Late Cenozoic Paleoclimate And Silicate Chemical Weathering Research In The Qaidam Basin

Analysing the process and mechanism of tectonic-climate interaction is one of the most challenging scientific problems in the field of earth science.Among them,the relationship between the regional/global climate and the uplift of Tibet Plateau is a typical case for researching the interaction between tectonic and climate.The uplift of Tibet Plateau not only has a significant impact on the onset and evolution of the Asian monsoon,but also can further reduce atmospheric carbon dioxide concentration by enhancing silicate chemical weathering,leading to global climate cooling.Therefore,reconstructing reliable long-term climate change history and silicate chemical weathering records in the northeastern Tibet Plateau is an important way to fully understand the above-mentioned problems.The Qaidam Basin is a typical Cenozoic sedimentary basin in the northeastern Tibetan Plateau.It is located not only in the tectonic active area,but also in the northwest inland arid area,the eastern Asian monsoon region and the unique high cold region of the Tibetan Plateau,the intersection hinge of three nature geography.In addition,the Cenozoic strata in the basin is thick and almost continuous bearing many layers of mammal fossils,which have recorded rich tectonic deformation,climate change and silicate chemical weathering signal.So,it is an ideal area to solve the above problems.In this research,we chose the well exposed Dahonggou section from the Qaidam Basin as the study targets.Based on precise paleomagnetic age,the analysis of environmental magnetism and element geochemistry proxies were performed to reconstruct the paleoclimate and silicate weathering intensity records in Qaidam Basin.Furthermore,we combined heavy mineral assemblages with published U-Pb chronology of detrital zircon and paleocurrent analysis data to ascertain the uplift history of Qilian Mountain.Finally,we compared paleoclimate and chemical weathering records with tectonic events to explore the major driving force of Asia paleoclimate and silicate chemical weathering since Late Cenozoic,and further understand the interaction between tectonic uplift and global/regional climate.The following conclusions were obtained:(1)The ?fd to hard isothermal remanence magnetization ratio(?fd/HIRM)-based precipitation variation of Qaidam Basin intensified during middle Miocene climate optimum(17-14 Ma)and late Miocene(11-5 Ma),and show similar pattern with East Asia summer monsoon record from China Loess Plateau and South China Sea,suggesting strong precipitation variations at Dahonggou as the product of East Asian summer monsoon(EASM)variations.(2)The results of elements geochemistry of bulk and size-fractions samples in Dahonggou section in the northeastern Qaidam Basin indicate strong chemical weathering intensity during 17-14 Ma and a decreasing chemical weathering intensity trend since 14 Ma.(3)Provenance analyses based on heavy mineral compositions combined with published paleocurrents,and detrital zircon U-Pb ages show three times provenance shifts of Dahonggou at ~19 Ma,~11 Ma and ~8 Ma,which reveal the Qilian Shan was activated since 19 Ma,followed by rapid southward propagation of the Qilian-Nan Shan thrust system starting at ca.11 Ma and 8 Ma.(4)We compared the EASM records with tectonic events and global climate record for a comprehensive analysis.Results show that strong summer monsoon precipitation coincided with global climate warm during middle Miocene climate optimum(17-14 Ma),supporting high concentration of CO2 was responsible for the intensification of EASM.In addition,the intensification of summer monsoon precipitation during the Late Miocene(~11-5 Ma)and Late Pliocene(4-2.7 Ma),occurred simultaneously with the tectonic activity in the northeastern margin of the Plateau,supporting the uplift of TP intensified the summer monsoon.(5)We compared weathering intensity record with global climate(benthic?18O and surface sea temperature record)and tectonic events since Late Cenozoic.Results show that chemical weathering intensity exhibits similar trend with the long-term global climate cooling,suggesting that temperature provided the first-order control on long-term chemical weathering intensity.