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 × 10 5 yr (τ Mo ) and 1.3 × 10 5 yr (τ Re 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 -SO 2− 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 187 Re -187 Os decay, Re isotope measurements have a reproducibility of ±0.05 for analyte concentrations of 20 ng Re mL −1 . Total Re isotopic variability to date is 0.9 . This includes 0.3 across five commercially available Re products, and 0.5 across a black shale weathering profile. δ 187 Re variability across the weathering profile was systematic with the most weathered samples showing the most significant 187 Re depletions. The 3 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.

Fault-charged mantle-fluid contamination of United Kingdom North Sea oils: Insights from Re-Os isotopes: Figure 1.

“…Osmium is organophyllic , and our data suggest that Os can easily be complexed with oil during interaction with an Os-bearing fl uid, thereby overprinting the original radiogenic Kimmeridge Clay Formation ratio (>1) with an unradiogenic mantle-like ratio (~0.12). Hydrothermal fl uids are reported to be essentially bereft of Re (−0.92 -5.6 pmol kg -1 ; Miller, 2009). This may be supported by our results that show that Re abundances and 187 Re/ 188 Os are similar in oils that have unradiogenic 187 Os/ 188 Os (0.22-1.4 ppb, 1.5-339, respectively) to oils bearing radiogenic 187 Os/ 188 Os (0.13-4.2 ppb, 17-653, respectively; Table DR1).…”

Spatial and temporal dynamics of biogeochemical processes in the Fraser River, Canada : a coupled organic-inorganic perspective

“…This sampling protocol has been described previously (Miller, 2009;Miller et al, 2011). Nutrient concentration samples were collected in PE scintillation vials, pre-cleaned first with laboratory soap solution, then soaked for 3 days in 10% HCl, followed by 3 rinses with Milli-Q (Millipore) water.…”

Tracing river chemistry in space and time: Dissolved inorganic constituents of the Fraser River, Canada

“…certification by the U.S. Environmental Protection Agency). This sampling protocol has been 135 described previously (Miller, 2009;Miller et al, 2011 are presented in Table 4 for comparison.…”