Mineral magnetic properties of surface soils and sands across four North African transects and links to climatic gradients

In light of their earlier success in characterizing and differentiating soils, sediments and atmospheric particulates, magnetic measurements were carried out on samples of North African surface soils and sands. The main aims were to document spatial variations in magnetic properties, explore their possible links to climatic gradients and their likely basis, and thereby help to differentiate potential sources for Sahara/Sahel-derived dusts. Samples were collected along four broadly north-to-south orientated transects located in Egypt, Niger, Mali, and Benin and southern Togo. These transects cover major climatic gradients ranging from hyper-arid to tropical. Across the rainfall range spanned by the Niger and Mali Transects (800 mm/yr) there are significant north-to-south gradients in magnetic mineral concentrations. This is seen most clearly in the concentrations of the finest ferrimagnetic minerals (magnetite/maghemite), which are strongly represented in the clay fraction. Concentrations of the imperfect antiferromagnetic minerals (predominantly haematite) also increase with increasing rainfall. By analogy with studies spanning a similar rainfall gradient in areas of more uniform lithology, we suggest that the most likely cause for these trends is increased chemical weathering linked to the progressively wetter climate across the Sahara/Sahel transition. Within the set of samples from Egypt (rainfall >100 to