A joint research team led by the Institute of Earth Environment of the Chinese Academy of Sciences (CAS) has reported the spatial riverine magnesium isotope (δ²⁶Mg) variation in the Three Rivers (the Jinsha Jiang, the Lancang Jiang, and the Nu Jiang), three extremely-high-erosion catchments in the southeastern Tibetan Plateau. 

They found that riverine magnesium isotope had undergone “in-stream” fractionation driven by cation exchange associated with SPM delivered to rivers in large amounts under extremely-high erosion. A novel mechanism was proposed to reveal the negative correlation between riverine δ²⁶Mg and carbonate weathering intensity in the Three Rivers and large rivers globally, leading to a refined riverine δ²⁶Mg tool for the quantification of carbonate weathering at the continental scale. 

This work was published in Geochimica et Cosmochimica Acta on 15 December 2023.

Riverine δ²⁶Mg values were determined by a Thermo-Fisher Neptune Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS), ranging from -1.39‰ and -0.50‰ within the Three Rivers catchments. Based on constraints of an inversion mixing model, the researchers has partitioned riverine Mg²⁺ sources and confirmed that Mg²⁺ budget was dominated by carbonate weathering, followed by evaporite dissolution in the Three Rivers. Then, they employed a conservative mixing analysis for riverine magnesium isotope, and found that riverine magnesium isotope has been fractionated by Mg²⁺-Na⁺ exchange during river transport associated with SPM or suspended loads. 

Moreover, the team has calculated, based on the Rayleigh fractionation model, that at least 6-33% (and up to 58-73%) of the initial Mg²⁺ within the Three Rivers Basin was removed by cation exchange, with isotopically light Mg (²⁴Mg) preferentially adsorbed into the riverine clay exchange pool under extremely-high erosion and suspended loads.

Extending the finding to global large rivers, they suggested that the negative correlation between riverine δ²⁶Mg and carbonate weathering intensity could 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 global Mg cycling. 

Contact: BAI Jie, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China. Email: baijie@ieecas.cn