Surface-cycling of rhenium and its isotopes

Abstract

The application of elemental and isotopic metal palaeoredox tracers to the geologic past rests on an understanding of modern metal cycles. This study reevaluates the surface-cycling of Mo and Re in near-surface reservoirs. Revised river averages of Mo and Re are 1.8- and 7.9-fold larger than previous estimates. The river concentrations of 8.0 nmol Mo kg−1 and 11.2 pmol Re kg−1 (pre-anthropogenic), result in shorter seawater response times of 4.4×105 yr (τMo) and 1.3×105 yr (τRe pre-anthropogenic). These metals, especially Re, are more sensitive to changing source and sink fluxes than previously thought. Evaluation of Mo and Re concentrations in high temperature fluids from the Manus Basin indicate that Re is essentially absent from the hydrothermal end member and Mo is present at concentrations considerably lower than ambient seawater. The sink fluxes represented by hydrothermal circulation are negligible in comparison to the revised river source fluxes. Anthropogenic contributions to the Re flux to seawater are seen in the high concentrations of certain impacted water samples such as those associated with mining sites. It may also be seen in a significant, variable, Re enrichment feature in the Hudson River estuary. This Re enrichment feature is not the result of estuarine mixing or the remobilization of sediment-hosted Re. On the basis of a Re - SO2− 4 correlation we are able to quantify and correct for the anthropogenic Re, which corresponds to ~33% of the modern river average. This study documents the development of an analytical method for stable Re isotopes. Though complicated by analyte requirements and 187Re – 187Os decay, Re isotope measurements have a reproducibility of ±0.05h for analyte concentrations of 20 ng Re mL−1. Total Re isotopic variability to date is 0.9h. This includes 0.3h across five commercially available Re products, and 0.5h across a black shale weathering profile. δ187Re variability across the weathering profile was systematic with the most weathered samples showing the most significant 187Re depletions. The Re isotopic weathering profile is well described by both two-component mixing and Rayleigh fractionation. There are currently insufficient data to discriminate between the two models.