The Optically Stimulated Luminescence Chronology And Environmental Significance Of Palaeoflood In The Qaidam Basin

Located in the northeast of Qinghai-Tibet Plateau,Qaidam Basin is located at the boundary between Asian Summer Monsoon and the Westerlies.It is an arid and semi-arid region.The seasonal rivers developed in the basin are likely to occur large-scale flooding during the flood season under its special natural environment.Therefore,flood is an important surface process in arid areas.The Tiekui Desert is the largest desert in the basin,and a large number of palaeo-flood sediments are distributed in and around the Tiekui Desert.The widely distributed palaeo-flood sediments are important archives to reveal the regional palaeoenvironmental and geomorphological changes of the Qaidam Basin.However,there are few specific descriptions and researches on flood sedimentary characteristics,and the chronology study of floods are still limited in Qaidam Basin,which seriously affects the recognition of flood sediments and the in-depth study of regional environmental changes.Grain size is a relatively mature paleoenvironmental surrogate index in sediments.The in-depth study of grain size characteristics is of great significance to the recognition,evolution process and paleoenvironmental significance of paleoflood sediments.In this study,two paleoflood sections in the middle reaches of the Xiariha river in the eastern of Qaidam Basin was studied to understand its grain size characteristics and its difference with other types of sediments.Its paleoenvironmental implications were discussed as well.Chronology is a key method to study the evolution process of palaeo-flood and its environmental background.In this study,the accurate and reliable chronological data of flood sediments are obtained by means of Optically Stimulated Luminescence technique and the appropriate chronological model is selected to establish the chronological framework of flood sediments in the research area.In obtaining accurate flood sediment data of sedimentology and chronology based on the analysis of the flood frequency in different periods and size differences,analyze its evolution process,andby combining other high resolution paleoclimatic record of climate change information carrier,reveals the environmental background of the palaeo-flood event and discusses the response of regional palaeo-flood event to global climate change and its possible driving mechanism.The results show that:(1)Two paleoflood sections in the middle reaches of the Xiariha river in the eastern of Qaidam Basin was studied to shows that there are two types of grain size distribution curves,namely single-peak distribution and bimodal distribution.The flood sediments were poorly sorted,and its grain size characteristics are obviously different with fluvial sediments,aeolian sand and loess,which could be used as a criterion to identify different sedimentary facies.They are different in transpotational processes as well.(2)The coarse-fine cycles which are common in flood sediments were proved to be caused by different grainsize fractions and their ratios.The finer and coarser layers are different in the shapes of grain size curves(the finer with bimodal distribution,while the coarser with single peak),and transpotational processes as well,which was caused by the changes of the hydrodynamic changes during the flood.Consequently,the coarse-fine cycles could be used as a method to identify the flood events.During the different sedimentary stages,the frequency and magnitude of the floods were different,and the sedimentary environment changes from flood to aeolian sand,which might be associated with the geomorphological and climatic changes.(3)The frequency and scale of floods vary in different stages,which may be related to the combined effects of geomorphic evolution and climate change.During the period of frequent flooding caused by climate change,the influence of river on the terrace was gradually reduced with the river cutting down and the flood deposition increasing.(4)The results of the study showed that there was a standard growth curve of the palaeo-flood sediments in the middle reaches of the Xiariha river in the eastern of Qaidam Basin,and the growth rate was linear,and the regeneration dose was far from saturation,which indicated that IRC-SAR/SGC method was suitable for the chronological test of the palaeo-flood sediments in the Xiariha river.And we suggest MAM is more appropriate for OSL dating of fluvial/flood sediments.(5)Combining two paleoflood sections in the middle reaches of the Xiariha river OSL dating and field stratigraphic classification,were divided into 4 floods/events,respectively in 4.0ka,2.7-2.5 ka,1.8-1.5 ka,1.1-0.68 ka,background analysis and field geomorphic characteristics based on the climate environment comprehensive inference,the intensified Asian summer monsoon is the main cause of the ha river flooding in the summer due to the increased water decline in the humid environment background of the holocene.Besides,due to the warming climate,the mountain glaciers in the source region are melting,and the amount of melting snow and ice continues to increase.The runoff of melting snow and ice water into rivers also provides the possibility for the occurrence of floods.