Fractionation of rhenium isotopes in the Mackenzie River basin during oxidative weathering

Rhenium (Re) is a trace element whose redox chemistry makes it an ideal candidate to trace a range of geochemical processes. Here, we report the first rhenium isotopic measurements (δ187Re) from river-borne materials to assess the influence of chemical weathering on Re isotopes at continental scale. The δ187Re was measured in water, suspended sediments and bedloads from the Mackenzie River and its main Arctic tributaries in Northwestern Canada. We find that the δ187Re (relative to NIST SRM 989) of river waters ranges from −0.05‰ to +0.07‰, which is generally higher than the corresponding river sediment (−0.25‰ to +0.01‰). We show that the range of δ187Re in river sediments (∼0.30‰) is controlled by a combination of source bedrock isotopic variability (provenance) and modern oxidative weathering processes. After correcting for bedrock variability, the δ187Re of solids appear to be positively correlated with the amount of Re depletion related to oxidative weathering. This correlation, and the offset in δ187Re between river water and sediment, can be explained by preferential oxidation of reactive phases with high δ187Re (i.e. rock organic carbon, sulfide minerals), but could also result from fractionation during oxidation or the influence of secondary weathering processes. Overall, we find that both basin-average bedrock δ187Re (∼−0.05‰) and dissolved δ187Re (∼−0.01‰) in the Mackenzie River are lower than the δ187Re of Atlantic seawater (+0.12‰). These observations provide impetus for future work to constrain the Re isotope mass balance of seawater, and assess the potential for secular shifts in its δ187Re values over time, which could provide an additional isotopic proxy to trace current and past redox processes at Earth's Surface.