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

Carbonate weathering plays a significant role in regulating global carbon budget at short timescales, and thus needs to be better constrained in the context of global warming. Riverine magnesium isotopes (δ26Mg) 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 δ26Mg is rare. In this study, spatial riverine δ26Mg 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 δ26Mg 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 Mg2+ sources and confirmed that the riverine Mg2+ budget was dominated by carbonate weathering, followed by evaporites dissolution. However, conservative mixing could not explain riverine δ26Mg variation within the Three Rivers catchments, because measured δ26Mg values (δ26Mgmeasured) systematically deviated from the modeled ones (δ26Mgmodeled), with Δ26Mgmeasured–modeled (δ26Mgmeasured – δ26Mgmodeled) up to 0.79‰. The positive correlations between Δ26Mgmeasured–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, Mg2+-Na+ exchange is proposed for the first time as an explanation for the observed δ26Mg variations in the rivers draining the Tibetan Plateau, although the role of carbonate precipitation could not be excluded. The strong positive correlations between riverine δ26Mg 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 Mg2+-Na+ exchange, driving riverine δ26Mg towards higher values. Expanding our finding to global rivers, the negative correlation between riverine δ26Mg and CWI can be interpreted by a competition between the fast dissolution of carbonates leading to the enrichment of 24Mg in waters and Mg isotope fractionation induced by cation exchange leading to the depletion of 24Mg in the residual waters. This study provides new insight into cation exchange as a regulator of riverine δ26Mg and Mg cycling, highlighting the robustness of riverine δ26Mg in tracing CWI and constraining the carbon cycle.