The Mid-late Cenozoic Paleotopography And Paleoenvironment Of Tibetan Plateau:Constraint From Leaf Wax Compound-specific Isotope

The uplift and growth of Tibetan Plateau is one of most important geological events in Cenozoic,which affect the regional and global climate significantly.Reconstructing the paleoelevation and paleotopography can help constrain the uplift history of Tibetan Plateau.The Cenozoic sediments on Tibetan Plateau also record the regional and global climate change.Therefore,the paleoelevation and paleoenvironment studies in Cenozoic basins on Tibetan Plateau is of great significance.However,problems remain in paleoelevation and paleoenvironment studies of Tibetan Plateau:(1)the paleoelevation studies are concentrated in the southern Tibetan Plateau,while the topography history of north-central Tibetan Plateau is still under debated;(2)paleoenvironment studies focus on the northern and northeastern boundary,while the southern Tibetan Plateau lack paleoenvironment studies and effective paleoenvironment index.Aiming at the above problems,we conducted paleoelevation and paleoenvironment research in the hinterland and southern part of Tibetan Plateau utilizing the leaf wax compound-specific isotopes.Compared with the carbonate isotopes,the leaf wax n-alkanes are mainly sourced from terrestrial plants,widely spread in sediments and less affected by diagenesis.We firstly combined the provenance analysis and leaf wax n-alkane hydrogen isotopes for constraining the late Eocene-Oligocene paleotopography of north-central Tibetan Plateau.The provenance analysis shown that the Northern Qiangtang is the prominent source region for Yaxicuo Group.We reconstructed the paleowater isotopic composition and got the elevation of?3800 m from leaf wax hydrogen isotopes.Modern observations in Ganges,Amazon and Congo river systems indicate that the leaf wax isotopes in river sediments integrated the isotopic signals of the catchment.Therefore,we prefer that the elevation calculated from leaf wax isotopes in Yaxicuo Group represents the high elevation of the source region,that is the Northern Qiangtang.Pollen evidence indicate that the elevation of Hoh Xil Basin was lower than 2 km in the late Eocene.Thus,the northern boundary of proto-Tibetan Plateau was characterized by high-relief topography.We applied leaf wax n-alkane ?13C and ?2H,carbonate ?13C and ?18O,and total organic carbon(TOC)?13C values in the paleoenvironment study of Zhada Basin.The-800 m late Cenozoic terrestrial sediments are well preserved in the Zhada Basin,recording the paleoenvironment history since late Miocene.The carbonate and TOC isotopes shown positive shift at-5.5 Ma,consistent with the depositional environment change.The leaf wax hydrogen isotopes shown large oscillation,and covaried with carbon isotopes,both reflecting the variation of precipitation amount.As the Zhada Basin is located in the northern boundary of South Asian monsoon circulation,thus,the basin paleoenvironment is sensitive to monsoon strength.The well correlation between leaf wax n-alkane isotopes and global benthic foramina ?18O curve,along with the?100 kyr sedimentary cycles observed in Zhada Basin,indicate that global ice volume might be the prominent driving force for the evolution of basin environment and South Asian monsoon during late Miocene to early Pliocene.