Transport Characteristics And Indicative Significance Of Pollen In The Middle And Lower Reaches Of The Yellow River And Sediment

Understanding the processes determining the composition of alluvial pollen assemblages and its relationship with watershed vegetation is a prerequisite for reconstructing the paleoenvironment and paleovegetation.The middle and lower reaches of the Yellow River are the core areas of the origin and prosperity of Chinese civilization.Since the late Holocene,the Yellow River has been frequently diverted and flooded with sediment,making it difficult to find an ideal place for pollen research in the Yellow River floodplain,resulting in a very weak reconstruction of the ancient environment in this area,which hinders our understanding of the relationship between the origin,development,evolution of the Yellow River civilization and the natural environment.Since the material source of the alluvial deposits in the lower Yellow River is mainly the middle reaches of the Yellow River,does the spore and pollen deposited along with the sediment also come from the middle reaches of the Yellow River? How does the spore and pollen transport after entering the Yellow River? The key to solving the above problems is to carry out research on the modern process of river pollen in the middle and lower reaches of the Yellow River.Only by clarifying the source and transmission mechanism of modern pollen in the Yellow River,can we "discuss the present from the ancient" and use the pollen from the alluvial deposits in the lower reaches of the Yellow River to scientifically reconstruct the ancient environment.In this study,the section of the Yellow River from entering the Shanxi-Shaanxi Gorge to Kaifeng was selected as the research section,with 31 main stream sections and 17 tributary sections arranged.A total of46 river water samples and 48 river sediment samples were collected during the flood season.The geographical detectors were used to explore the influencing factors in the process of pollen transmission in the middle and lower reaches of the Yellow River.Based on the correlation analysis between the land use data extracted from remote sensing images and the pollen assemblage of the tributary section,the research on the pollen source of the main stream in the middle reaches of the Yellow River and its relationship with vegetation was carried out.The method of dissimilarity coefficient was used to identify the pollen source area in the lower Yellow River and reveal its environmental indication significance.The main conclusions are as follows:(1)Based on the geographical detector and particle size analysis,it is found that the content of suspended solids is the main factor affecting the pollen concentration of the Yellow River,which is particularly evident in the sample points of the main and tributaries outside the Shanxi-Shaanxi Gorge.The high concentration of suspended solids and low concentration of pollen in the sample points of the main stream in the ShanxiShaanxi Gorge may be caused by the large amount of coarse sand and too fast flow velocity.The clay content is the main factor that affects the sediment pollen concentration,and the clay content and flow velocity have an interactive way of enhancing the sediment pollen concentration by two factors.After the Yellow River is located in the Shanxi-Shaanxi Gorge,the terrain gradually flattens,the river velocity slows down,and the hydrodynamic force weakens,resulting in a significant increase in sediment pollen concentration and clay content.(2)The pollen combination of river water samples and sediment samples in the study area is mainly herbaceous.The tributaries that contribute more to the tree pollen in the main stream of the Yellow River are Xinshui River,Sanchuan River,and Qin River,while the tributaries that contribute more to the tree pollen in the Wei River basin are Luo River.Herbaceous pollen mainly originates from the flood plain,and from channel bars and point bars,and there is no obvious relationship between herbaceous pollen and tributary inputs.(3)The relative proportions of the various land use classes in the middle reaches of the Yellow River can be ordered as follows: grassland(GL)> cultivated land(CL)> forest(FO)> shrubland(SH)> water(WA).The herbaceous pollen of the Huangfuchuan River,Kuye River and Wuding River are closely related to the coverage of GL;cereal pollen is not fully representative of the CL coverage in the watershed,which is closely related to the yield and preservation capacity of pollen;and the pollen of woody plants is extremely over-represented compared to the coverage of FO and SH in the watershed,which is related to the high yield and strong dispersal ability of some woody pollen.Because higher pollen yield will increase its proportion in the pollen combination,the actual vegetation abundance will be exaggerated when interpreting the pollen signal.(4)It is found that the pollen concentrations of sediment samples decreased more significantly compared with water samples.Although the reservoir has a certain sedimentation effect on the upstream sediment or pollen,in the process of water and sediment regulation before flood season and flood discharge in flood season every year,the huge hydrodynamic force will still bring a large amount of sediment and pollen to the downstream.Combined with the results of the dissimilarity coefficient,the pollen combination characteristics of the sample points at Kaifeng downstream are more similar to those of the sample points above the Xiaolangdi Reservoir.In summary,we think that the pollen at Kaifeng in the lower reaches of the Yellow River comes from the river section above the Xiaolangdi Reservoir during the flood season.