Geochemistry and geochronology of modern river sediments from the Heihe River on the northeastern Tibetan Plateau: Implications for weathering and sediment source-to-sink processes

Abstract

Deciphering the sediment source-to-sink processes in modern and dynamic fluvial systems serves as a key to understanding earth surface processes; however, the sedimentary signal preservation and propagation within sediment routing systems are complicated and heterogeneous. We conducted systematic geochemical and SrNd isotopic analysis and zircon UPb geochronology on fluvial sediments as well as Cretaceous sandstone from the Heihe River drainage basin, located in the northeastern Tibetan Plateau. Fluctuations in geochemistry (i.e., alkali and alkaline-earth metals) close to the average unweathered bedrocks suggest that the silt-sized sediments in the drainage basin form during the early stage of chemical weathering of source rocks. The UPb ages of detrital zircons from sand-sized sediments show two significant shifts in the upper and middle reaches, i.e., the disappearance of the Tonian-Stenian age group and the dramatic increase of the Cisuralian components in mainstream sediments. Our integrated assessment of source-rock lithology, sedimentary processes, and geomorphological and climatic conditions of the drainage basin led to two important insights: (1) In arid and semiarid environments like Heihe River, chemical weathering in the drainage basin is primarily controlled by geomorphologic and climatic factors; and (2) tributary dilution and en-route storage during the sediment routing systems are key factors responsible for changes in zircon age signatures. The confluence of representative tributaries and geomorphological and climatic transformations significantly influence sediment weathering and transport processes.