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

• 0.3‰ of δ 187 Re variation across rivers from the Mackenzie River basin. • For each river, the δ 187 Re of the dissolved load is higher than that of the corresponding river sediment • The δ 187 Re of river water and sediments is controlled by both provenance and modern oxidative weathering processes • The average δ 187 Re of Mackenzie bedrock (∼ − 0.05 ‰) and dissolved load (∼ − 0.01 ‰) are lower than Atlantic seawater δ 187 Re. 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 (δ 187 Re) from river-borne materials to assess the influence of chemical weathering on Re isotopes at continental scale. The δ 187 Re was measured in water, suspended sediments and bedloads from the Mackenzie River and its main Arctic tributaries in Northwestern Canada. We find that the δ 187 Re (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 δ 187 Re 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 δ 187 Re of solids appear to be positively correlated with the amount of Re depletion related to oxidative weathering. This correlation, and the offset in δ 187 Re between river water and sediment, can be explained by preferential oxidation of reactive phases with high δ 187 Re (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 δ 187 Re (∼−0.05‰) and dissolved δ 187 Re (∼−0.01‰) in the Mackenzie River are lower than the δ 187 Re 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 δ 187 Re values over time, which could provide an additional isotopic proxy to trace current and past redox processes at Earth's Surface. [ABSTRACT FROM AUTHOR]