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The effects of desert dust upon climate and ecosystems depend strongly on its particle size and size-resolved mineralogical composition. However, there is very limited quantitative knowledge on the particle size and composition of the parent sediments along with their variability within dust-source regions, particularly in dust emission hotspots. The lower Drâa Valley, an inland drainage basin and dust hotspot region located in the Moroccan Sahara, was chosen for a comprehensive analysis of sediment particle size and mineralogy. Different sediment type samples (n= 42) were collected, including paleo-sediments, paved surfaces, crusts, and dunes, and analysed for particle-size distribution (minimally and fully dispersed samples) and mineralogy. Furthermore, Fe sequential wet extraction was carried out to characterise the modes of occurrence of Fe, including Fe in Fe (oxyhydr)oxides, mainly from goethite and hematite, which are key to dust radiative effects; the poorly crystalline pool of Fe (readily exchangeable ionic Fe and Fe in nano-Fe oxides), relevant to dust impacts upon ocean biogeochemistry; and structural Fe. Results yield a conceptual model where both particle size and mineralogy are segregated by transport and deposition of sediments during runoff of water across the basin and by the precipitation of salts, which causes a sedimentary fractionation. The proportion of coarser particles enriched in quartz is higher in the highlands, while that of finer particles rich in clay, carbonates, and Fe oxides is higher in the lowland dust emission hotspots. There, when water ponds and evaporates, secondary carbonates and salts precipitate, and the clays are enriched in readily exchangeable ionic Fe, due to sorption of dissolved Fe by illite. The results differ from currently available mineralogical atlases and highlight the need for observationally constrained global high-resolution mineralogical data for mineral-speciated dust modelling. The dataset obtained represents an important resource for future evaluation of surface mineralogy retrievals from spaceborne spectroscopy. [ABSTRACT FROM AUTHOR]

The effect of mineral dust emitted from arid and semiarid surfaces upon climate and ecosystems depends fundamentally on their particle size distribution (PSD) and size-resolved mineralogical composition. However, soil mineralogy atlases used for mineral-speciated dust modelling are highly uncertain as they are derived extrapolating mineralogical analyses of soil samples that are particularly scarce in dust-source regions. This extrapolation neglects the processes affecting the formation of different dust-emitting surface sediments, such as dunes, crusts, and paved sediments. The Lower DrÃa Valley, an inland drainage basin and preferential dust-source located in southern Morocco, was chosen for a comprehensive analysis of sediment grain size and mineralogy. Different sediment types samples were collected, including paleo-sediments, paved surfaces, crusts, and dunes, and analysed through PSD analysis of minimally and fully dispersed samples, and X-ray diffraction mineralogical analysis of bulk samples. We also performed Fe sequential wet extraction to characterize Fe mineralogy, including the contents of (oxyhydr)oxides (goethite and hematite), key to dust radiative effects, and poorly crystalline pool of Fe (readily exchangeable ionic Fe and nano-Fe-oxides), relevant to dust impacts upon ocean biogeochemistry. Based on the results we propose a conceptual model where both particle size and mineralogy are segregated by transport and deposition of sediments during runoff of water across the basin, and by the precipitation of salts, which causes a sedimentary fractionation. Coarser particles substantially richer in quartz are more present in elevated areas, and finer particles rich in clay, carbonates, and Fe-oxides are present in depressed areas, where dust emission is maximized. When water ponds and evaporates, secondary carbonates and salts precipitate, and the clays are enriched in readily exchangeable ionic Fe, due to sorption of dissolved Fe by illite. Our results differ from currently available mineralogical atlases and highlight the need for observationally-constrained global high-resolution mineralogical data for mineral-speciated dust modeling. [ABSTRACT FROM AUTHOR]