Uranium Isotopic Constraints On The Nature Of The Prehistoric Flood At The Lajia Site,China

According to China’s early historiographies,the establishment of Xia Dynasty by Yu the Great who successfully controlled a lengthy and devastating flood of the Yellow River(Great Flood),is the first national dynasty in Chinese history and also marks the beginning of Chinese civilization.However,the truth of the Great Flood and the origin of Xia Dynasty have been controversial.Previous studies have shown that the Lajia site,in the upper Yellow River in China,was destroyed by a prehistoric earthquake and outburst mega-flood caused by this earthquake.If this outburst flood is the same one controlled by Yu in legend,the well-dated Lajia site could provide an accurate constraint on the beginning time of the Xia dynasty.The key link between the prehistoric earthquake and the followed mega-:flood is a set of blackish sand layer(BSL)filling in seismic fissures around Lajia site.It is proposed that the BSL may originate from the main channel in Jishi Gorge in the upper Yellow River,however,it also may from other geological process such as the mudflows in local tributaries and gullies in north of Lajia site or from the underlying gravel layer squeezed through cracks in the terraces by the earthquake-induced liquefaction.Hence,tracing the source of the BSL is the key to find out where the mega-flood occurred.It is pretty difficult to discriminate the process-origin of the BSL due to the similar geochemical compositions of sediments between the Yellow River and local tributaries.The Yellow River and local tributaries have the same geological background,which prevent the application of traditional geochemical tracers.Uranium(U)isotopic comminution chronology may have great potential on tracing the provenance of the BSL.The 230U/238U activity ratio,normally expressed as(234U/238U),of fine detrital particles,caused by alpha decay recoil,records the time sinece the particle broken from the bedrock and has no relationship with the composition of the original rocks.Different surface process often correspond to different deposition age and transport time,so the producing particels have different comminution age and thus different(234U/238U)values.Therefore,U isotope could distinguish different end-members which have similar geochemical components but different surface process.This study has measured the(234U/238U)values of the BSL in multiple places of the Lajia site,and also the sediments of local gullies and ancient dammed lake in the Yellow River,respectively.The result shows that the BSL have homogeneous(234U/238U)value and is similar to that of sediments in the main channel of the Yellow River but different from the sediments in local gullies and tributaries,suggesting the BSL come from the upstream Yellow River outburst flood,but has nothing to do with the mudflows in the local gullies.Thus,if the outburst flood is the same flood controlled by Yu the Great,the age of destruction of Lajia site could be used to constraint on the beginning time of Xia Dynasty.