Processes And Mechanisms For The Yellow River Traversing The Sanmen Gorge:Insight From A Drill Core Study

The Yellow River is the second largest river in China and the mother river of Chinese civilization.Its formation and evolution are closely related to events such as the uplift of the Tibetan Plateau,global climate change,and the evolution of the Asian monsoon.The history and mechanism of the Yellow River’s formation and evolution are major scientific questions of great interest in the earth sciences.The Sanmen Gorge is the last gorge of the main channel of the Yellow River,and the eastward flow of the Sanmen Gorge into the sea marks the formation of the modern Yellow River.but there is a discrepancy on the incision time,including the Pliocene(～5 Ma),no later than 1.2Ma,and ～0.15 Ma.The reason for this is that in the past,studies mainly relied on natural sections,which are not fully exposed in the Sanmen Gorge,resulting in multiple interpretations in terms of stratigraphic alignment,age determination,and sedimentary phase interpretation.Therefore,we drilled a continuous core(core number: SMX19)with a length of 150 m,a core rate of 95%,and a chronological span of ～1.96 Myr in the Sanmen Gorge.Based on the paleomagnetic dating framework established by the research team,this thesis carried out a systematic sedimentary phase and source analysis study on this core to identify the process of the Yellow River through Sanmen Gorge in order to provide scientific support for understanding the formation era and mechanism of the modern Yellow River,which include the following insights:(1)The results of sedimentary facies,grain size,soluble salt,and trace boron content analysis show that from ～1.96 to ～1.25 Ma,closed brackish-saline lake deposits without fluvial sediments were dominant in the Sanmen Gorge,and the possibility of large river development can be excluded due to limited sedimentation space;at ～1.25 Ma,brackish-saline lake sediments eventually died out and typical fluvial sediments began to develop,recording the significant transition process from closed lakes to open river systems.(2)The results of Sr-Nd isotope and heavy mineral analyses indicate that the sediments in the Sanmen Basin at ～1.96-1.25 Ma mainly originated from the surrounding Qinling and Zhongtiao Mountains and other proximal mountains;starting at ～1.25 Ma,clastic material from the Ordos Block in the middle and upper reaches of the Yellow River began to flood in,indicating the connection and penetration of the mid-Yellow River system into the sea east of the Sanmen Gorge.(3)The age of the Yellow River through the Sanmen Gorge revealed by the borehole cores is comparable to the age of the formation of the highest Yellow River terraces such as the Jin Shaan Gorge and the Kouma Basin,suggesting that the modern Yellow River was formed at ～1.25 Ma,which may be related to the uplift of the Tibetan Plateau and the accelerated lowering of global sea level during the Mid-Pleistocene transition.This thesis provides key evidence to clarify the formation age of the modern Yellow River based on environmental drilling studies in the Sanmen Gorge and serves as a scientific support for further studies on the ecological and environmental effects of the Yellow River formation and evolution.