Spectral signs of aeolian activity around a sand-dune belt in northern Algeria

Abstract

Dune color has been used to investigate the provenance, stabilization, and age of the sand. Here, we look for signs of aeolian activity in the colors of a sand-dune belt in northern Algeria. On the one hand, visible and nearinfrared spectral analyses of satellite images and laboratory samples showed reddish, yellowish, and whitish sands, depending on the amount of gypsum particles and Fe oxides coating quartz grains. Specifically, the dithionite-extractable Fe content was related to a redness index calculated from remote-sensing data and the abundance of hematite, estimated in the second derivative of the Kubelka-Munk function, paralleled the CIELAB hue-angle of sand samples. On the other hand, a spatiotemporal analysis showed that the reddish sand had undergone a continuous remobilization and dispersion throughout the area, reaching two large salt flats (sabkhas) with seasonal water. Yellowish and whitish sands appeared as patches on the periphery of these sabkhas and along the dune belt, exhibiting percussion marks and dissolution pits on the surface of quartz grains. Taken together, the results suggest that the reddish sand partially loses its Fe-oxide coatings by mechanical abrasion in the entrainment and reductive dissolution in the sabkhas during waterlogging, becoming yellowish. The periodic reactivation by wind of reddish and yellowish grains, together with whitish gypsum particles formed by evaporation as the sabkhas dry up, may explain the sorting of grains according to their mineralogy and size along the sand-dune belt, resulting in striking color changes. Accordingly, color reflects sand movements and chemical processes taking place in this dune system.