Coarse Sediment Characteristics And Its Provenance Implication In The Ningxia-inner Mongolia Sections Of The Yellow River

The Yellow River has the world‘s hightest sediment load,which is one of disastrous rivers on earth with frequent dike breaching and flood disasters.A large area of deserts distribute on both sides of the Yellow river in the Ningxia-Inner Mongolia sections of the upper reach,thereby it is representative of desert-gully alluvial reaches with the interplay of aeolian-sand and flow-sand,and characterized by frequent channel migration.In recent 50 years,controlled by both the global warming and human activities,siltation in the channel of the Ningxia-Inner Mongolia sections caused the riverbed shrinkage evidently and this reach has appeared as a?hanging river?,which resulting in unprecedented pressure on flood management.Therefore we should systematic study on the characteristics and provenance of sediments deposited on the riverbed and floodplain.It was the premise of river restoration,and also played an important role in the implementation of regional hydrographic engineering and reasonable development of water resources in the Yellow River.To address these issues,we collected a series of samples from different geomorphic units including riverbed,floodplain,alluvial lake,and potential sources such as the Loess Plateau,Tengger desert,Hedong desert,Ulanbuh desert,Kubuq desert and Ten great Gullies.All the samples have been analyzed to obtain the sedimentary physicochemical characteristics and further established the tracing indexes of coarse sediment including the grain size,mineralogy,grain morphology and geochemical elements in the Ningxia-Inner Mongolia sections of the upper Yellow River.Finally,we preliminary calculated the contributions of coarse sediment from potential sources based on the End member model and statistical methods.The spatial and temporal patterns of coarse sediment variation in the Qiantao basin have also been discussed.The results show that riverbed materials are characterized by moderate-poor sorting,with the mean grain size mainly distributed between 20-60?m?51.8%?and80-250?m?41.7%?.The distribution characteristics of median particle size?D50?show that there are not obvious downstream fining trend in the Ningxia-Inner Mongolia sections of the upper Yellow River.There is obvious grain coarsening in the section of Bayangaole-Sanhuhekou,where the 53.5%of riverbed sediments with median grain size greater than 80?m,and the cumulative probability curves exhibit two sections,dominated by bedload?75-98%?.While the median grain size of riverbed sediments in the section of Sanhuhekou-Toudaoguai are less than 80?m,and the bedload content decreases to 70-75%.The mean grain size of floodplain sediments are dominately distributed in the range from 30?m to 60?m,and belong to poor sorting materials.However,there are significant variations along the channel,the finest materials are distributed in the section of Bayangaole-Sanhuhekou and the cumulative probability curves with one-two sections,characterized of suspension transport,which suggest that the coarse sediment with the size larger than 80?m have not effectively carried downstream or deposited in the floodplain.Secondly,the riverbed sediments are composed of opaque mineral,amphibole,garnet and epidote,with minor zircon,tourmaline,rutile,and the diagnostic light mineral of illite and smectite.The mineral assemblages are respectively dominated by epidote-illite and garnet-smectite in the Shizuishan-Bayangaole and Bayangaole-Sanhuhekou.Thirdly,scanning electron microscope?SEM?imaging of quartz grains show that the coarse sediment are characterized by composite microtextures of V-shaped percussion features and butterfly pits,pitted surface and polishing surface.Lastly,major element concentration normalised to upper continental crust?UCC?are strong depletion in Na₂O,Al₂O₃,Fe₂O₃,MgO,K₂O,Nd,Ni,Rb,Sr,Y and Zn,and enrichment of CaO,Ce,Cr,Hf,Nd and Zr in the riverbed and floodplain sediments.There is a significant increase of Zr,Hf with the coarser grain size in respond to the local input of desert sand in the section of bayangaole-sanhuhekou,while the Rb,Ni and V have an opposite trend controlled by finer fraction from sanhuhekou to Toudaoguai.We try to select the sensitive indicators based on the geophysical-chemical characteristics of coarse sediment.Firstly,the results of mineral assemblages demonstrate there is a good consistency between the riverbed and potential source sediments in different sections of the Yellow River.The riverbed samples from the upper reach?Xiaheyan-Shizuishan?are characterized by high content of epidote,amphibolite,illite and chlorite,which are similar to the mineral assembalges of Qingshui river sediments originated from the Loess Plateau and Northeast Tibet Plateau.Our data suggest that seasonal tributaries?the??Ten great Gullies??carrying detritus from the Ordos Plateau may account for the localized high concentration of garnet and smectite in the section of Sanhuhekou–Toudaoguai.Secondly,scanning electron microscopy?SEM?of quartz grains may prove useful in interpreting the contribution of Hedong desert,Ulanbuh desert and Kubuq desert to the coarse sediment by the high frequency of composite structures and subcircular particles.Thirdly,the spatial pattern of chemical elements in the Ningxia-Inner Mongolia sections of the Yellow river indicates that the Qingshui river sediments have high concentration of calcium,and the Ulanbuh and Kubuq desert with low titanium content,while high concentration of potassium and strontiam distribute in the Ten great Gullies and present decreasing trend to Kubuq desert.This variation suggests the Sr/Ca could be used as a tracering index to differentiate the Hedong desert,Qingshui river and Ten great Gullies.Similarly,low value of Ti/K could be used to distigush the sediments between Ulanbuh desert,Kubuq desert and Ten great Gullies.Consequently,Sr/Ca and Ti/K could be regared as indicators of potential sources in the Ningxia-Inner Mongolia sections of the Yellow river.Contributions of different sources to coarse sediment deposits on the riverbed are calculated by the mixing model with the Kruskal Wallis H-test and discriminant analysis method.The relative contributions from the Tengger desert,Qingshui river and Heong desert are respectively 84.2%,49.9%and 57.6%in the Yinchuan basin.Almost 72.8%coarse sediment in the river channel are carried by the Ulanbuh desert in the Hetao basin,and the contribution of Kubuq desert and Ten great Gullies are72.8%and 61.6%,respectively.The contributions of potential sources to coarse sediment in different sections calculated from two-end member linear method agree with the results of multivariate mixing model.The results also show that some inconsistency only occur in the Xiaheyan,which mainly due to the less sample numbers of riverbed and Tengger desert sediments.Based on the grain size analysis of twelve cores,our study indicates that the sediments in the Houtao basin?Bayangaole-Sanhuhekou?are mainly composed of fine sand and medium-coarse sand,with the mean grain size distribute in 10-40?m and 150-350?m,respectively account for 23.2%and 57.4%,and belong to medium-poor sorting.The sediments in the Qiantao basin?Sanhuhekou-Toudaoguai?mainly consist of fine sand and silt,dominated by the mean grain size of 10-40?m and most of sediments are poorly sorted with a few being moderately sorted.The median grain size of core sediment displays significant zonal distribution around the boundary of 80?m with the depth,which further prove that the median size of 80?m as a criticle threshold in the Ningxia-Inner Mongolia sections of the upper Yellow River.Furthermore,provenance studies on the core sediments of Qiantao basin suggestes that the contributions of Ten great Gullies are controlled mainly by climate change and human activities.The average contribution of Ten great Gullies was 33-47%during 10-4 ka,and decreased to 22-43%during the 4-2ka,which in response to low flood frequency caused by dry-cold climatic conditions.The contribution of the Ten great Gullies is significantly incresed to 43-71%since nearly 2ka due to the land desertification and desert expansion in the Hetao basin with the continuous increase in population and human activities.