Characteristics Of Chemical Weathering And Similarities And Differences In The Northeast Tibetan Plateau Since Holocene

The northeastern Tibetan Plateau,with an altitude of more than 3200 m,is high in the west and low in the east.The climate belongs to the alpine arid-semi-arid continental climate and the alpine semi-arid grassland climate,with annual precipitation below 600 mm.The northeastern Tibetan Plateau is located in the northwestern margin of the Asian monsoon region.It is affected by the monsoon circulation,westerly circulation and plateau monsoon,and is more sensitive to climate.Combined with the special geological structure and landform features,the combination of climate water and heat characteristics in the region determines chemical weathering.The degree is not only similar but also different in space.In this paper,three aeolian sediment profiles（LQ profile,MS profile,QDST profile）in the northeast of the Tibetan Plateau are taken as the research objects,combined with ¹⁴C and OSL dating and constant elements and CIA,Na/K,Rb/Sr,A-CN-K chart and migration rate and other chemical weathering indicators are used to analyze and discuss the degree of chemical weathering in the northeast of the Tibetan Plateau and the differences in weathering in different regions.The following research results are obtained:（1）The geochemical element combinations in the LQ section,MS section and QDST section in the northeast of the Tibetan Plateau are consistent,and the constant element variation coefficient（CV）values of each section are low,indicating that the composition of sediment sources in each section of the northeast of the Tibetan Plateau is basically the same.The chemical weathering parameters Ki,Rb/Sr and CIA of each section showed a consistent change trend in the stratum,and the high values all appeared in the paleo-soil layer,and the low values corresponded to the paleo-sand layer;And the change trend of Saf,Na/K,Ba,w of each sectionin the stratum is basically the same,the low value corresponds to the ancient soil layer,and the high value corresponds to the ancient aeolian sand layer and fluvial sedimentary layer.However,the CIA and Na/K values of different profiles are different.The CIA values of LQ,MS and QDST profiles are ranked as LQ（50.45）>MS（40.01）>QDST（32.42）;the Na/K values of LQ,MS and QDST profiles The results of QDST（2.38）>MS（1.80）>LQ（1.46）indicate that the LQ profile has the strongest chemical weathering and is in the primary stage of chemical weathering.The MS profile has weaker chemical weathering and the QDST profile has the weakest chemical weathering.（2）The A-CN-K triangle diagram and element migration characteristics show that the LQ,MS and QDST profiles in the northeast of the Tibetan Plateau are basically in the stage of elementary chemical weathering,but the degree of weathering is slightly different.The plagioclase was chemically decomposed during the chemical weathering of each section.Among them,the LQ section has completed the early decalcification and Na stage,and has not reached the middle stage of K removal;the MS section is undergoing decalcification and Na stage;the QDST section has not reached Ca and Na stages.（3）The process of weathering and soil formation in each section of the northeastern Tibetan Plateau is basically the same,but the formation time of the sedimentary facies of the same section is different.All profiles have experienced the paleo-aeolian sand period with cold and dry climate and weak chemical weathering in the early Holocene（11-8 ka BP）;the overall climate is warm and humid with strong chemical weathering in the mid-Holocene（8-4 ka BP）The period of consolidation and soil formation;the climate and environment of the late Holocene（4 ka BP-present）gradually deteriorated and tended to be cold and dry,and the period of chemical weathering was weak.（4）The elemental composition of the LQ,MS,QDST section and the comparative section（DQ,LC,XF section）in the northeast of the Tibetan Plateau are basically the same,but the content of SiO₂ and Na₂O is higher than that of the comparative section,Al₂O₃,Fe₂O₃,CaO,MgO and K₂O The low content indicates that the climate was relatively cold and dry during the formation of the three profiles.The chemical weathering degree of each section is obviously different,which generally shows the change trend of XF section>LC loess>LQ section>upper crust UCC>MS section>DQ section>QDST section.The CIA values of the LQ,MS,and QDST sections are significantly lower than those of the LC loess and XF sections;however,the CIA values of the LQ,MS,and QDST sections are similar to those of the DQ section,indicating that the chemical weathering degrees of the sections located in the northeast of the Tibetan Plateau are similar.