Source To Sink Process Of Aeolian Loess In Lower Reaches Of Yangtze River And Yellow River And Late Quaternary East Asian Aridification

The long-ranged transports of fine-grained dust directly or indirectly has an important impact on the climate,the ecological environment and human health.Therefore,the transportation of atmospheric dust has been widely concerned.Quaternary eolian loess was deposited by wind-borne dust.It directly recorded the ancient sand-dust storm activity and atmospheric circulation information.Therefore,research on the provenance of aeolian loess can help to examine the atmospheric circulation patterns,clarify the complex dust transport process on the earth's surface,and provide insight into the ancient sandstorm activity and aridification history in the source area.The source to sink process of eolian loess has become a hot spot in geomorphology and Quaternary science.To better understand the deposition process of aeolian loess and clarify the paleoclimatic significance for the occurrence of loess deposition,this study takes Xiashu loess,Xuancheng yellow-brown soil and Taohuayu loess as the research objects,and conducts a detailed study of tracing source by trace,rare earth element geochemistry and detrital zircon U-Pb age spectrum.Furthermore,combining the provenance results and the available chronological data,this study first establishes the sedimentary mode of aeolian loess in the Yangtze River and the Yellow River downstream,and discusses the stepwise expansion of Quaternary East Asian aridification and its potential driving mechanism.The main knowledge gained in this study are as follows:(1)The elements of V,Cr,Zr,Hf,and Cs in Xiashu loess,Xuancheng loess,Taohuayu loess and their potential provenance samples have obvious grain-size effects.Sc,Th,Y,Nb,Ta,U,and Ti are trace elements of independent grain-size in the Yangtze River downstream sediments.Th is an independent grain-size element in Luochuan loess;Sc,U,Ta,Nb,Y,and Ti are independent grain-size elements in<20?m components of Luochuan loess.Sc,Y,Nb,Ta,Th,U,and Ti are independent grain-size elements in Xuancheng loess and Shuiyang River sediments.Ti,Rb,Y,Nb,and Ta are independent grain-size elements in modern Yellow River Sanmenxia and downstream sediments,as well as Taohuayu loess.The chondrite-standardized distribution models of rare earth element from Xiashu loess,Xuancheng loess,Taohuayu loess and their potential provenance samples all show the characteristics of upper crust material,indicating that the rare earth element distribution model is difficult to be used for the tracing provenance of aeolian loess.Rare earth element parameters and stable element ratios are potential tracers.(2)Sc-Th-La and Ta*10-Nd-Cr/10 element ternary diagram;the element ratios of Zr/Sc,Th/Sc,Cr/Th,Sc/Th,Ta/Th,La _N/Yb _N and Eu/Eu*can be used to distinguish Luochuan loess and Yangtze River sediments.The element ratios of Th/Sc,Ta/Th,Sc/Th,La/Sc,Cr/Ta,Nd/Ta,Cr/Th,La _N/Sm _N and Gd _N/Yb _N can be used in discrimination of the Shuiyang River and the Yangtze River sediments.Cr/10-Nd-Ta*10 element ternary diagram;the element ratios of Nb/Ta,La/Nb,Co/Th,Ta/Th,(La/Sm)_N,(Gd/Yb)_N and(La/Yb)_N can be used for discrimination of Luochuan loess and Yellow River downstream sediments.(3)The characteristics of elemental geochemistry in<5?m and some 5?10?m components of Xiashu loess along the Yangtze River are similar with Luochuan loess,indicating that the fine-grained components of Xiashu loess are mainly derived from arid areas of inland Asia.The sedimentary mode of the fine-grained components of Xiashu loess is mountain source-river transport-desert transition-Asian inland sandstorm transport.The geochemical characteristics of the 10?20?m and 20?50?m components of the Xiashu loess are similar to the Yangtze River sediments;The zircon U-Pb age spectrum of the Xiashu loess shows the six main age components of 300?180Ma,540?360 Ma,900?720 Ma,1080?900 Ma,2040?1800 Ma,and 2600?2400 Ma,which are similar to the Yangtze River sediments;These evidences indicate that the coarse-grained components of Xiashu loess are mainly derived from the Yangtze River downstream sediments;The sedimentary mode is mountain source-Yangtze River transport-wind transport in the Yangtze River downstream.(4)The characteristics of elemental geochemistry in Xuancheng loess are similar to the Shuiyang River sediments,and only the<5?m component has signals of the Yangtze River downstream sediments;The zircon U-Pb age distribution of Xuancheng loess shows 6 age peaks:136 Ma,440 Ma,844 Ma,944 Ma,1800 Ma and 2476 Ma,which are different from the Chinese Loess Plateau loess,the Xiashu loess and the lower Yangtze River sediments;These evidences indicate that the Xuancheng loess is mainly composed of the local river sediments.The Yangtze River downstream sediments contribute only to the<5?m component;The sedimentary mode of Xuancheng loess is mountain source-river transportation-local wind transportation in Xuancheng area.(5)The characteristics of element geochemistry in the four grain fractions of the Taohuayu loess are very similar to the Yellow River downstream sediments;The zircon U-Pb age distribution of the Taohuayu loess shows five characteristic peaks:298 Ma,444 Ma,910 Ma,1924 Ma and 2559 Ma,which are consistent with the lower Yellow River sediments;These evidences indicate that Taohuayu loess originated from the lower Yellow River sediments;The characteristics of elemental geochemistry in modern Yellow River Sanmenxia sediments are very similar to Luochuan loess,indicating that the Chinese Loess Plateau loess has become the main source of modern sediments in the middle and lower reaches of the Yellow River;The sedimentary mode Taohuayu loess is mountain and Chinese Loess Plateau loess source-Yellow River and its tributaries transport-wind transport in the lower Yellow River.(6)The nearby-source deposition and initial deposition of Taohuayu loess at0.9?1.0 Ma indicate that aridification has occurred in the lower reaches of the Yellow River at?1.0 Ma;The near-source deposition of coarse-grained components and initial deposition of the Xiashu loess at around 0.9 Ma indicate that the 32°N area of southern China has formed an arid environment at?0.9 Ma;The local wind-transported dust deposition and initial deposition of Xuancheng loess at around 70 Ka confirm the 30°N areas of southern China begin to appear arid environment during the last glacial period.The tectonic uplift of the Qinghai-Tibet Plateau is difficult to explain the Quaternary arid environment of southern China.The beginning of aridification in the lower Yangtze River,the lower Yellow River and the Xuancheng area of Anhui corresponded to the expansion of the ice sheet during the Middle Pleistocene and the last glacial period,respectively.Therefore,ice sheet expansion and global cooling are considered the potential driving mechanism for the stepwise expansion of Quaternary East Asian aridification.