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

The magnesium isotopic composition (8(26)Mg) of river water is a promising indicator of continental chemical weathering. While many studies have investigated the factors that influence riverine 8(26)Mg, 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 8(26)Mg in stream water increases (0.14 +/- 0.05 %o) from dry to wet seasons, but decreases (0.21 +/- 0.05 %o) 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 8(26)Mg in stream water, while during rainfall-free periods in wet seasons carbonate deposition elevates the 8(26)Mg. 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 8(26)Mg variations. Overall, our findings suggest that the response of 8(26)Mg to hydrology is typically associated with extreme hydrologic events and has important implications for tracing continental weathering.