Unravelling the controls on the molybdenum isotope ratios of river waters

Horan, K., Hilton, R.G., McCoy-West, A.J., Selby, D., Tipper, E.T., Hawley, S., and Burton, K.W. (2020) Unravelling the controls on the molybdenum isotope ratios of river waters. Geochemical Perspectives Letters, 13.

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Abstract

The molybdenum (Mo) isotope ratios (δ98/95Mo) of river waters control the δ98/95Mo values of seawater and impact on the use of Mo isotope ratios as a proxy of past redox conditions. The δ98/95Mo values of river waters vary by more than 2 ‰, yet the relative roles of lithology versus fractionation during weathering remain contested. Here, we combine measurements from river waters (δ98/95Modiss), river bed materials (δ98/95MoBM) and soils from locations with contrasting lithology. The δ98/95Mo values of river bed materials (δ98/95MoBM), set by rock type, vary by ~1 ‰ between rivers in New Zealand, the Mackenzie Basin, and Iceland. However, the difference between dissolved and solid phase Mo isotopes (Δ98/95Modiss-BM) varies from +0.3 ‰ to +1.0 ‰. We estimate Mo removal from solution using the mobile trace element rhenium and find that it correlates with Δ98/95Modiss-BM across the sample set. The adsorption of Mo to Fe-Mn-(oxyhydr)oxides can explain the observed fractionation. Together, the amount of Mo released through dissolution and taken up by (oxyhydr)oxide formation on land may cause changes in the δ98/95Mo values of rivers, driving long term changes in the Mo isotope ratios of seawater.

Item ID: 65313
Item Type: Article (Research - C1)
ISSN: 2410-3403
Keywords: molybdenum isotopes, river catchments, weathering, redox, fractionation, rhenium
Copyright Information: © 2020 The Authors. This work is distributed under the Creative Commons Attribution 4.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Additional information is available at http://www.geochemicalperspectivesletters.org/copyrightand-permissions.
Funders: Natural Environment Research Council (UK), European Research Council (ERC), Durham University, British Society for Geomorphology, Carnegie Trusts (ETT), Aurora Research Institute
Projects and Grants: ERC-StG, 678779, ROC-CO2, Durham University Grant (Building Research Links in New Zealand)
Date Deposited: 15 Dec 2020 04:36
FoR Codes: 37 EARTH SCIENCES > 3703 Geochemistry > 370303 Isotope geochemistry @ 60%
37 EARTH SCIENCES > 3709 Physical geography and environmental geoscience > 370901 Geomorphology and earth surface processes @ 40%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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