Investigating Mg Isotope Fractionation During Cation Exchange Processes

Mg stable isotopes are known to fractionate during secondary mineral formation, diffusion, and bio-uptake. Compared to these processes, Mg sorption-desorption and associated isotope fractionations are less well documented. To resolve this, a series of batch experiments were firstly conducted, which have revealed insignificant fractionation of Mg isotopes (< 0.2 ‰) during exchange with clay minerals (kaolinite and montmorillonite), cation resin (an analogue of organic matter) and natural regolith sediment. The lab experiment observation was further corroborated by our field research showing similarly insignificant difference in Mg isotope composition between concomitant dissolve Mg and exchangeable Mg in riparian zone, river channel and marine environments. Our findings suggest that Mg isotope exchange behaves as a simple mixing process, resulting in nearly identical δ26Mg values in both dissolved and exchangeable phases once equilibrium is achieved. One implication of this result is that the potential for exchangeable Mg to alter the δ²⁶Mg of water—or vice versa—depends on the relative masses of Mg in these two phases.

Table S1 Experimental Investigation of Mg Isotope Fractionation During Adsorption onto Cation Exchange Resin
Table S2 Experimental Investigation of Mg Isotope Fractionation During Cation Exchange on Natural Samples
Table S3 Mg Concentration and Isotope Composition of Paired Natural Water Samples and Exchangeable Fractions from Sediments
Table S4 Concentration of Exchangeable Mg in Regolith Sediments Along a Weathering Profile in the Mulanxi Catchment
Table S5 Concentration and Isotope Composition of Exchangeable Mg in Samples Used for Exchange Experiments
Table S6 Mg concentration and isotope composition of leachates from sediment leaching experiments using artificial rainwater
Table S7 Data quality control (reference materials measured in this study)