Factors controlling Mg isotopes in meltwater and suspended sediments of Arctic rivers

Enhanced chemical weathering in glacial regions, driven by climate change, is projected to increase the delivery of dissolved and particulate matter to the ocean, significantly disrupting biogeochemical cycles of critical elements which exert a strong influence on the global carbon cycle. This study investigates the elemental and Mg isotope geochemistry of meltwater, suspended particulate matter (SPM), and bedrock samples from Ny-Âlesund, Svalbard, in order to elucidate the link between glacial weathering processes and Mg isotope variations within this glacial environment. Magnesium isotopic compositions (δ²⁶Mg) in meltwaters and SPMs exhibit significant variability, in which meltwater δ²⁶Mg values are in isotopic equilibrium with corresponding SPM values, yielding two distinct isotope fractionation factors depending on the drainage lithology.

A global comparison of water δ²⁶Mg values in Arctic rivers reveals that variability in waterδ²⁶Mg can be attributed to two primary factors, which are a global isotopic equilibrium state that is consistent with what is observed in Svalbard, and an influence of drainage lithology (silicates versus dolomite). Globally, riverine Mg, on average, exhibits a consistent Mg isotopic signature that closely resembles that of the upper continental crust, regardless of the diverse environmental conditions encountered by these river systems. This observation strongly suggests that dynamic interactions between erosion and weathering processes rapidly drive the system towards isotopic equilibrium, which is well supported by this study. 

Overall, this study highlights that the difference in δ²⁶Mg between waters and SPMs can be used as a novel indicator for predicting weathering disequilibrium induced by global warming and other factors influencing the Earth's surface evolution.