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

Abstract

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 (Re) from river-borne materials to assess the influence of chemical weathering on Re isotopes at continental scale. The 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 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 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 Re of solids appear to be positively correlated with the amount of Re depletion related to oxidative weathering. This correlation, and the offset in Re between river water and sediment, can be explained by preferential oxidation of reactive phases with high 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 Re (∼−0.05‰) and dissolved Re (∼−0.01‰) in the Mackenzie River are lower than the 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 Re values over time, which could provide an additional isotopic proxy to trace current and past redox processes at Earth's Surface.