Temporal And Spatial Processes Of Alluvial Systems Revealed By Single Grain Luminescence Of Potassium Feldspar

The luminescence signal of mineral grains is attenuated by sunlight when exposed to surface transport,and accumulated by irradiation when deposited and buried.Therefore,the luminescence signal of sediment mineral grains is a time capsule that seals the history of deposition,burial and transport sun decay:The mineral grains whose luminescent signals are zeroed before burial serve as the timekeeper of the deposition process,while the characteristic of the mineral grains’luminescent signals being zeroed in the sample indicates the transport exposure process before deposition.At the same time,the luminescence signal sensitivity,which reflects the information of the source region of the sample,as an inherent material property of the mineral grains,is relatively stable in the process of transport.Therefore,on the basis of measuring the equivalent dose of sediment samples to obtain the burial age,combining the sundecay characteristics of mineral grains（equivalent dose distribution）with the luminescence sensitivity,it is possible to vividly reproduce the surface process of sediment from source to sink.In this study,the alluvial and diluvial systems of different spatial scales in front of Altyn-Tagh and front of Tianshan Mountain in northwest China are taken as examples to explore the potential of restoring the source region,transport and deposition processes of potassium feldspar by using the luminescence signal and age distribution characteristics of single grains.The sedimentary facies of the 14m-deep channel section of the Wuzunnir paleoearthquake in front of Altyn-Tagh reveal the evolution of the environmental landscape near the piedmont fault from mainly wind-sand activity to relatively still water deposition.High-resolution post-IR₁₁₀ IRSL₁₇₀ infrared luminescent dating of single-grain feldspar in the trough section reveals that the deposition age is 11～2ka,and the deposition rate decreases significantly at～8 ka,which is consistent with the stratigraphic boundary of the 8.2ka BP event.With the exception of the samples from the eolian dune at the bottom,the dispersion of OD value and Tn in yellow strata is smaller than that in the livid gray strata,and the separation coefficient is larger than that in the livid gray strata,which represents the slow near-source fluvial-facies deposition with single source and uniform sundecay of grains.The sensitivity of the livid gray strata is very different,and the poor properties of the minerals themselves and the uneven sundecay result in the dispersion of the equivalent dose.It indicates that the dispersion and sensitivity of luminescence age distribution can reveal the difference of provenance and the change of sedimentary environment.Different from the time-varying sedimentary environment and transport dynamics before the Altyn-Tagh,the Guertu River at the northern foot of the Tianshan Mountains cut down the diluvial fan after exiting the mountain pass.Down the Guertu River,there are not only mineral grains from the interior of the Tianshan Mountain,but also the diluvial fan material from the bank up to 70m.The intermixing of riverbank diluvial material is not only an important contribution to the process of surface material redistribution,but also the result of the widening of the river to shape the landscape of the diluvial fan.In this study,paired surface sediment samples were collected from the channel center and the west bank of the Guertu River 2-3km downstream from the mountain pass.By analyzing various infrared luminescence signals with different sensitivity of potassium feldspar single grains to sunlight,the process of diluvial material inflow and the characteristic distance of suspended material transport along the riverbank were tracked and explored.The statistical distribution characteristics of the same sample and ripari-center luminescence signals with different sun-sensitive characteristics(MET-IRSL_50,MET-p IR IRSL₁₂₅,MET-p IR IRSL₂₂₅)reveal the disturbance of riparian diluvial material input on the gradual decay trend of luminescence signals downstream.The input of diluvial material in different parts of the channel is significantly different,and is significantly controlled by the slope of the channel.This result shows that it is possible to obtain the information of the bank broadening characteristics of sedimentary strata through the combined analysis of the equivalent dose distribution characteristics of various luminescence signals.The variation of the mean value of MET-p IR IRSL225 signal intensity in the downstream direction indicates that the characteristic transport distance of suspended material in the Gurtu River is at least 38km.In addition to being a traditional chronological tool,the infrared luminescence signal of single grains of potassium feldspar has potential to reveal the source,transport and redistribution of sediment materials,and is expected to provide a new quantitative tool for the study of surface processes.