Iron isotope fractionation in river colloidal matter

Temporal variations in the iron isotopic composition, δ56Fe between − 0.13‰ and 0.31‰, have been measured in the suspended fraction in a Boreal river. The major mechanism behind these variations is temporal mixing between two types of particles-colloids, Fe-oxyhydroxides and Fe-C colloids. Data in this study indicate that these two types of colloids have different Fe-isotope composition. The Fe-C colloid has a negative δ56Fe value whereas the Fe-oxyhydroxide colloid is enriched in 56Fe.These two types of colloidal matter have different hydrogeochemical origin. The Fe-C colloid reaches the river during storm events when the upper sections of the soil profile (O and E horizons) are flooded by a rising water table. Colloidal Fe-oxyhydroxides reach the river via inflow and subsequent oxidation of groundwater enriched in dissolved Fe(II).