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Unravelling the controls on the molybdenum isotope ratios of river waters

K. Horan1,

1Department of Earth Sciences, Durham University, South Road, Durham, DH1 3LE, UK

R.G. Hilton2,

2Department of Geography, Durham University, South Road, Durham, DH1 3LE, UK

A.J. McCoy-West1,3,

1Department of Earth Sciences, Durham University, South Road, Durham, DH1 3LE, UK
3School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, 3800, Australia

D. Selby1,4,

1Department of Earth Sciences, Durham University, South Road, Durham, DH1 3LE, UK
4State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Resources, China University of Geosciences, Wuhan, 430074, Hubei, China

E.T. Tipper5,

5Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK

S. Hawley1,

1Department of Earth Sciences, Durham University, South Road, Durham, DH1 3LE, UK

K.W. Burton1

1Department of Earth Sciences, Durham University, South Road, Durham, DH1 3LE, UK

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Cite this letter as: Horan, K., Hilton, R.G., McCoy-West, A.J., Selby, D., Tipper, E.T., Hawley, S., Burton, K.W. (2020) Unravelling the controls on the molybdenum isotope ratios of river waters. Geochem. Persp. Let. 13, 1–6.

KH was funded by a Natural Environment Research Council (NERC, UK) PhD award. RGH was supported by a European Research Council Starting Grant (ERC-StG, 678779, ROC-CO2). Fieldwork in New Zealand was funded by a Durham University Grant (Building Research Links in New Zealand) to RGH. Fieldwork support for Canada came from the British Society for Geomorphology (RGH) and the Carnegie Trusts (ETT).

Geochemical Perspectives Letters v13  |  doi: 10.7185/geochemlet.2005
Received 13 August 2019  |  Accepted 07 December 2019  |  Published 19 February 2020
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY 4.0




Figure 1 Molybdenum isotope ratios (δ98/95Mo, NIST-SRM3134 = 0 ‰) for this study (Southern Alps, Iceland, Mackenzie Basin and Yukon), alongside published measurements with: R = rocks, BM = river bed materials (grey); SL = suspended load (pink); S = soils (yellow); W = water (blue). Measurements are shown as grey dots, bars show the ±2 s.e. and whiskers ±2 s.d.
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Figure 2 The Mo isotope ratios of materials from the western Southern Alps, New Zealand, versus the Mo to Re concentration ratios for river waters (light blue), river bed materials (grey), suspended load (purple) and soils (yellow). Black diamond is the mean of the bed material samples. Shaded domains show the expected fields of soil and water compositions if preferential dissolution of sulfides was occuring, but data lie perpendicular to this trend implying an alternative mechanism is responsible for fractionation patterns observed.
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Figure 3 The fraction of Mo remaining in river water, fModiss, estimated using the ratio of Mo to rhenium (Re) in the dissolved load relative to parent materials, versus the difference in δ98/95Mo between river water and river bed materials. Lines are a batch fractionation model using fractionation factors between a solution and secondary mineral phases, based on fractionation factors of -0.8 ‰ (black) to -1.4 ‰ (grey) (Goldberg et al., 2009

Goldberg, T., Archer, C., Vance, D., Poulton, S.W. (2009) Mo isotope fractionation during adsorption to Fe (oxyhydr) oxides. Geochimica et Cosmochimica Acta 73, 6502-6516.

; Barling and Anbar, 2004

Barling, J., Anbar, A.D. (2004) Molybdenum isotope fractionation during adsorption by manganese oxides. Earth and Planetary Science Letters 217, 315-329.

). Error bars indicated for fModiss are the propagated 2 s.e. errors on (Mo/Re)BM, which is the main source of uncertainty. Error bars for Δ98/95Modiss-BM incorporate the 2 s.d analytical error on δ98/95Modiss and the 2 s.e. of the mean δ98/95MoBM.
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