| The interaction processes and mutual feedback mechanisms between tectonic,climatic and surface process are one of the preface topics of geoscience research.As an important form of surface process,rivers can respond sensitively to tectonic activities and climate change,resulting in behavioral changes such as accumulation,undercutting,and lateral erosion,so that water systems reorganize,evolve,and shape and transform landforms.Therefore,revealing the formation and evolution of river systems,especially megarivers or continental-scale rivers,is an ideal window to discuss the above frontier topics.In addition,their formation and evolution also involve the interaction of multiple factors such as tectonic,climate,sea level,vegetation,paleontology,human activities,etc.,and their formation and evolution is of great significance for revealing the mutual feedback mechanism between these elements,and has been widely valued by geoscience circles at home and abroad.In recent years,with the development of provenance tracing technology,the study of river formation and evolution in academia has risen from a single river section to the level of the evolution process of the whole catchment pattern.Nevertheless,research based on this method remains highly controversial.The reason is that,in addition to the unclear nature of material transport in the river system in different geological and historical periods,the inconsistency of understanding of the representative material source markers is also an important reason for the differences.The core of solving these problems lies in strengthening the research on the source characteristics of riverbeds in geological history.As the cradle of Chinese civilization and the mother river,the Yellow River flows through China’s three major geomorphological steps,communicating different active tectonic zones and natural zones along the way,and its material transport laws are not only complex but also lack of relevant research.The layered landform surfaces represented by fluvial terraces are riverbeds or floodplains in geological history,and their material composition characteristics can well record the source and material transport law.Among them,geochemical neodymium isotopes have the characteristics of not being affected by weathering,biology,sedimentation,metamorphism and other processes,which can well retain the properties of the original rock,and are ideal indicators for tracing the source of clastic in fluvial clastic.Compared with other provenance tracing methods,this isotope is also easy to accumulate fine particles of fluvial clastic,making its hydrodynamic response insensitive,and has the significance of long-distance source tracer.In this paper,five geomorphological cross-sections were selected from the middle reaches of the Yellow River from Hekou Town to Taohuayu to study the overlying fluvial clastic neodymium isotopes.The results showed that:(1)the neodymium isotopes characteristics of modern riverbed materials in the upper,middle and lower reaches of the Yellow River were different,indicating that the material contribution of tributaries and local potential source areas had a greater impact;(2)There were differences in the detritus of fluvial clastic overlying the Planation surface of Tang County period,middle reaches of the Yellow River,and neodymium isotopes from the lower Yellow River terraces and modern riverbed sediments,mainly derived from local materials from red clay,East Ordos,Lüliang Mountain,Taihang Mountain and East Qinling Mountains;(3)Materials from the northeast of the Tibetan Plateau in the upper reaches of the Yellow River,even < 0.074 mm riverbed materials,are difficult to reach downstream through the Jinshaan Gorge in the middle reach.Therefore,tracing the evolution process of upstream water systems based on the changes of source information in downstream basins or delta strata requires careful consideration of the transport law of riverbed materials in geological history.In addition,the results of this paper reveal that the riverbed materials in the lower reaches of the Yellow River may mainly come from the middle reaches of the Yellow River,East Ordos,Lüliang Mountain,Taihang Mountain and East Qinling Mountains,rather than the upper reaches,which clarifies the source of materials for the downstream "Suspended River" treatment. |
