15-year variability of desert dust optical depth on global and regional scales

This study aims to investigate the global, regional and seasonal temporal dust changes as well as the effect of dust particles on total aerosol loading, using the MIDAS fine resolution dataset. MIDAS delivers dust optical depth (DOD) at fine spatial resolution (0.1° × 0.1°) spanning from 2003 to 2017. Within this study period, the dust burden has been increased across Central Sahara (up to 0.023 yr−1) and Arabian Peninsula (up to 0.024 yr−1). Both regions observed their highest seasonal trends in summer (up to 0.031 yr−1). On the other side, declining DOD trends are encountered in Western (down to −0.015 yr−1) and Eastern (down to −0.023 yr−1) Sahara, Bodélé Depression (down to −0.021 yr−1), Thar (down to −0.017 yr−1) and Gobi (down to −0.011 yr−1) Deserts and Mediterranean Basin (down to −0.009 yr−1). At spring, the most negative seasonal trends are recorded in Bodélé Depression (down to −0.038 yr−1) and Gobi Desert (down to −0.023 yr−1) whereas in West (down to −0.028 yr−1) and East Sahara (down to −0.020 yr−1), and Thar Desert (down to −0.047 yr−1) at summer. Over western and eastern sector of Mediterranean Basin, the most negative seasonal trends are computed at summer (down to −0.010 yr−1) and spring (down to −0.006 yr−1), respectively. The effect of DOD to the total aerosol optical depth (AOD) changes is determined calculating the DOD to AOD ratio. Over Sahara Desert the median ratio values range from 0.83 to 0.95 whereas in other dust affected areas (Arabian Peninsula, South Mediterranean, Thar and Gobi Deserts) is recorded approximately around 0.6. In addition, a comprehensive analysis of the factors effecting the sign, the magnitude and the statistical significance of the calculated trends is conducted. Firstly, the implications between the implementation of geometric mean instead of arithmetic mean to trend calculations are discussed revealing that the arithmetic-based trends tend to overestimate compared with the geometric-based trends both over land and ocean. Secondly, an analysis interpreting the differences in trend calculations under different spatial resolutions (fine and coarse) and time intervals is conducted, which sounds a critical aspect when satellite-based measurements are utilized.

Contrasting Genetic Footprints among Saharan Olive Populations: Potential Causes and Conservation Implications

The Laperrine’s olive is endemic to the Saharan Mountains. Adapted to arid environments, it may constitute a valuable genetic resource to improve water-stress tolerance in the cultivated olive. However, limited natural regeneration coupled with human pressures make it locally endangered in Central Sahara. Understanding past population dynamics is thus crucial to define management strategies. Nucleotide sequence diversity was first investigated on five nuclear genes and compared to the Mediterranean and African olives. These data confirm that the Laperrine’s olive has a strong affinity with the Mediterranean olive, but it shows lower nucleotide diversity than other continental taxa. To investigate gene flows mediated by seeds and pollen, polymorphisms from nuclear and plastid microsatellites from 383 individuals from four Saharan massifs were analyzed. A higher genetic diversity in Ahaggar (Hoggar, Algeria) suggests that this population has maintained over the long term a larger number of individuals than other massifs. High-to-moderate genetic differentiation between massifs confirms the role of desert barriers in limiting gene flow. Yet contrasting patterns of isolation by distance were observed within massifs, and also between plastid and nuclear markers, stressing the role of local factors (e.g., habitat fragmentation, historical range shift) in seed and pollen dispersal. Implications of these results in the management of the Laperrine’s olive genetic resources are discussed.

Geochemical reconnaissance of the Guéra and Ouaddaï Massifs in Chad: evolution of Proterozoic crust in the Central Sahara Shield

In 1964, W.Q. Kennedy suggested that the crust of Saharan Africa is different from the rest of Africa. To date, the geologic evolution of this region remains obscure because the age and composition of crystalline basement are unknown across large sectors of the Sahara. Most of Africa comprises Archaean cratons surrounded by Palaeo- to Mesoproterozoic orogenic belts, which together constitute Africa’s three major shields (the Southern, Central and West African Shields), finally assembled along belts of Pan-African rocks. By contrast, central Saharan Africa (5.3x106 km2), an area just over half the size of Europe, is considered either as a Neoproterozoic region constructed of relatively juvenile crust (0.5 to 1.0 Ga), or as an older (North African) shield that was reactivated and re-stabilized during that time, a period commonly referred to as “Pan African”. Here, using U-Pb zircon age determinations and Nd isotopic data, we show that remote areas in Chad, part of the undated Darfur Plateau stretching across ¾ million km2 of the central Sahara, comprise an extensive Neoproterozoic crystalline basement of pre-tectonic gabbro-tonalite-granodiorite and predominantly post-tectonic alkali feldspar granites and syenites that intruded between ca. 550 to 1050 Ma. This basement is flanked along its western margin by a Neoproterozoic continental calc-alkaline magmatic arc coupled to a cryptic suture zone that can be traced for ~2400 km from Tibesti through western Darfur into Cameroon. We refer to this as the Central Saharan Belt. This, in a Gondwana framework, is part of a greater arc structure, which we here term the Great Central Gondwana Arc (GCGA). Inherited zircons and Nd isotopic ratios indicate the Neoproterozoic magmas in the central Sahara were predominantly derived from Mesoproterozoic continental lithosphere. Regional deformation between 613 to 623 Ma marks the onset of late alkaline granite magmatism that was widespread across a much larger area of North Africa until about 550 Ma. During this magmatism, the region was exhumed and eroded, leaving a regional peneplain on which early Palaeozoic (Lower-Middle Cambrian) siliciclastic sediments were subsequently deposited, as part of a thick and widespread cover that stretched across much of North Africa and the Arabian Peninsula. Detrital zircons in these cover sequences provide evidence that a substantial volume of detritus was derived from the central Sahara region, because these sequences include ‘Kibaran-age’ zircons (ca. 1000 Ma) for which a source terrain has hitherto been lacking. We propose that, in preference to calling the central Sahara a “ghost” or “meta” craton, it should be called the Central Sahara Shield.

The enigmatic continental crust of North-Central Africa: Saharan Metacraton or Central Sahara Shield?

The continental crust of North-Central Africa between the Tuareg and Arabian-Nubian shields and south to the Central African Orogenic Belt is enigmatic due to the few bedrock exposures especially within the central region. The current understanding, based on a review of geochronology and isotope geochemistry, is that the central Sahara region is a large, coherent craton that was ‘highly remobilized’ during the Late Neoproterozoic amalgamation of Gondwana and referred to as the Saharan Metacraton. However, new data from the Guéra, Ouaddaï, and Mayo Kebbi massifs and the Lake Fitri inlier of Chad suggest that it may be a composite terrane of older cratonic blocks or microcontinents with intervening Mesoproterozoic to Neoproterozoic domains and referred to as the ‘Central Sahara Shield’. It is postulated that the older crust and juvenile crust were sutured together along a Pan-Gondwana collisional belt (Central Sahara Belt) that bisects the central Sahara region. The ‘Central Sahara Shield’ hypothesis suggests the Chad Lineament, a narrow arcuate gravity anomaly within central Chad, could be a collisional belt suture zone and that it may explain the existence of the relatively juvenile crust that typifies southern and eastern Chad. The new data improves upon the existing knowledge and challenges the lithotectonic paradigm of the Saharan Metacraton. Further investigations are required to fully characterize the crust of the central Sahara region and to test the contrasting hypotheses.

Exploring Environments through Water: An Ethno-Hydrography of the Tibesti Mountains (Central Sahara)

An ethno-hydrography, studying the organization of space through water, can provide a key to understanding how people conceive their environments in a holistic way. Based on mapping as a dynamic process, different representations of river systems among the Tubu Teda, who live in the Tibesti mountains (Central Sahara), are described in this paper. I first discuss a large-scale subdivision of the mountains into drainage basins, and then representations of a sub-regional and local river system, including an engraving on a sandstone rock. Finally, I discuss these case studies in the context of holistic experiences of environments and the dynamic processes of mapping.

Between Imajaɤen (Warrior) and Timogoutar (Helplessness)

This chapter is drawn from a much larger qualitative phenomenological inquiry into the Kel Tamashek of the Central Sahara and its Sahelian transition zone. The impetus for this larger research was driven by US Army Generals John Mulholland (Ret), James Linder (Ret), and US Navy Admiral Brian Losey. These senior military leaders foresaw the coming clash between this powerful ethnic community and the rapid spread of globalization into the vast spaces of the Sahel and Sahara Desert. This ethnic community lives in an alternate reality in the northern parts of Niger and Mali, and the southern parts of Algeria and Libya. This alternate reality is of their own design and is well over a millennium in the making. The Kel Tamashek are of extreme interest to regional and international security forces because of their tendency to resist political control. After fighting the French Colonial governments to a standstill in the 17th and 18th centuries, they went on to overthrow the African-based governments in Mali and Niger several times each.

The First Emergence of Ceramic Production in Africa

The discoveries at Ounjougou (Mali), an open-air site in the Dogon Country, shed new light on the “early Neolithic” in Africa. The stratigraphic sequence and a cluster of absolute dates established a terminus ante quem of 9400 cal bc for ceramic sherds associated with a small bifacial lithic industry. The emergence of this typo-technical complex corresponds to one of the wet phases of the Pleistocene–Holocene transition in West Africa, most probably that of the climatic upturn at the beginning of the Holocene, between 10,200 and 9,400 cal bc. Paleoenvironmental results, particularly archaeobotanical ones, indicate that the landscape was in a state of change and that, for several millennia, the surfaces covered by desert overlapped an open steppe with grasses, some of which were edible. This environmental situation allowed the dispersion of prehistoric groups over the continent and probably encouraged a new behavior: the practice of intensive selective gathering (i.e., the targeted and rational harvesting of wild grasses for their seeds). However, not only must seeds be kept dry and protected from rodents, they must also be processed through cooking or fermentation. This process helps the human body to assimilate the starch, as the digestive enzymes necessary for its digestion are not naturally present. Ceramics would have been particularly useful in this process. Ceramics emerged in sub-Saharan Africa and seem to have spread toward the central Sahara during the early Holocene at the end of the 10th and the beginning of the 9th millennium cal bc, while the desert zone became increasingly greener. It has yet to be understood whether the Nile Valley was an important corridor for the diffusion of this technology or if ceramics appeared as the result of a second independent process of innovation.