Flow paths control on spatiotemporal weathering dynamics in high-relief regions, eastern Qinghai-Tibet Plateau: Insight from dissolved Li-Sr-O isotopes

Abstract

Mountain-building is generally acknowledged to accelerate chemical weathering of silicate rocks by reshaping geomorphology, tectonics, hydrology, and climate. Yet, the dominant driving forces of terrestrial silicate weathering remain unclear. To reveal the mechanisms of silicate weathering during mountain-building, we present hydrochemistry, water isotopes (δD and δ¹⁸O), radiogenic strontium isotopes (⁸⁷Sr/⁸⁶Sr), and lithium isotope (δ⁷Li) data of river waters from the tectonically active, fault-rich Bailong River catchment on the northeastern Qinghai-Tibet Plateau. Results show a wide spatial range of dissolved δ⁷Li values from +8.4 to +20.8 ‰ and a relatively narrow temporal range from +14.0 to +16.8 ‰. Spatially, δ⁷Li values negatively correlate with Li/Na ratios, suggesting significant Li uptake into secondary minerals. Downstream increases in δ⁷Li values corresponding with decreasing Si/(Na_sil + K) and K/(Na_sil + K) ratios (Na_sil refers to the Na concentration derived from silicate weathering) further suggest gradually enhanced secondary mineral formation and intensified weathering. We attribute these spatial variations in δ⁷Li values to changes in water flow paths that influence weathering intensity and secondary mineral formation. Specifically, longer flow paths of surface and subsurface waters in the lower reaches are facilitated by increased precipitation, greater relief, and fault-related hydrological pathways and could lead to prolonged water-rock interactions and higher δ⁷Li values. Temporally, δ⁷Li values, Li flux, silicate weathering rates, and physical erosion rates all correlate with river discharge, underscoring the influence of hydrological conditions on Li mobilization and δ⁷Li variations. Seasonal changes in water flow paths, driven by variations in precipitation and discharge, result in a δ⁷Li fluctuation of ∼2.8 ‰. During the wet season, increased discharge could extend water flow paths connected to deep groundwater and long-distance groundwater, promoting enhanced water-rock interactions and more Li incorporation into secondary minerals that lead to elevated δ⁷Li values. In contrast, shorter water flow paths (shallow groundwater) during the dry season can limit these processes, resulting in lower δ⁷Li values. Overall, these results highlight the importance role of water flow paths in regulating riverine spatiotemporal δ⁷Li variations and offer insights into modern and paleo-weathering in tectonically active regions.