Monsoonally hydrological control on seasonally riverine magnesium isotopes in the tectonically-active Tibetan Plateau

Chemical weathering plays a crucial role in Earth’s geochemical cycles, affecting the climate and surface environment. Magnesium isotope ratios (δ²⁶Mg) are emerging as a powerful tracer of chemical weathering processes, but the impact of hydrology on δ²⁶Mg variations in river systems remains still controversial. This study aims to investigate the seasonal variations of riverine δ²⁶Mg over a full hydrological year in the upper Min Jiang, a tectonically active zone on the eastern Tibetan Plateau, to address how the monsoonal hydrology affects Mg cycling. Weekly water samples were collected at the Zhenjiangguan station and analyzed for Mg isotopes, with the δ²⁶Mg values ranging from –1.20‰ to –1.00‰. Sensitive to storm events, riverine δ²⁶Mg show rapid increases with increasing water discharge and suspended sediment concentrations. After ruling out the influences of lithology and secondary mineral formation through Mg lattice incorporation, we attribute the rapid fluctuations in riverine δ²⁶Mg to Mg²⁺–Na⁺exchange processes occurring on clay inter-layer sites or surfaces, which are regulated by monsoonal hydrology. Moreover, correlation of time-series riverine δ²⁶Mg data with runoff (r² = 0.50) in the tectonically-active Tibetan Plateau further confirms hydrological control on riverine δ²⁶Mg by instantaneous high suspended sediment concentration and rapid water–rock/particulate interaction. Our findings suggest that hydrological variability, driven by monsoonal runoff, is a key modulator of Mg sources and isotopic fractionation in tectonically-active river systems, particularly during storm periods with high-runoff, which is essential for deciphering climate effects on the global Mg cycle.