Hard, Confusing, Entertaining is any attempt to understand James’ joys, or the details of the commodius vicus recirculation of molybdenum. Molybdenum (Mo) is a redox-sensitive element whose isotope fractionation in oceanic sediments is being established as a proxy for paleo-oceano-graphic conditions. In order to constrain the oceanic Mo cycle, it is necessary to understand its sources. The average Mo riverrun influx to the oceans is not well established. Crystalline continental crust has a lighter isotopic composi-tion than oceanic Mo by about 2 permil. We report here Mo isotopic compositions and concentrations from several closely-spaced river catchment profiles. We selected our sampling in such a way to monitor, for each sample suite, one or more of the following potential controlling factors: base-ment geology; fractionation during weathering; within-river precipitation (including lakes); anthropogenic contamination. Our results show variable deviations of dissolved river Mo from average crystalline continental crust, between 0 and 1.9 permil. We observe a positive, but not simple, cor-relation between Mo concentration and isotopic composi-tion, indicating that the riverine Mo system and thus global continental runoff are highly complex. Neither steady-state soil adsorption nor within-river precipitation cause a major isotopic fractionation. Weathering and labo-ratory leaching may lead to transient fractionation in some cases. The control by basement geology in most other cases is evident and can be documented in great detail along river transects and in tributaries; rock analyses supplement water analyses and confirm lithological control. Two very prominent effects are weathering of sedimentary (evaporitic) sulfates and oxidative weathering of sulfides. Several sub-economic molybdenite (MoS2) occurrences in the Swiss Alps contribute isotopically variable Mo. Disseminated pyrite occurs in most magmatic, metamorphic and sedimentary rocks, with variable Mo concentrations. The weathering style (glacial vs. non-glacial) has an extremely high influence on dissolved Mo concentration in rivers. In our samples anthropogenic effects appear to be subordinate except in areas of intensive use of agricultural fertilizers.
Villa, I., Greber, N., Haudenschild, E., Heri, A., Hofmann, B., Neubert, N., et al. (2009). Riverrun, past Eve and Adam’s: Molybdenums Wake. In Goldschmidt Conference Abstracts 2009 (pp.A1384-A1384). Pergamon.
Riverrun, past Eve and Adam’s: Molybdenums Wake
VILLA, IGOR MARIA;
2009
Abstract
Hard, Confusing, Entertaining is any attempt to understand James’ joys, or the details of the commodius vicus recirculation of molybdenum. Molybdenum (Mo) is a redox-sensitive element whose isotope fractionation in oceanic sediments is being established as a proxy for paleo-oceano-graphic conditions. In order to constrain the oceanic Mo cycle, it is necessary to understand its sources. The average Mo riverrun influx to the oceans is not well established. Crystalline continental crust has a lighter isotopic composi-tion than oceanic Mo by about 2 permil. We report here Mo isotopic compositions and concentrations from several closely-spaced river catchment profiles. We selected our sampling in such a way to monitor, for each sample suite, one or more of the following potential controlling factors: base-ment geology; fractionation during weathering; within-river precipitation (including lakes); anthropogenic contamination. Our results show variable deviations of dissolved river Mo from average crystalline continental crust, between 0 and 1.9 permil. We observe a positive, but not simple, cor-relation between Mo concentration and isotopic composi-tion, indicating that the riverine Mo system and thus global continental runoff are highly complex. Neither steady-state soil adsorption nor within-river precipitation cause a major isotopic fractionation. Weathering and labo-ratory leaching may lead to transient fractionation in some cases. The control by basement geology in most other cases is evident and can be documented in great detail along river transects and in tributaries; rock analyses supplement water analyses and confirm lithological control. Two very prominent effects are weathering of sedimentary (evaporitic) sulfates and oxidative weathering of sulfides. Several sub-economic molybdenite (MoS2) occurrences in the Swiss Alps contribute isotopically variable Mo. Disseminated pyrite occurs in most magmatic, metamorphic and sedimentary rocks, with variable Mo concentrations. The weathering style (glacial vs. non-glacial) has an extremely high influence on dissolved Mo concentration in rivers. In our samples anthropogenic effects appear to be subordinate except in areas of intensive use of agricultural fertilizers.
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