Tectonic and climatic forcing of chemical weathering intensity in the northeastern Tibetan Plateau since the middle Miocene

Abstract

Chemical weathering of continental silicates is a crucial geological process in regulating Earth’s long-term climate and participating in global carbon cycle. The increasing aridity and active tectonic settings in the northeastern Tibetan Plateau since the middle Miocene, combining the global cooling background, make this region ideal for studying how the weathering process interacts with climate and tectonics. We focus on a well-dated Cenozoic sedimentary profile in the northeastern Qaidam Basin and use sandstone petrography, heavy minerals, mudstone clay minerals and geochemical data to interpret sediment provenance and to reconstruct paleo-chemical weathering history (15.3–1.8 Ma) in the northeast Tibet. Our results demonstrate unchanged sediment sources (the South Qilian Shan and the North Qaidam belt) and moderate chemical weathering intensity (CIA values of 67–80) for the sediment source-to-sink system since the middle Miocene. The high chemical weathering intensity in 15.3–12 Ma corresponded to relatively warm and humid climate, indicating minor efforts of global cooling (14–12 Ma) to the local weathering processes. The weakening chemical weathering in 12–9 Ma was closely related to rapid exhumation of the South Qilian Shan and accelerated growth of the northeastern Tibetan Plateau. Uplift-induced enhancement of physical denudation and regional aridity had triggered the transition of chemical weathering regime from supply-limited to kinetic-limited. The inspiring enhanced chemical weathering in 9–6 Ma was attributed to orographic rainfall or intensified East Asian monsoon precipitation. The warm periods and onset of the Northern Hemisphere Glaciation during the Pliocene-Pleistocene controlled the evolution of chemical weathering in the Qaidam Basin. Collectively, we propose that the late Cenozoic chemical weathering intensity in the northeastern Tibet was both constrained by tectonic deformation and climate change. Our findings emphasize the discordance between chemical weathering intensity and climate in arid, tectonically-active regions and thus caution should be exercised while using paleo-weathering intensity to reconstruct paleoclimate history.