Decoupling of Neogene Seawater Lithium Isotopes from Uplift-driven Weathering

The ~9‰ increase in seawater lithium isotope composition (δ⁷Li) during the Cenozoic is widely interpreted as evidence for uplift-driven intensification of continental silicate weathering, particularly associated with major orogenic systems such as the Tibetan Plateau. However, this interpretation remains largely untested due to the lack of long-term riverine δ⁷Li records from tectonically active regions. Here we present the first Neogene paleowater δ⁷Li records spanning the past ~15 Myr from both the southern and northern Tibetan Plateau, a region that today contributes ~18% of the global riverine Li flux. Our dataset is derived from a 3500-m-thick fluvial sequence (15-5 Ma) in the Siwalik foreland basin (southern, monsoon-dominated Plateau) and a 1700-m drill core (7.3-0.1 Ma) from the Qaidam Basin (northern, arid Plateau). These two archives capture contrasting climatic, lithological and denudation regimes associated with Neogene uplift and cooling. Reconstructed paleowater δ⁷Li values reveal persistently low values in the southern Plateau and a long-term decrease in the northern Plateau, indicating reduced silicate weathering intensity under conditions of climatic cooling and rapid exhumation. These trends contrast with the coeval rise in seawater δ⁷Li, challenging the view that enhanced silicate weathering from uplifted mountain belts directly drives the marine lithium isotope record. By integrating our δ⁷Li reconstructions and reconstructed Li fluxes from the entire Tibetan Plateau into a global lithium cycle model, we show that continental silicate weathering from tectonically active mountains alone is unlikely to account for the observed Neogene increase in seawater δ⁷Li. Our results highlight the need for direct continental records from major orogenic systems to robustly constrain the links between tectonics, weathering, and the long-term carbon cycle.