Temporally And Spatially Variable Provenance Of The Chinese Loess Plateau Revealed By Integrated Provenance Analysis

Eolian deposits provide a window into late Cenozoic climate and environments.Dust,for instance,may record the history of inland aridification and past atmospheric circulation patterns.In addition,dust can affect Earth’s radiative forcing budget and ocean biogeochemical cycles.Loess and underlying Red Clay sequences of the Chinese Loess Plateau compose one of the most complete late Cenozoic terrestrial eolian paleoclimatic archives.However,there are still debates on the provenance of loess-Red Clay sequences and whether there are temporal and spatial variations.In this thesis,single-grain detrital zircon U-Pb dating analysis supplemented with stable heavy mineral ratio,and Nd isotopic geochemistry has been used to obtain the provenance information of several sedimentary outcrops across the Chinese Loess Plateau since the late Miocene.These results have improved the understanding of the formation and the source-to-sink processes of the Chinese Loess Plateau.Details are as follows:(1)By generating and compiling large-observation detrital zircon U-Pb geochronology data of Quaternary loess strata,at least three distinct provenance zones are recognized for Quaternary loess strata: central western,eastern,and northeastern.These zones received the coarse-grained sediment from their neighboring river systems in addition to the arid regions.The Quaternary ice age accelerated river erosion and enhanced dust emission,which may be a reason for the rapid increase in dust accumulation rate on the Chinese Loess Plateau.(2)Large-observation detrital zircon U-Pb geochronology data of the late Miocene-Pleistocene dust sequences were generated from the Jiaxian site from the northeastern Chinese Loess Plateau and compared the results with those from the other Red Clay sections across the Chinese Loess Plateau.It is found that the spatial provenance differences on the Chinese Loess Plateau were slight during the late Miocene,with a trend of increasing spatial provenance contrast from the late Miocene to the Pleistocene.This is mainly driven by the significant provenance variations on the northeastern Chinese Loess Plateau,while the provenance on the central Chinese Loess Plateau has less variations over time.By comparing with regional tectonic activities and paleoclimate records,it is shown that the temporal provenance changes on the northeastern Chinese Loess Plateau were controlled jointly by significant uplift and denudation of the Qilian Shan and northeastern Tibetan Plateau in the late Miocene,global warming and wetting in the early Pliocene,and the increased climatic variability in the late Pliocene,which caused the changes in sediment supply,sediment availability,and possibly wind strength at different periods.These factors produced the changes in dust sources and dust accumulation rates in the northeastern Chinese Loess Plateau.(3)By re-analyzing the detrital zircon U-Pb geochronology data of the late Miocene-Pliocene Red Clay sequence on the central Chinese Loess Plateau and dividing the data into two groups for comparison according to their grain sizes.It is found that zircon U-Pb age signatures do differ between different grain sizes.The coarse-grained(> 20 μm)fraction appears to receive more sediment from the dominant wind direction from Qilian Shan and northeastern Tibetan Plateau,while the fine-grained(12-20 μm)fraction receives more sediment from the relatively weak wind-dominated eastern Mu Us desert and middle Yellow River regions.However,the differences due to the grain size effect are much smaller than those between different potential source areas.(4)By re-analyzing the heavy mineral assemblage data from the published loess-Red Clay sequences on the Chinese Loess Plateau and finding several stable heavy minerals’ ratios of similar density can be used to trace the provenance changes since the late Miocene.The results indicate the central Chinese Loess Plateau has been sourcing dominantly from the northeastern Tibetan Plateau since the late Miocene,and the stable heavy minerals’ ratios show spatial variation between the northeastern and the central Chinese Loess Plateau.This result is consistent with the results revealed by the detrital zircon U-Pb geochronology method in the Chinese Loess Plateau.It further indicates that mineral fertility is not the dominant factor controlling the provenance variations on the Chinese Loess Plateau.(5)Nd isotopic geochemical analysis of the late Miocene-Pliocene Red Clay sequence from the Chaona site on the central Chinese Loess Plateau,and the Jiaxian site in the northeastern Chinese Loess Plateau revealed spatial variation between the northeastern and the central Chinese Loess Plateau.The results further suggest that the conclusion of spatial variation between the northeastern and central Chinese Loess Plateau is still valid even when the contribution of finer grained(<12 μm)fractions is considered.These findings deepen the understanding of the spatial and temporal variations and drivers of the source to sink processes of the Chinese Loess Plateau,emphasize the genetic link between dust accumulation on the Chinese Loess Plateau and reorganization of the Yellow River system as well as tectonics,and global and regional climate over the late Miocene–Pleistocene.In addition,these findings provide a new perspective for reconstructing paleoclimate and paleoenvironment changes from eolian deposits on the Chinese Loess Plateau.