Cation exchange controls riverine magnesium isotopes in extremely-high-erosion catchments

Carbonate weathering plays a significant role in regulating global carbon budget at short time scales, and thus needs to be better constrained in the context of global warming. Riverine magnesium isotopes (δ 26 Mg) have the potential to trace carbonate weathering intensity (CWI) but require further testing under various climatic and geological settings. Cation exchange is an important mechanism buffering river water chemistry, especially in catchments characterized by high erosion. However, field evidence on the influence of cation exchange on riverine δ 26 Mg is rare. In this study, spatial riverine δ 26 Mg variation within the Three Rivers (i.e., the Jinsha Jiang, the Lancang Jiang, and the Nu Jiang), three extremely-high-erosion catchments in the southeastern Tibetan Plateau, was investigated to address this issue. The results showed that riverine δ 26 Mg values present a wide range from –1.11 ‰ to –0.59 ‰ in the Jinsha Jiang, from –1.39 ‰ to –0.65 ‰ in the Lancang Jiang, and from –1.19 ‰ to –0.50 ‰ in the Nu Jiang. An inversion model was used to partition riverine Mg 2+ sources and confirmed that the riverine Mg 2+ budget was dominated by carbonate weathering, followed by evaporite dissolution. However, conservative mixing could not explain riverine δ 26 Mg variation within the Three Rivers catchments, because measured δ 26 Mg values (δ 26 Mg measured ) systematically deviated from the modeled ones (δ 26 Mg modeled ), with Δ 26 Mg measured–modeled (δ 26 Mg measured – δ 26 Mg modeled ) up to 0.79 ‰. The positive correlations between Δ 26 Mg measured–modeled and suspended particulate matter (SPM) concentrations indicate Mg isotopic fractionation was related to high suspended loads owing to extremely-high erosion rates. Given the significant cation exchange capacity of SPM, Mg 2+ -Na + exchange is proposed for the first time as an explanation for the observed δ 26 Mg variations in the rivers draining the Tibetan Plateau, although the role of carbonate precipitation could not be excluded. The strong positive correlations between riverine δ 26 Mg and exchangeable Mg/Na ratios in the Three Rivers further support that light Mg isotopes may be preferentially retained in the riverine exchange pool during Mg 2+ -Na + exchange, driving riverine δ 26 Mg towards higher values. Expanding our finding to global rivers, the negative correlation between riverine δ 26 Mg and CWI can be interpreted by a competition between the fast dissolution of carbonates leading to the enrichment of 24 Mg in waters and Mg isotope fractionation induced by cation exchange leading to the depletion of 24 Mg in the residual waters. This study provides new insight into cation exchange as a regulator of riverine δ 26 Mg and Mg cycling, highlighting the robustness of riverine δ 26 Mg in tracing CWI and constraining the carbon cycle.