Variation Of Quaternary Dust Composition And Source In East Asia Revealed By The Soluble Salt Of Loess-paleosol Sequence In The Chinese Loess Plateau

Chinese loess is an important record that preserved abundant information about the dust cycle in East Asia and even on globle scale,however,the variations of the source and composition of the Chinese loess are still controversial.In particular,the dust emission potential and long-term variation of different point sources in dust source area,such as lakebed,gobi,sandy desert,alluvial fan and bedrock was not well understood,which inhibit us to accurately quantify and simulate the dust emission potential,emission rate and the variation of the dust composition.Soluble salt is an important component of the global dust,of which the generation,transport and deposition are related to a series of climatic and environmental processes from the source area to atmosphere and to depositional regions.Therefore,the analysis of soluble salt and its variation in loess is not only of great significance for further understanding the emission potential and evolution of point sources,especially lake landform,but also can provide geological evidence for understanding the environmental effects of soluble salt.In this paper,we systematically measured and analyzed the soluble salt(including Na⁺,K⁺,Mg²⁺,Ca⁺,Cl^-,SO₄^2-,NO₃^-)of the loess-paleosol sequence from the Jiuzhoutai section in the western Loess Plateau and from Lingtai section in the central Loess Plateau.Moreover,the soluble salts of lakebed in the potential source area of loess was also analyzed and compared with that from the Loess Plateau.Combined with numerous geomorphic,stratigraphic and paleoenvironmental evidence from the dust source area,we finally have identified the types and spatio-temporal changes of soluble salts in Quaternary loess-paleosol of the Chinese Loess Plateau.Then,the major control factors determining the soluble salt variation in the loess-paleosol was addressed and the driving mechanism of soluble salt variation in loess-paleosol was also discussed.The following is the major conclusions of this study.1.The soluble salts in soil of the Chinese Loess Plateau increased at～1 Ma and～0.5 Ma of which the increase of Na⁺,Cl^-and SO₄^2-was the most obvious,indicating that the soluble salts in Chinese Loess Plateau were mainly sulfate and chloride.The content of soluble salts in the western Chinese Loess Plateau is several to tens of times as much as that in the central Loess Plateau.2.In comparison with the soluble salts of the lakebed,surficial soil and lakewater in the dust source area,and with the paleoclimate evidence of the Chinese Loess Plateau,we found that the soluble salts in the loess-paleosol are mainly originated from dust input since the Middle Pleistocene（～1 Ma）.Abundant geomorphological and sedimentological evidence confirmed that the lake evolution in the dust source area is closely related to the enhancement of the soluble-salt-bearing dust at 1 Ma and 0.5 Ma in East Asia.3.The increasing emission of soluble-salt-bearing dust at 1 Ma and 0.5 Ma from the Lingtai section and Jiuzhoutai section,which indicats a change in the composition of Asian dust,manifested by a gradual increase in salinity and alkalinity.4.Global climate change and regional tectonic activity determined the evolution of lakes in the dust source area,and then affected the emission flux of soluble-salt-bearing dust in East Asia.Firstly,the joint effect of global cooling caused by the Middle Pleistocene Transition and the the Tibetan Plateau uplift promoted the aridity of the dust source area since～1 Ma,resulting in large-scale exposure of the lakebed and the emission of soluble-salt-bearing dust.Secondly,the Middle Brunhes Transition enhanced the East Asian summer monsoon during the interglacial periods,which induced the expansion of lakes in the dust source area.Meanwhile,the high temperature strengthened the potential evapotranspiration around the lake,forming extensive erodible salt crust and saline soil to provide rich soluble salts for the Chinese Loess Plateau.During the glacials,the weakened Asian summer monsoon caused desert expansion and lake shrank,and the exposed lakebed became the soluble-salt-bearing dust source again.5.In the future,due to global warming,EASM will continue to increase,not only increase the lake area,but also increase the evapotranspiration process.Under the influence of wind erosion,more soluble salt is discharged into the atmosphere,further changing the atmospheric components.