Early Miocene expansion of C₄ vegetation on the northern Tibetan Plateau

The proliferation of C ₄ photosynthesis in land plants marks the most important ecological shift in tropical and subtropical terrestrial ecosystems during the Cenozoic. Despite the wealth of geochemical data indicating an increased C ₄ biomass in the paleovegetation since the Late Miocene spatial and temporal variations of C ₄ plants abundance are not well known. The driving force for their expansion such as CO ₂ starvation, aridification and fire frequency are controversially discussed. Here we present an integrated review of the stable isotope stratigraphy, trace elements, palynology, lithological and phytolith record from early-middle Miocene lacustrine sediments in the Hoh Xil Basin. We report the first known silicified wood fossils from the genus Picea and indicate the Hoh Xil region might have already reached an elevation above 2500 m at least in the early Miocene, representing the early uplift stage of northern Tibetan Plateau. Our results show that the expansion of C ₄ vegetation and the appearance of Picea in the interior of the Tibet during the early Miocene may be related to early uplift of the northern Tibetan Plateau and the consequent cooling event, _P CO ₂ decrease, and aridification. We suggest that the C ₄ signal in vertebrate faunas predating the early Miocene is due to the ecology of the herbivores (who preferred plants with C ₃ metabolism) rather than the lack of C ₄ plants in the vegetation.