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The high field strength element budget of atmospheric aerosols (puy de Dôme, France)

Abstract : High field strength elements (HFSE), including Zr, Hf, Nb, Ta and Ti have low solubility in aqueous fluids and partition into dense and resistant minerals. HFSE proved useful in studying terrestrial weathering and sediment transport, but little is known about their behavior during atmospheric processes, which play an important role in global sedimentary cycles. The atmospheric budget of HFSE is evaluated from the sequential dissolution of aerosol samples collected between 2011 and 2014 at puy de Dôme (1465 m elevation, French Massif Central). Aerosols were sampled during nighttime, while the site is generally located above the planetary boundary layer. Systematic, partial recovery of HFSE during gentle dissolution of aerosols indicates that resistant minerals are ubiquitous in air samples. Total dissolution of aerosols in pressure vessels reveals that Zr and Hf occur on average in sub-crustal abundance, which is consistent with the sampling site being dominantly influenced by oceanic air masses depleted in zircons. Conversely, zircon excess occasionally occurs in continental air masses, in particular those originating from northern Africa. Overall, the Hf/Nd ratio, a proxy for zircon fractionation, varies from 0.26 to 3.94 times the Upper Continental Crust (UCC) value, encompassing the range of worldwide loess. This wide compositional range is consistent with (1) the occurrence of coarse zircons (10–30 μm) in dust source, with possible local enrichments relative to bulk UCC in residual wind-winnowed soils, and (2) gravitational settling of coarse zircons during long-distance (>ca. 1000 km) transport. Niobium and Ta are systematically more abundant (by a mean factor of ∼3) in puy de Dôme aerosols than expected from average crustal or soil concentrations. The volume-weighted average Nb/Ta ratio of 15.5 ± 2.6 (1σ) is also higher than in bulk UCC (11.4–13.3). The positive Nb–Ta anomaly of free troposphere aerosols unlikely reflects a net Nb–Ta enrichment but might result from loss of more water-soluble elements during weathering of aerosols in clouds. Depletion in Zr–Hf (coarse zircons settling) and Nb–Ta enrichment (cloud processing) might occur during large-scale transport of mineral dust over ocean basins, which could explain the peculiar HFSE distribution in some Hawaiian soils showing inputs of Asian dust.
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Ivan Vlastélic, Krzysztof Suchorski, Karine Sellegri, Aurélie Colomb, François Nauret, et al.. The high field strength element budget of atmospheric aerosols (puy de Dôme, France). Geochimica et Cosmochimica Acta, Elsevier, 2015, 167, pp.253-268. ⟨10.1016/j.gca.2015.07.006⟩. ⟨hal-02115272⟩



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