Downstream Variation Of Luminescence Signal Of Modern Fluvial Sediment And Its Implications For River Transport Process And Dating Applications

Rivers,as the main carrier of terrigenous detrital material migration over long distances and with large fluxes,have a profound impact on the formation and evolution of landform.The study of detrital material transport in rivers has become an important issue in river dynamics and river geomorphology.Luminescence signals of sediment has a special advantage in which reflect sediment burial-exposure history sensitively,which can instruct the bleaching degree of luminescence signals of upstream material is good or bad and transport speed,the exchange process between channel and long-term storage center,which led to the index of interpretation of the luminescence signals in the research on the application of river process and luminescence dating.In this study,fine suspended load from the Yellow River and boulders and cobbles from Shiyanghe river were taken as the main subjects to analyze the details of suspended and bed load transport process by means of luminescence method.For fine suspended load of the Yellow River,about 11 sampling locations below the Lanzhou section of the Yellow River and 2 sampling locations in the Weihe River,a tributary of the Yellow River,were selected in this study to collect suspended sediment samples of surface water in flood period and later flood period,and the equivalent dose of luminescence signal was measured by MET-IRSL method（for polyminerals samples）and SAR method（for quartz samples）.The results indicate that the residual D_e of luminescence signals for 4～11μm suspended-load sediments remain low level in the upstream and middle reaches（Jinshaan gorge）of the Yellow River,and it is significantly changed in Fen-wei basin because of the afflux of tributaries material of Fenhe river and Weihe river,thus result in the differentiation of the middle and upper reaches of the Yellow River from the lower reaches.The D_e values of different types of luminescence signals in the lower Yellow River show a significant exponential downstream decay trend,and the comparison of the D_e has the potential to indicate the exchange process of sediments between deposition system（storage center）and transport system（channel）.Although factors such as discharge,sediment concentration and slope may significantly affect the transport-bleaching process and exchange process of sediments,the source of sediment is the decisive factor of the downstream variation trend for the D_e value of 4～11μm suspended-load sediment in the Yellow River.For gravels of Shiyanghe,boulders with larger grain size（40-60 cm）were added on the basis of cobbles（10-20 cm）in this study.The upper and lower side of surface samples and known-age surfaces samples were collected,IRSL₅₀signals were measured and luminescence signal-depth curves of upper and lower side surfaces were constructed.The results show that transport distance and gravel diameter are the important factors affecting the luminescence signal-depth curve of river gravel.The luminescence signal of the boulders surface from modern river bed tends to be shallower downstream,while that of cobbles is slightly deeper,which indicates that the influence of abrasion on boulders may be greater than that of cobbles.It is expected to establish the numerical relationship between the luminescence signal of gravel surface and the transport distance by determining the abrasion rate of gravel under different gravel diameters,lithology and hydrodynamic conditions in modern riverbed.According to the characteristics of the luminescence signal-depth curve of upper and lower side surfaces,the three modes of long-term staying,sliding dominant and rolling dominant and the transition between the modes can be identified.The BDI index is expected to be applied to the analysis of dominant transportation mode of river gravel.Towards the lower reaches of Shiyang River,the bleaching difference between the upper and lower side surfaces of modern bed boulders decreases,and the main transport mode of gravel may change from sliding dominant mode to rolling dominant mode.Boulders are difficult to transport under normal hydrodynamic conditions in wet season,while cobbles can reach the critical starting condition relatively easily.The hydrodynamic conditions required for the sliding start of boulders are sufficient to transport cobbles by rolling or even saltation.The comprehensive results show that the index of luminescence signal has a good indicative significance for the transport process of suspended or bed load,which depends on further exploration.