Surface-Cycling of Rhenium and Its Isotopes

Christian Miller, Ph.D., 2009
Bernhard Peucker-Ehrenbrink, Advisor

The application of elemental and isotopic metal palaeoredox tracers to the geologic past rests on an understanding of modern metal cycles. This study re-evaluates the cycling of Mo and Re in near-surface reservoirs. Revised average river concentrations of 8.0 nmol Mo kg–1 and 16.5 pmol Re kg–1 are 1.8- and 7.9-fold larger than previous estimates, and result in shorter seawater response times of 4.4 x 105 (tMo) and 8.2 x 104 yr (tRe, pre-anthropogenic). Anthropogenic contributions to the Re river water flux are seen in the high concentrations of certain impacted water samples, such as those associated with mining sites. On the basis of a global Re–SO42– correlation we estimate the contribution of anthropogenic Re to be ∼33% of the modern river average.

This study describes the first analytical method for stable Re isotope analysis. Rhenium isotope measurements have a reproducibility of ±0.05‰ for analyte concentrations of 20 ng Re mL–1. Total d187Re variability observed to date is 0.9‰ including a systematic 0.5‰ variation across a black shale weathering profile.  The Re isotopic weathering profile is well described by both two-component mixing and Rayleigh fractionation. There currently aren’t sufficient data to discriminate between the two models.