Isotopic Fingerprints Of Mountain Uplift And Global Cooling In Paleoclimatic And Paleoecological Records From The Northern Tibetan Plateau

Post-middle Miocene changes in climate and ecology in the northern Tibetan Plateau have been attributed to tectonic uplift and global climate change. However, the relative roles of tectonism and global climate change have been a long-standing debate. To untangle the complex influences of global climate change versus the tectonism on regional climate and ecology in the northern Tibetan Plateau, we studied carbon isotopes (delta C-13(wax)) of leaf wax long-chain n-alkanes and carbon (delta C-13) and oxygen isotopes (delta O-18) of carbonates from three stratigraphic successions (ca. 16 Ma to 2 Ma) in the Hexi Corridor (foreland basin) and compare them with paleoclimate records in the Qaidam Basin (intermontane basin). Our isotopic results can be divided into two stages. In Stage 1 (16-12 Ma), a decrease of delta C-13(wax) values in the Hexi Corridor and Qaidam Basin is synchronous with global cooling after the Mid-Miocene Climate Optimum (MMCO). We interpret the 2 parts per thousand decrease in delta C-13(wax) values to be associated with the decline in C-4 plants due to the global cooling. In Stage 2 (since 12 Ma), plant carbon isotope discrimination (epsilon(CO2-wax)) and delta O-18 values in the Hexi Corridor reflect relatively stable hydroclimate condition, which contracts with the enhanced aridity in Qaidam Basin during the same interval. We attribute the difference in paleohydrological conditions to the uplift-induced basin isolation and the enhancement of water deficiency in the Qaidam Basin. The effects of late Miocene uplift also manifest as persistent intra-basinal differences in the Hexi Corridor, indicating changes associated with orographic precipitation and ecology.