Environmental Evolution Of The Region In Qinghai Lake Since The Last Glacial Interglacial Stage Recorded By Chromaticity And Clay Minerals

The Qinghai-Tibet Plateau is one of the special geographical units in the world.Located in the northeastern part of the Qinghai-Tibet Plateau,the Qinghai Lake region has an elevation of 3169-4649 m,due to the common influence of South Asia,East Asia,Plateau monsoon and westerly circulation,the sediments in the Lake District are sensitive to climate and the climate environment is relatively complex.Moreover,the geographical location of the sediments in the Lake District makes them a good information carrier for the study of palaeo-environmental evolution and Plateau uplift.Therefore,the sedimentary environment of Qinghai Lake area was recorded by using chroma and clay mineral index.In order to explore the significance of the sedimentary environment recorded by the soil chroma,to let chromaticity studies more useful to Quaternary environ mental researches,and also to enrich the available surrogate indexes of paleoen viron mental changes in northeastern Qinghai-Tibetan Plateau,this study investigate an exposed profile(Dashuitang section)in a wind formed sand dune that lies in the east lake margin of Qinghai Lake.This article takes the Qinghai Lake area Dashuitang profile(QDST)with a thickness of 10 m as the research object,and use s the CM-5 spectrophotometer and the Rigaku D/Max 2500 PC X-ray diffractomet er to measure the chromaticity parameters of 232 soil samples(a*,b*,L*,C*,h*)and clay mineral content,analyzed the soil color and clay mineral change charact eristics,supplemented by SC/D(grain sensitivity index),magnetic susceptibility,Rb/Sr,CO₃^2-,organic matter,p H value and other indicators,combined with OSL dating,restored the paleoen vironmental evolution process in the Qinghai Lake area since the ice stage(32 ka)during the last ice period.The results show that:(1)From the bottom of profile upwards,a*,b*,and C*first decreased then increased and finally decreased again.The high value indicates a warm and humid climate,while the low value indicates a cold and dry climate.From the bottom of profile upwards,L*,h*decreased sharply and then increased slightly,and shown that the value in paleosol layer is slightly lower than the aeolian sand layer,and the high value shown a cold and dry climate.L*can reflect the precipitation during the formation period of different sedimentary layers;the high value of h*indicates the cold and dry climate,and the low value indicates the warm and humid climate.(2)There are 8 main clay minerals in the QDST profile,with obvious content changes,which can record certain paleoenvironmental information and reveal the paleoclimate characteristics of different sedimentary formation periods.Clay min erals in the profile are mainly dominated by illite(I)content(67.10%),followed by(kaolinite+chlorite)combination(27.13%)and chlorite(16.95%),kaolinite(10.18%)and montmorillonite(0.39%)content is lower.The content of clay mine rals is:illite>(kaolinite+chlorite)>chlorite>I_C/S>kaolinite>C/S(green-smudge mixe d layer)>I/S(Immonite mixed layer)>Mongolian Off the stone.Among them,illite(I)and chlorite(C)in the QDST section can be used as signs of dry and cold clim ate;montmorillonite(S)can be used as a direct indicator of atmospheric prec ipitation;high content of kaolinite(K)in the profile indicates better hydrothermal conditions in climatic environment;the content of kaolinite+chlorite(K+C)can reflect the drought degree of the climate in Qinghai Lake;I/S minerals can be used to represent the change of the cold,warm,and dry climate since the formation of QDST profile.(3)QDST profile chromaticity have good comparisons with typical climate substitute indicators such as particle size,magnetic susceptibility,geochemical elements,organic matter,and p H value.The results show that a*and b*are signifi cantly positively correlated with organic matter,a*is significantly negatively correlated with p H,and L*corresponds well to the peak and valley regions of p H;a*,b*,C*and magnetic susceptibility,L*and Rb/Sr are significantly negatively correlated,L*and carbo nate are significantly positively correlated and consistent with the SC/D value changes;carbonate contributes more to L*.(4)Since the last glacial interglacial stage(32 ka),the climate in Qinghai Lake region has been developing in a cold and dry direction.Qinghai Lake region has experienced several times of warm-wet and cold-dry climate alternations during the past 32 ka.It can be further divided into 4 time periods:Firstly,a*,C*are higher,b*is the highest,h*is lower,the content of illite and chlorite is relatively low,the content of kaolinite is relatively high,and the content of montmorillonite is the mo st,the climate was warmer and wetter during the Interstadial of the Last Glacial(32.0-23.3 ka).Secondly,a*,b*,C*is low,L*,h*are high,illite content is the highest in the whole profile,kaolinite content is low,the Last Glacial Maximum(23.3-15.2 ka)climate was generally cool and moist.Thirdly,a*and b*are lower,L*is the lowest,illite and chlorite content are low,kaolinite content is higher,the Last Deglaciation(15.2-10.4 ka)was cool and humid Finally,and the chromaticity parameters and clay mineral curve trough peaks fluctuated significantly,the Holocene(10.4 ka to date)climate varied frequently between warm and cold stages.The results of palaeo-environment evolution since the last glacial interglacial stage(32 ka)in Qinghai Lake are in good agreement with the typical summer monsoon index(EASMI)and the oxygen isotope EA^?18O index reconstructed from stalag mite data,and the results of palaeo-environment evolution inversion are in good agreement.