Theoretical modeling of rhenium isotope fractionation, natural variations across a black shale weathering profile, and potential as a paleoredox proxy

- Predictions of equilibrium isotope fractionation for rhenium (δ187Re) are ‰ level.
- Fractionation is predominantly mass dependent.
- 0.3‰ of δ187Re variation is observed across a modern black shale weathering profile.
- 0.5‰ of variation is seen across several million years of Late Devonian stratigraphy.
- These observations suggest the potential utility for δ187Re as a paleoredox proxy.

We present the first data documenting environmental variations in the isotope composition of Re, and the first theoretical models of equilibrium Re isotope fractionation factors. Variations of δ187Re at modern surface temperatures are predicted to be ‰ level for redox (ReVII ⇌ ReIV) and perrhenate thiolation reactions (ReVIIO4−⇌ReVIIOXS4−X−⇌ReVIIS4−). Nuclear volume fractionations are calculated to be smaller than mass dependent effects. Values of δ187Re from New Albany Shale samples presented in this work and in a previous study show a range of 0.8‰ over a stratigraphic interval of ∼20 m. The magnitude of variation is consistent with theoretical predictions and may provide evidence for changing δ187Re of seawater in the geologic past. A −0.3‰ change in δ187Re across a 14 m horizontal black shale weathering profile is accompanied by a hundred-fold decrease in Re concentration and a 75% decrease in organic carbon associated with the transition from reducing to oxic weathering environment. We attribute decreasing δ187Re to the loss of organically bound Re component (δ187Re = −0.28‰). The Re isotope composition of the complementary detrital silicate fraction varies from −0.59 to −1.5‰, depending on the choice of silicate Re concentration.