This dataset consists of supplementary data tables (S4 to S15) with all underlying data and information on sampling localities. The data derive from a study of reactive phases in suspended particulates in a major Arctic river, the Mackenzie River (largest sediment source to the Arctic Ocean). Data derive from various sampling localities and six sediment cores. Data tables include information on: mass balance verification of leaching method; elemental concentrations of filtrates, sediment residue concentration data and sediment leachate concentrations (ICP-OES); neodymium and strontium isotopic data on river sediments and filtered water; REE concentration data on suspended sediment leaches and residues; REE concentrations in standards via isotope dilution (ICP-MS). The paper associated with this dataset is available by subscription at https://doi.org/10.1016/j.epsl.2021.116933. The accepted manuscript and a document of supplementary materials can be accessed via https://www.repository.cam.ac.uk/handle/1810/324773.