| Under the background of global warming,extreme natural disasters have attracted media and public attention from around the world.Palaeoflood slackwater deposit(SWD)is a major evidence used for inference about information of the past flood events.Palaeoflood study provides a complete catalog of flood events at a watershed scale that is very important in understanding the regional hydrological response to monsoonal climatic change.Sedimentary identification and integrative dating of slackwater deposits play an important role in palaeoflood study.Reliable and accurate ages are crucial to reconstruct complete,detailed time series of climate change andpalaeoflood events.At present,there is an urgent need for reliable annual data to support the in-depth study of environmental evolution in the source region of the Yellow River and the upper reaches of Ruhe River.In addition,the study of cross-basin correlation analysis of palaeoflood events in China is not yet mature.Based on the detailed field investigation of Maqu section in the source region of the Yellow River and Suiping section in the upper reaches of Ruhe River,we select TWR and DRQ sites in the source region of the Yellow River,ZSX site in the upper reaches of Ruhe River in order to do comparative research objects.Grain-size distribution,magnetic susceptibility,geochemical elements,micromorphology were analyzed in the laboratory in order to reveal the sedimentary characteristics of palaeoflood slack water deposits.Combined OSL dating with additional stratigraphic correlations,a chronological framework is established.This paper aims to reveal the different sedimentary characteristics,provenances supply,sedimentary environment,granularity parameters,physical property indexes,causation and climate background of palaeoflood SWDs which were in different basins.It is confirmed that the palaeoflood events in the source region of the Yellow river is mainly by ice-snow melting water and heavy rainfall contribution,while the palaeoflood events in the upper reaches of Ruhe River is mainly influenced by the heavy rainfall contribution.We make a comparative analysis of the palaeoflood flood events in the source region of the Yellow river and the upper reaches of Ruhe River.The main conclusions are as follows:(1)Palaeoflood slackwater doposits(SWDs)have been found at the TWR and DRQ sites in the source region of the Yellow River,ZSX site in the upper reaches of Ruhe River.Based on field observations and laboratory analysis including grain-size distribution,susceptibility and geochemical elements,in combination with the characteristicsof modern large flood sediment,well-sorted suspend sediments were identified as palaeoflood SWDs.(2)Palaeoflood chronology was established in the source region of the Yellow River and upper Ruhe River by combining the OSL dates and peostratigraphcal correlations.There are two periods of palaeoflood events recorded in TWR and DRQ profiles were dated to 13.6?13.0 ka BP,11.0-9.6 ka BP.The climatic background of the two period of palaeoflood events were corresponded to transition period from the last glacial period to the early Holocene.The three periods of extraordinary overbank flooding evens recorded by the palaeoflood SWDS in ZSX profile were dated to 12.0?11.2 ka BP,4.2?4.0 ka BP,3.2?3.0 ka BP,respectively,by combining the OSL dates and pedostratigraphical correlations with the dated Holocene loess-soil profile(GDZ)in the study area.These extraordinary flood events occurred upper Ruhe River were synchronous with the climatic event defined by various paleoclimatic proxies from other parts of China and the world.It is therefore inferable that these extraordinary flood events represent obvious responses to the global climatic events during periods of highly variable and instable climate.(3)By Comparative analysis of geochemical elements between loess-soil layer in the ZSX profile with loess layer in the YPC profile(previous study),we find that geochemical element content and trend of UCC standardization curve between the loess-soil layer in ZXS profile and loess layer in YPC profile are almost identical.The contrasting on geochemical elements for these two profiles revealed that there were the same dust sources,which belonged to the same dust transporting aeolian systems.The loess in the upper reaches of the Ruhe River was a wind blown deposition of near source,while the coarser dust mainly came from loose alluvial deposits of alluvial and proluvialfans of the Yellow River.(4)On this basis,we make comparative analysis of sedimentological characteristics in the fieldwork,grain-size distributions,geochemical elements,magnetic susceptibility and micromorphology characteristics between palaeoflood SWDs recorded in the source region of Yellow River with palaeoflood SWDs recorded in the upper reaches of Ruhe River.The results show that the grain-size composition of the palaeoflood SWDs in TWR and DRQ profile in the source region of the Yellow River is thicker than that in ZSX profile in the upper Ruhe River,indicating that the flood hydrodynamic force in the source region of the Yellow River is stronger than that in the upper Ruhe River.By fieldwork investigations,we find that some parts of palaeoflood SWDs in the TWR and DRQ profile show obvious sedimentary characteristics of the sand bedding.The reasons for this phenomenon is that fine particle in the palaeoflood SWDs were carried away by wind after deposition and the coarse particles remained in situ accumulation.In TWR and DRQ profiles,the geochemical element content and UCC normalized curve of palaeflood SWDs are significantly different from those of loess-soil in this area.These results indicate that the material source of the palaeoflood SWDs are different from that of loess-soil.They were deposited from the suspended sediment load of overbank flooding.While in ZSX profile,geochemical element content and the curves of geochemical element by UCC are similar to loess-soil.These features indicate that the palaeoflood SWDs are mainly decided by local soil parent material.They were deposited by suspended sediment-load of the floodwater sourced from surface soils distributed over the Ruhe River valley.(5)Paleo-hydroclimatic correlations are the focus in the paleohydrological study.The palaeoflood events response to climatic variation and possible driving factors of the catastrophic floods were discussed by using historical data,the chronology framework of palaeoflood events and the climatic records from all over the world.These results indicate that the occurrence of palaeoflood events(13.6?13.0 ka BP,11.0-9.6 ka BP)in the source region of the Yellow River are caused by snow melting or superimposed rainstorm factor.The warming of the Tibet Plateau has led to the melting of the mountain glaciers,the shrinking of the frozen soil and the continuous increase of the melt water during the last deglaciation.The massive discharge of ice melt water fluxed into the Yellow River led to extraordinary palaeoflood events.While the occurrence of palaeoflood events(12.0?11.2 ka BP,4.2?4.0 ka BP and 3.2?3.0 ka BP)in the upper Ruhe River are closelyrelated to monsoonal climate frequent shift phase.They belong to rainstormfloods. |
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