Study On The Late Pleistocene Paleofloods Events In The Maqu Reach Of Yellow River Source Area,NE Tibetan Plateau

Flood disaster,as an extremely important environmental and social economic problem,seriously threatens the survival of human beings and the development of society,causing serious casualties and economic losses to modern human society.Paleoflood deposits is an important ideal carrier for recording past climatic hydrological events and climate changes.Paleoflood hydrology provides an effective way to master the time rule of super-long scale large floods and its relationship with global climate change,invert the paleoclimate change process,and deeply understand the environmental and disaster effects of the interaction of hydrological process,climate change,geomorphic factors and human activities.The Yellow River source area,as the largest flow-producing region and an important hydrologic and ecological function region,is very sensitive to global climate change.The flood has the characteristics of large flood peak volume,wide range,long duration,high frequency and serious disaster.Moreover,there are a substantial amount of people living along the valley in the Yellow River source area.However,modern scientific instrument flood observation is only a few decades,and a limited number of historical floods can be accurately recorded,the flood data and information is slightly insufficient.It is difficult to satisfy the needs of the occurrence regularity of extreme flood events in the Yellow River source area,the development and evolution trend of the basin hydrological system under the background of climate warming only with limited measured data and few historical documents.Therefore,it is urgent to explore the response of the hydrologic system of the Yellow River source area to global change from the perspective of palaeoflood hydrology.Palaeohydrological investigations were carried out in the Yellow River source area on the northeast（NE）Tibetan Plateau.During our fieldwork investigations,two units of overbank flood deposits（OFD）with thickness up to 2-6 m were found at CMQ and ZHK sites on the right bank of the Yellow River in the Maqu reach.On the basis of the field identification of the OFD,sedimentology and chronology samples from the two profiles were collected systematically.These OFD units were studied using a multi-index approach,including magnetic susceptibility,hygroscopic water,grain size distribution and micro-morphological features.It is revealed that these OFD units well recorded two episodes of paleoflood events in the Maqu reach in the Yellow River source area.Using the optically stimulated luminescence（OSL）and AMS ¹⁴C dating techniques,the first episode was dated to34680±1880～29000±1790 yr BP that correlated with the late Marine isotope stage（MIS）3a with a warm and wet climate.The second episode occurred at 16500±1250～14000±1280 yr,probably in the B（?）lling-Aller（?）d warm stage during the last deglaciation.At these two period,mountain glaciers on the Tibetan Plateau accelerated large-scale melting,frozen soil intensified and shrank,ice and snow melted water continued to increase.A large amount of ice melted water flowed into the Yellow River or combined with large-scale precipitation,which led to the occurrence of overbank flooding events.At the same time,these large floods overflowed the river banks and deposited abundant fluvial sediment on the glacial outwash/flashflood deposits,which intensified the river channel aggradation.These results not only enrich the flood records in the Yellow River source area and are contribute to understanding the hydro-climatic and geomorphic developmental process response of the Tibetan Plateau Yellow River source area to global climate change.But also have the important practical significance in developing and utilizing hydrology and water resources in Yellow River source area,as well as flood control and disaster reduction in the Yellow River Basin.