Concentration-discharge relationships of dissolved rhenium in Alpine catchments reveal its use as a tracer of oxidative weathering
Abstract
Chemical weathering of rocks acts to exchange carbon dioxide (CO2) with the atmosphere. Hydrology plays a central role in these reactions, supplying reactants and transporting them away from reaction sites. Many studies have examined the weathering of silicate and carbonate minerals, from the context of CO2 transfer to dissolved inorganic carbon via carbonic acid weathering of these phases. However, many sedimentary rocks contain rock organic carbon (OCpetro). Oxidation of OCpetro acts to transfer carbon from the lithosphere and plays an important role in the global carbon cycle. Despite this recognition, there remain very few estimates of catchment-scale rates of OCpetro oxidation.
The element rhenium (Re) has been proposed as a tracer of rock organic carbon (OCpetro) oxidation, and the dissolved fluxes of Re used to estimate OCpetro weathering rates across river catchments. However, the sources and pathways of Re through river catchments remain poorly understood. Here we examine the behaviour of dissolved Re across a time series of samples collected from three instrumented alpine catchments which all drain sedimentary rocks (Erlenbach and Vogelbach in Switzerland, and the East River, Colorado), but differ in terms of erosion rate and hydro-climate. We examine relationships between Re concentration and water discharge, and ion ratios and water discharge. The two paired catchments in Switzerland have contrasting Re fluxes (with higher Re flux in the catchment with higher erosion rates) but reveal similar [Re]/[Na+] and [Re]/[SO42-] ratios, which suggest a dominant source of Re from OCpetro, mixing with Re from silicates and sulfides. In all three catchments, these ratios are positively correlated with water discharge. We suggest preferential routing of Re from a near-surface oxidative weathering zone, which is decoupled from the sulfide oxidation front and consistent with a source of Re from OCpetro. Low flow samples suggest that fluids with longer residence times are more enriched in silicate- and sulfide-derived ions in these catchments. Together, these observations support the use of Re as a proxy to trace rock-organic carbon oxidation. In the Erlenbach, we apply the Re proxy, and estimate carbon dioxide production by OCpetro oxidation of 5.7 +6.6/-2.0 tC km-2 yr-1, with a weathering intensity of ~40%.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H11J1639H
- Keywords:
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- 1899 General or miscellaneous;
- HYDROLOGY