Hydrological regulation of seasonal Mg isotopic variation in the upper Jinsha River draining the eastern Tibetan Plateau

Abstract

The compositions of magnesium isotopes (δ²⁶Mg) in the river systems are primarily controlled by the sources of Mg²⁺ and the processes of fractionation. However, how these factors govern the riverine δ²⁶Mg values under different seasons remains unclear. To better understand the influences of weathering and hydrology on water chemistry, a systematic investigation was conducted on the seasonal variation of the riverine Mg and Sr isotopes, using water samples collected weekly throughout the year 2021 in the upper Jinsha River draining the eastern Tibetan Plateau. The result showed that both δ²⁶Mg and ⁸⁷Sr/⁸⁶Sr exhibited a significant sensitivity to seasonally hydrological conditions linked to the monsoon climate. A forward model was used to partition riverine Mg²⁺ sources and confirmed that the riverine Mg budget was dominated by carbonate weathering, followed by evaporite dissolution and silicate weathering. However, the conservative mixing of different endmembers could not fully explain riverine δ²⁶Mg in the upper Jinsha River, thus the role of secondary carbonate precipitation associated with hydrological changes was proposed here for the observed variations in riverine δ²⁶Mg values. It showed that higher water discharge triggered by monsoonal rainfall reduced Mg isotope fractionation owing to a shorter water residence time, leading to low riverine δ²⁶Mg values. Conversely, during the dry seasons and the less rainy periods both with lower water discharge, longer water residence time would result in high riverine δ²⁶Mg values that is subjected to increased Mg isotope fractionation. This study provides new insights into the potential influence of hydrological variations on the riverine δ²⁶Mg values.