Riverine Li isotopes response to a flood event on the Chinese Loess Plateau

Silicate weathering is essential to supply nutrients from land to ocean and to sustain the long-term habitability of our planet via consumption of atmospheric CO₂. Riverine Li isotopes (δ⁷Li_riv) are proposed a promising silicate weathering tracer. However, how silicate weathering and riverine Li isotopes respond to an extreme hydrological event has rarely been investigated. Owing to homogeneous geochemical compositions, loess provides a unique setting to understand how riverine Li isotopes trace silicate weathering under an extreme hydrological event. Here, we collected and analyzed river water samples within the Beiluo River catchment that fully covered by loess in the dry and wet seasons, in order to capture the response of silicate weathering to a flood event that accounting 1/5 annual runoff in 9 days. The results showed that δ⁷Li_riv values have distinct seasonality between the regular dry and wet seasons and the flood event. In the regular dry and wet seasons, δ⁷Li_riv values were controlled by two processes: 1) mixing between groundwater and silicate weathering in the upper stream, and 2) Rayleigh fractionation processes caused by loess weathering in the middle-lower reaches. In contrast, mixing between extruded porewater and groundwater controlled the δ⁷Li_riv values throughout the flood event. Our findings provides new clues into silicate weathering processes and the response of δ⁷Li_riv to extreme hydrological events.