Provenance and recycling of Sahara Desert sand

We here present the first comprehensive provenance study of the Sahara Desert using a combination of multiple provenance proxies and state-of-the-art statistical analysis. Our dataset comprises 44 aeolian-dune samples, collected across the region from 12°N (Nigeria) to 34°N (Tunisia) and from 33°E (Egypt) to 16°W (Mauritania) and characterized by bulk-petrography, heavy-mineral, and detrital-zircon U–Pb geochronology analyses. A set of statistical tools including Multidimensional Scaling, Correspondence Analysis, Individual Difference Scaling, and General Procrustes Analysis was applied to discriminate among sample groups with the purpose to reveal meaningful compositional patterns and infer sediment transport pathways on a geological scale. The overall homogenity across sand samples, however, precluded a detailed narrative. Saharan dune fields are, with a few local exceptions, composed of pure quartzose sand with very poor heavy-mineral suites dominated by durable zircon, tourmaline, and rutile. Some feldspars, amphibole, epidote, garnet, or staurolite occur closer to basement exposures, and carbonate grains, clinopyroxene and olivine near a basaltic field in Libya. Relatively varied compositions also characterize sand along the Nile Valley and the southern front of the Anti-Atlas fold belt in Morocco. Otherwise, from the Sahel to the Mediterranean Sea and from the Nile River to the Atlantic Ocean, sand consists nearly exclusively of quartz and durable minerals. These have been concentrated through multiple cycles of erosion, deposition, and diagenesis of Phanerozoic siliciclastic rocks during the long period of relative tectonic quiescence that followed the Neoproterozoic Pan-African orogeny, the last episode of major crustal growth in the region. The principal ultimate source of recycled sand is held to be represented by the thick blanket of quartz-rich sandstones that were deposited in the Cambro-Ordovician from the newly formed Arabian-Nubian Shield in the east to Mauritania in the west. Durability of zircon grains and their likelihood to be recycled from older sedimentary rocks argues against the assumption, too often implicitly taken for granted in provenance studies based on detrital-zircon ages, that their age distribution reflects transport pathways existing at the time of deposition rather than inheritance from multiple and remote landscapes of the past.