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The molybdenum (Mo) isotope budget of the surface environment has been well characterized in recent decades, facilitating accurate mass balance modeling of the ancient redox-driven Mo cycle. One yet unresolved component is the range of possible processes and sources that result in isotopically heavy river waters relative to continental sources. Following a recent hypothesis that isotopically heavy δ98Mo of precipitation may control the final δ98Mo of river water relative to its continental source bedrock, we investigated the δ98Mo composition of 19 snow samples from three locations in Central Europe: the Swiss Alps ("Alpine" samples) from the Hochalpine Forschungsstation Jungfraujoch (HFSJ) in both summer and winter, the Swiss Jura Mountains in winter and the French Vosges Mountains in winter. Stream waters from two snowmelt-fed streams were additionally collected from the Alpine site. Snow sample δ98Mo compositions were highly variable, ranging from −0.03 to +1.93 ‰, with no clear mixing trends indicating complex sources, source pathways, or post-depositional processing. Only the winter snow samples from the high-altitude HFSJ site had δ98Mo values consistently heavier than typical continental crust. The δ98Mo results were coupled with radiogenic Sr isotopic, major ion, and trace element compositions as tracers for three major sources of airborne ion inputs: sea salt, mineral dust, and anthropogenic aerosols. We found that the most likely source of Mo to precipitation in the lower elevation Vosges and Jura Mountains samples was isotopically light soil mineral dust, which reflected the underlying bedrock sources, with a likely additional anthropogenic component. The isotopically heavy winter snow samples at the elevated HFSJ site were attributed to a higher input of carbonate mineral dust from long-distance transport of carbonate-rich Saharan dust, which was overwritten in the summer by an influx of low elevation sources transported upwards during higher vertical thermal mixing. Finally, we concluded that precipitation has a negligible direct effect on the overall Mo content and isotopic composition of inner continental European streams and rivers relative to other sources, such as bedrock weathering. Future time-series studies may augment these conclusions to show possible heterogeneity of precipitation as well as the addition of sample sites closer to marine sources, where a larger flux of isotopically heavy marine aerosols to streams might be expected. • Highly variable δ98Mo in precipitation ranging from −0.03 to +1.93 ‰. • Seasonal δ98Mo variation with complex sources (natural, anthropogenic) observed. • Negligible contribution of marine aerosols to the δ98Mo of rainwater. • Strongest δ98Mo signal from carbonate-rich mineral and soil dust. • Airborne sources have a negligible impact on the overall riverine [Mo] and δ98Mo. [ABSTRACT FROM AUTHOR]

Abstract: Concerning the question of how and when free oxygen started to accumulate in the Earth’s atmosphere and hydrosphere, we report Mo concentrations and isotopic compositions as well as Platin group element and Rhenium (PGE-Re) data from black shales of the Transvaal Supergroup (Ghaap Group, 2.64–2.5Ga, and Pretoria Group, 2.45–2.15Ga). For comparison we also include new data from the 3.23Ga Fig Tree Group, Barberton Greenstone Belt. This time range covers the period between the first robust evidence for cyanobacteria at 2.7Ga and the disappearance of mass independent sulfur isotope fractionation (MIF) at 2.32Ga. Due to the redox dependent solubility of Mo, Re and Os, such data are important proxies for changes in oxygen levels of the early atmosphere and oceans. In particular, Mo isotope fractionation can only occur when Mo is in solution as oxyanions, requiring free oxygen. In the Fig Tree Group samples, Mo is not fractionated relative to the continental crust, and PGE-Re abundance patterns reflect those of komatiites, indicating a purely detrital input of these element, a sign of no free oxygen. A general increase in Mo concentration and isotope fractionation, as well as an enrichment in Re, compared to the continental crust and Fig Tree Group, can be seen within the Ghaap Group indicating a gradual rise of oxygen between 2.64 and 2.5Ga. However, Mo concentrations and δ98/95Mo values vary strongly in this period. Local conditions of sedimentation (changes in redox and input conditions) and/or global oxic/anoxic fluctuations could have caused these Mo variations. Samples of the overlying Pretoria Group (2.45–2.15Ga) show Mo concentrations and isotopic compositions somewhat below the continental input, while PGE-Re abundance patterns are quite similar to those for the Ghaap. The apparent contradiction between the decoupled Mo and PGE-Re values can be resolved assuming a stratified ocean with almost total scavenging of Mo. Such an increased scavenging can result from enhanced biological sedimentation between and especially after glacial events. This period coincides with the Lomagundi-Jatulian carbon isotope excursion, which indeed indicates very enhanced organic carbon burial. [Copyright &y& Elsevier]