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

Abstract

[1] 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 (<100 to >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 <5 mm/yr), there is a marked gradient in the coercivity of remanence in the antiferromagnetic minerals present, with coercivity increasing southwards, paralleling the trend toward increasing aridity. Given the strong spatial discrimination demonstrated, especially in the Niger and Mali transects, these potentially climate-linked gradients hold out the promise of discriminating between dust source regions.

Source-to-sink magnetic properties of NE Saharan dust in Eastern Mediterranean marine sediments: review and paleoenvironmental implications

“…the hard isothermal remanent magnetization (HIRM) imparted with a field of 1 T and a back field of 300 mT) increases more than tenfold when precipitation rises from <25 to >800 mm/year ( Figure 2B), and its coercivity increases with decreasing rainfall (Lyons et al, 2010). Goethite might also be present in a minority of samples (Oldfield et al, 2014).…”

“…Nevertheless, the concentration of ferrimagnets (estimated from the anhysteretic remanent magnetization, ARM) increases tenfold between arid regions of the central Sahara (e.g., <25 mm/year) and wetter regions (>800 mm/year) further to the south (Figure 2A) (Lyons et al, 2010). Coarser [pseudosingle domain (PSD) and multi-domain (MD)] ferrimagnetic grains are also found in soils and dune sands (Lyons et al, 2010(Lyons et al, , 2012Oldfield et al, 2014). The high coercivity fraction is dominated by hematite.…”

“…The magnetic mineral assemblage of these surface sediments is characterized by the presence of both low and high coercivity magnetic minerals, which are dominantly associated with the fine grain-size fraction (e.g., silt and clay) (Lyons et al, 2010(Lyons et al, , 2012Oldfield et al, 2014). The low coercivity fraction corresponds to a ferromagnetic mineral whose size spans the superparamagnetic/stable single domain (SP/SD) boundary, regardless of variable background lithologies and climatic context (Lyons et al, 2010(Lyons et al, , 2012Oldfield et al, 2014). Rock magnetic data are consistent with the presence of both magnetite and maghemite, but do not enable a clear distinction between these minerals.…”

“…Mesozoic rocks include Triassic and Jurassic sedimentary sequences that are mainly represented by Upper Jurassic to Upper Cretaceous continental and marine sedimentary rocks, FIGURE 1 | Geological sketch map of the NE Sahara, with indication of mean annual precipitation (blue contours; Petit-Maire and Guo, 1997), total ozone mapping spectrometer (TOMS) annual aerosol index (red contours; Goudie and Middleton, 2001), preferential dust source areas (PSAs, Scheuvens et al, 2013) and the main paths for eolian dust transportation (red arrows; Trauth et al, 2009;Varga et al, 2014). Orange circles indicate the locations of surface sediment samples studied by Lyons et al (2010), and red squares indicate the locations of surface sediment and dust samples studied by Lyons et al (2012) and Oldfield et al (2014). AA, Al-Haruj al-Aswad massif; TM, Tibesti massif; JU, Jebel Uweinat; GK, Gilf Kebir; CSW, central Saharan watershed; NW, Nile watershed.…”

“…Magnetic property data for 103 surface materials exist for the easternmost part of PSA4 in Egypt, the Chad basin (PSA5) and for other dust source areas in Mali (Lyons et al, 2010(Lyons et al, , 2012Oldfield et al, 2014) (Figure 1). These surface materials include recent (e.g., Quaternary) distal alluvial and lacustrine sediments as well as soil and dune sands.…”