Grain-Size And Elemental Spatial Differentiation Of The Xiashu Loess In The Poyang Lake Region During The Last Glacial Age And Their Environmental Indicators

Since the Last Glacial Age,the loess in southern China has been widely distributed,which is considered to be similar to the Xiashu loess in Nanjing,and is collectively known as "Xiashu loess".Because of its geographical location and its uniqueness,it has attracted the attention of many researchers.As for its source,most studies are based on judging whether the source of loess in the Loess Plateau is the same as that in the Xiashu Loess Plateau.At present,it is believed that the source of loess in the south bank of the Yangtze River is the floodplain and the exposed riverbed of large and medium-sized rivers such as the Yangtze River during the glacial period.However,the regional distribution and differentiation characteristics of these loess are seldom discussed.Based on previous studies,the loess from the southern bank of the Yangtze River to the northeast of Poyang Lake in northern Jiangxi and the loess from the Houtian sand area to the Daguling Mountain in Xinyu in the southwest of Poyang Lake were selected for detailed field investigation and sampling.In these two regions,a section was arranged along the direction of the modern winter monsoon,along which several loess sections were collected.At the same time,some modern floodplain samples were collected along Yangtze River floodplain and Ganjiang floodplain respectively.The potential provenance of loess and the grain size and regional chemical differentiation of elements of loess were studied.Based on the analysis of the distribution of loess thickness,chromaticity,grain size characteristics,element geochemistry of loess samples less than 20 microns and modern floodplain samples,it is concluded that:(1)The loess in both regions gradually thinned from north to south.As the thickness of loess becomes thinner,the content of fine particles increases continuously,such as clay and end-member components EM1 and EM2,while the content of coarse particles decreases continuously,such as sand and end-member components EM5 and EM6.Meanwhile,the average particle size decreases gradually,which indicates that the particles become finer and finer.It shows that the loess in Xiashu was mainly transported from north to south;(2)The results of elemental geochemistry show that the geochemical composition of the Xiashu loess has obvious spatial differentiation.Along D-D section and H-D section,the ratio of stable element/stable element pair(constant)in the two regions changes regularly;(3)The characteristics of grain size parameters and geochemistry of major elements show that the material source of D-D section is Yangtze River floodplain,and the material source of H-D section is Ganjiang floodplain.The results show that during the last glacial maximum,the winter monsoon strengthened and the sea level fell,and the broad floodplain of the Yangtze River and Ganjiang River in the middle reaches of the Yangtze River provided a large amount of materials for the regional dust system,thus forming a number of regional wind-sand dust accumulation systems.