?39 ka,area of the Dengkou Subuplift was submerged by a high lake level,which might be originated from a northward shift of the center of the Jilantai-Hetao Basin as a response to the accelerated uplift of the Qinghai-Tibet Plateau.During the phase of 39?13 ka the Jilantai Basin was still covered by a shallow paleolake,and the water of the lake was extended to the Dengkou subuplift around 23 ka,but the area of the paleolake was limited in the Jilantai Basin and the high lake level was fluctuated between 1060-1050 m a.s.l.during the other phase of 39?13 ka.During the phase of 13?8 ka,the Dengkou Subuplift was flooded again by the inflowing of the Yellow River through the present southeast Ulan Buh Desert to form the namely Tushenze lake.(2)The sedimentary facies and the OSL dating results of the sections found in western Helan Mountain suggest that during the phase of 57?32 ka,there was a large scale of surface runoff caused by the rainfall of the western Helan Mountain.And the paper finds out that the surface runoff fed into the Jilantai Basin.Therefore the paper point out that the surface runoff had an importance contribution to the rising lake level of the Jilantai Basin during the phase of the MIS 3.(3)The paper was isolated pollens from the sediments in the borehole(named DK 12)and the sections S40,S42.The species identification and content analysis of the pollens suggest that the overwhelming majority of the pollens in the Dengkou subuplift were Chenopodiaceae,Artemisia,Poaceae,Typha,Cyperaceae,and the community's constructive species were composed of Chenopodiaceae during the phase of the MIS 3.The pollens types may indicate that the Dengkou subuplift had been contemporary covered by the paleolake and sahel,and the surrounding mountain forest might be characterized by Picea and Pinus during the phase of the MIS 3 by taking the relatively shortlived transport distance of these pollens into consideration.Besides,the sedimentary sequence and the pollens analysis results of the sections S40,S42 indicate that the most likely environment of the Jilantai Basin was shallow lake which was accumulated aeolian sediments in surrounding and the vegetation development might be poor during the phase of the MIS 3.