This study reports on the concentration of rare earth elements (REE) along with ancillary geochemical parameters at 11 locations across the Mackenzie River, its delta and coastal waters, both under ice and in open water. Specifically, we analyzed REE, carbon, and redox-sensitive elements (Fe, Mn) in 98 sediment samples and 96 porewater and overlying water samples collected under ice before the spring freshet (April-May) and in open water in early fall (August-September). While sediment REE concentrations remained relatively stable across seasons, results revealed a striking contrast between the two sampling seasons in the porewater, where REE concentrations were nearly two orders of magnitude higher under ice (avg. 216 nmolL-1) than under open water in the fall (avg. 3.20 nmolL-1). Similarly, dissolved organic carbon (DOC) concentrations were approximately one order of magnitude higher under ice than in the fall. Sediment REE concentrations were positively correlated to those of Fe and Mn, particularly under ice, consistent with control by adsorption processes onto their (oxy)hydroxides. In the porewater, winter and fall samples form distinct clusters based on concentration magnitudes. Chromophoric properties of dissolved organic matter (DOM) in the overlying water suggest that under-ice DOM was characterized by low aromaticity, older material compared to the more aromatic, humic-rich DOM measured in open water. We conclude that under-ice conditions, chiefly cold temperature, allow for DOM accumulation in the porewater, which, combined with other possible REE enrichment mechanisms in the porewater, such as REE-carbonate complex formation and exclusion during ice formation, contributes to the elevated winter REE concentrations observed here. To our knowledge, this is the first report of such large seasonal fluctuation in dissolved REE in the fluvial-marine transition zone of the Mackenzie, the largest riverine influence on the Arctic Ocean.
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