Size-resolved mineralogy of dry- and wet-sieved surface sediment samples collected during a field campaign in Morocco

Abstract:

Detailed studies linking the size-resolved mineralogy of emitted dust with its parent sediment are currently lacking, hindering possibilities to extend and constrain theories of dust emission. Within the FRAGMENT project (FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe), we conducted detailed sampling of surface and airborne sediment during a field campaign near M’hamid El Ghizlane, Morocco, in a border basin of the Sahara Desert. We use 42 samples to characterise the variability of particle size and size-resolved mineralogy across the basin. We find coarser particle sizes in the highlands and finer particles in the lowlands where flooding occurs. Our analysis shows three major types of surface sediments: original paleo-sediments, paved sediments and crusts. Quantitative mineralogical analysis using X-Ray Diffraction (XRD) and the internal reference quantitative approach (using Quartz as an internal reference), show presence of clays (Mica, Chlorite, Smectite, Palygorskite and Kaolinite), carbonates (Calcite and Dolomite), feldspars (Albite/Anorthite and Microcline), Quartz, iron oxides (Goethite and Hematite) and salts (Gypsum and Halite). We find mineralogical and particle size segregation in crusts, with surfaces enriched with finer particles, clays, iron oxides and salts. A conceptual model on the formation of these crusts is elaborated and validated with vertical micro-profiles of these crusts. We additionally analyse 20 wet-sieved (totally disturbed) and dry-sieved (minimally disturbed) samples for size-resolved mineralogy at high particle size resolution (>2000, 1000-2000, 500-1000, 250-500, 80-250, 63-80, 40-63, 20-40, 10-20 and <10µm). Important mineralogical differences between wet and dry size fractions are found and used to better understand the fragmentation of sediment aggregates. We compare our results with global atlases of soil composition that are currently used to simulate dust mineralogical composition at global scale.