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Geochemical signature of sources, recycling and weathering in the Last Glacial loess from the Rhône Valley (southeast France) and comparison with other European regions

Abstract : Pleistocene loess of the Rhône Valley and Provence region, disconnected from the two main European aeolian systems (North European Loess Belt and Danube basin), are of interest to test the impact of source variability, grain size sorting, weathering and sediment recycling on the geochemical composition. The concentration of major, trace and rare earth elements in loess and potential sources was analysed through Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and ICP-Mass Spectometry (ICP-MS). Analyses show a genetic relationship between loess and the Ca-rich river sediments from the Alpine Ice Sheet (AIS). The loess of Provence show distinctive properties with a higher CaO and MgO content and a higher Europium anomaly (Eu/Eu*), suggesting sources richer in dolomite together with a contribution from mafic rocks (Alpine ophiolite complex). The homogeneous loess composition, compared to alluvial sources, reflects particle mixing during transport in the atmosphere. At the European scale, the geochemical composition of loess changes according to regions, but remains consistent within each region. Such variability is mostly related to the composition of rocks outcropping in respective catchments and especially in areas covered by ice sheets. The Chemical Proxy of Alteration (CPA) and Rb/K ratio suggest a weak chemical weathering linked to the cold and dry conditions typical of the Last Glacial. Recycling of older aeolian formations possibly caused loess enrichment in resistant minerals and quartz, especially for loess derived mostly from the abrasion of aeolian sand (Aquitaine).
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Mathieu Bosq, Pascal Bertran, Jean-Philippe Degeai, Alain Queffelec, Olivier Moine. Geochemical signature of sources, recycling and weathering in the Last Glacial loess from the Rhône Valley (southeast France) and comparison with other European regions. Aeolian Research, Elsevier, 2020, ⟨10.1016/j.aeolia.2019.100561⟩. ⟨hal-02440380v2⟩

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