The response of riverine Mg isotope to hydrology and implications for continental weathering

The magnesium isotopic composition (δ²⁶Mg) of river water is a promising indicator of continental chemical weathering. While many studies have investigated the factors that influence riverine δ²⁶Mg, the impact of hydrology remains unclear. In this study, we collected eighty-four samples of stream water with nearly diurnal resolution in 2018 from a well-monitored, carbonate-rich catchment on the Chinese Loess Plateau. Our results demonstrate that δ²⁶Mg in stream water increases (0.14 ± 0.05 ‰) from dry to wet seasons, but decreases (0.21 ± 0.05 ‰) during rainfall events. These variations closely link to the dissolution and deposition of carbonates (i.e., source-related processes), and the adsorption and desorption of the exchangeable pool. Carbonate dissolution during rainfalls lowers the δ²⁶Mg in stream water, while during rainfall-free periods in wet seasons carbonate deposition elevates the δ²⁶Mg. Conversely, the exchangeable pool, reflecting carbonate weathering in the geological past, cannot be a source of Mg in stream waters, but act as a transfer Mg-pool. At an instantaneous picture, it releases the majority of Mg (>80 %) to stream water, and thereby has buffering effect on riverine Mg isotope. This highlights the significance of considering the buffering effects when studying riverine δ²⁶Mg variations. Overall, our findings suggest that the response of δ²⁶Mg to hydrology is typically associated with extreme hydrologic events and has important implications for tracing continental weathering.