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The location and geography of Lake Mega-Chad. (A) Location of Lake Mega-Chad and its catchment within Africa. (B) Lake Mega-Chad catchment, showing the maximum extent of the lake during the Holocene and key geographical features. (C) The stages of Lake Mega-Chad identified in this study. The Bhar el Ghazal River (dashed line) that feeds water from Lake Chad into the Bodélé Depression when the level of Lake Chad rises above the sill at 288 m is also shown. The elevations are given in meters above present-day sea level. The location and elevation of each sample site is shown in more detail in SI Appendix.
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North Africa was wetter 15,000–5,000 years ago than today, with wetlands and lakes formed in the Sahara due to an enhanced monsoon. We reconstruct the lake-level history of Lake Mega-Chad, when it was the largest African lake, and demonstrate that this humid period ended abruptly 5,000 years ago, indicating that the African monsoon exh...
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... sedimentary deposits associated with the rise and fall of Lake Chad and its earlier incarnation, paleolake Mega- Chad. The latter was the largest freshwater lake in Africa and probably the largest pluvial lake on Earth. During the Early to mid-Holocene wet phase, Lake Mega-Chad attained an area of 361,000 km 2 (16-18) and a depth of up to 160 m ( Fig. 1). Paleolake Mega-Chad has the potential to provide an important record of WAM dynamics because it is (i) sensitive to changing moisture balance due to its shallow depth and large surface area; (ii) dominated by fluvial inputs, unlike smaller North African lakes where groundwater can dominate; and (iii) Located in north central Africa, ...
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... the Early to mid-Holocene wet phase, Lake Mega-Chad attained an area of 361,000 km 2 (16-18) and a depth of up to 160 m ( Fig. 1). Paleolake Mega-Chad has the potential to provide an important record of WAM dynamics because it is (i) sensitive to changing moisture balance due to its shallow depth and large surface area; (ii) dominated by fluvial inputs, unlike smaller North African lakes where groundwater can dominate; and (iii) Located in north central Africa, thereby integrating moisture from tropical and desert latitudes ( Fig. 1 A and B). In addition, Lake Chad potentially acts as a climate driver in itself, projecting tropical moisture ∼1,000 km northward during humid periods, thereby providing an efficient moisture source in the absence of long- range atmospheric moisture delivery (19) and allowing dust production in the Bodélé Depression during arid periods (20,21). ...
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... moisture delivery (19) and allowing dust production in the Bodélé Depression during arid periods (20,21). This dust production may further enhance aridity by sup- pressing rainfall (22). This dual role as alternate moisture and dust source is made possible by the geography of the paleolake Mega-Chad catchment, which feeds two interlinked basins (Fig. 1C). The southern (Chad) basin contains present-day Lake Chad and is fed primarily by the tropical catchments of the Chari River. The northern (Bodélé) basin is presently hyperarid and is fed either by Saharan catchments or by overflow from the Chad Basin via the Bahr el Ghazal (BEG) sill at an elevation of 287 m (Fig. 1C). The BEG sill is ...
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... two interlinked basins (Fig. 1C). The southern (Chad) basin contains present-day Lake Chad and is fed primarily by the tropical catchments of the Chari River. The northern (Bodélé) basin is presently hyperarid and is fed either by Saharan catchments or by overflow from the Chad Basin via the Bahr el Ghazal (BEG) sill at an elevation of 287 m (Fig. 1C). The BEG sill is the lowest point in the watershed between the Chad and Bodélé basins. A single lake (Mega-Chad) is formed when lake levels exceed this elevation, and fluctuations observed in one basin are common to both. Conversely, ages for samples below 287 m only constrain lake levels in the basin where they were collected, except ...
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... indicates lake-level changes. The horizontal blue dashed line represents the elevation of the BEG sill, below which separate lakes exist in the Chad and Bodélé basins. Consequently, lake-level changes below this line represent the Bodélé Basin only. Separate lake-level re- constructions for the Chad and Bodélé basins are presented in SI Appendix, Fig. S1. (B) The 30°N June insolation and ODP core 658C terrigenous dust content (2). (C) Gulf or Guinea sea surface salinity (SSS), primarily reflecting discharge from the Niger-Benue and Sanaga rivers, which drain similar lat- itudes to the Lake Mega-Chad catchment (1). increased runoff from the Niger-Benue and Sanaga rivers from 14.5 ka, ...
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... a background of arid conditions in the Lake Mega-Chad Basin after 5.0 ka is a brief 290-m highstand of Lake Chad at 3 ka, which resulted in the deposition of the Ngelewa Ridge. This highstand reached the BEG sill and flooded the Bodélé Basin, although resulting in a much smaller-volume lake than was present in the Early Holocene Lake Mega-Chad (Fig. 1C). A short-lived return to wetter con- ditions is recorded across much of the Sahara-Sahel region at this time (6), although timing is quite variable between sites, ranging from ∼4.2 to 3.2 cal. ka B.P. (39). More northerly records, such as ODP core 658C and Lake Yoa, do not record this event. At this time, the Bodélé Basin would have ...
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... Palaeolake Mega-Chad basin is comprised of two separate but interlinked depressions (Figure 1c), termed the Chad and Bodélé basins. The northern (Bodélé) basin has a basal elevation of ~167 m, while the southern (Chad) basin has a basal elevation of ~276 m. ...
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... we used samples from both Chad and Bodélé to reconstruct the fluctuations of lakes in these basins, the two lake level histories are subtly different. Separate reconstructions of lake levels in the Chad and Bodélé basins are presented in Figure S1. It is important to note that the key fluctuations discussed in our paper (onset of humidity at ~15 ka, a Lake Mega-Chad highstand from 11.5-5 ka and an abrupt termination of the African Humid Period at 5 ka) are common to both records. ...
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... measurements, analysis and interpretation of data necessary to generate OSL ages were performed by SJA. Figure S1. Separate lake level reconstructions (black dashed line) for a) the Chad Basin and b) the Bodélé basin. ...
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... two new dates presented here are from the Bodélé Depression in Chad (SUERC-18366) and from near Monguno in Nigeria (SUERC-18367). The Bodélé Depression sample consists of in-situ bivalves from the lake bed sediments in the Bodélé Depression ( Figure S10). Samples were collected by hand from the surface of a thin diatomite bed. ...
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... BP), and represent the most recent dated occurrence of perennial standing water in the Bodélé Basin. Figure S10. In-situ, articulated bivalves in the Bodélé Basin, dated as sample SUERC-18366. ...
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... shells were found at an elevation of 291 m, 8 miles south of Mongonu, at a site close the Ngelewa Ridge. The molluscs were found in a pit that was dug for a soil survey ( Figure S11). The base of the pit consists of aeolian sand with fossil grass fragments that suggest stabilization after deposition. ...
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... Chad is dotted with elongated islands that trend NW to SE ( Figure S12). These islands are interpreted to be transverse dunes that have been drowned by the rising lake waters and exposed recently as the lake waters fell (25). ...
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... dating of the dune islands was undertaken to determine when the dunes were last active, and by inference to constrain when lake waters flooded the dunes. Three dune islands in the Nigerian sector of Lake Chad were sampled using a hand auger ( Figure S12). Sample NG33 is from a depth of 1.6 m in very fine grained pale buff coloured sands at Phulkime Island. ...
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... the Bodélé, the lake was 160 m deep, and the diatomite deposits are dominated by planctic species Aulacoseira spp. The contact between the diatomite and the dune sands is very sharp with no noticeable alteration of the underlying laminated dunes sands or roots ( Figure S13). Three exhumed barchan dunes were sampled. ...
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... exhumed barchan dunes were sampled. In each case, the OSL samples were collected from the downwind margins of the dune where the sand was overlain by diatomite ( Figure S13). Sample CH16 was collected from an outcrop close to the campsite at Chicha where fine grained, pale buff coloured, aeolian sand is overlain by around 1m of diatomite. ...
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... CH16 was collected from an outcrop close to the campsite at Chicha where fine grained, pale buff coloured, aeolian sand is overlain by around 1m of diatomite. Sample CH22 is from a well exposed section of cross-stratified yellow dune sand overlain by 1.25 m of diatomite around 29 km downwind from Chicha ( Figure S13). Sample CH51 is a further 60 km downwind where pale pink coloured, medium to coarse grained, laminated aeolian sand is overlain by 0.5 m of diatomite. ...
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... CH51 is a further 60 km downwind where pale pink coloured, medium to coarse grained, laminated aeolian sand is overlain by 0.5 m of diatomite. Figure S13. Outcrop photographs of exhumed dune sample locations: a) CH22 and b) CH51. ...
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... this study we sampled the ridge in the suburbs of Maiduguri (Dalori Quarters) and at locations to the northwest of the town (Kurimari and Magumeri) where the ridge expands forming multiple sub-parallel ridges ( Figure S3). OSL samples were collected from aggregate pits in the ridge, which allowed access to sections in fresh exposures, well beneath the weathering zone ( Figure S14). Figure S14. Outcrop photograph of sand pit section Kurimari II. ...
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... samples were collected from aggregate pits in the ridge, which allowed access to sections in fresh exposures, well beneath the weathering zone ( Figure S14). Figure S14. Outcrop photograph of sand pit section Kurimari II. ...
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... strata within the granules dip at 30° towards the NNE (25°-30°) and are interpreted to be offshore directed bedforms. Sample NG11 was collected from a depth of 5 m in very coarse grained cross-stratified sands ( Figure S14). Samples NG9, NG10 and NG11 yield OSL ages of 5.4 ± 0.5, 5.7 ± 0.4, and 9.4 ± 0.9 ka respectively. ...
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... In contrast, years with minimum wind speeds correspond to La Niña events, which are associated with anomalies less than −0.5°C. These findings for the Sahel are consistent with those of Okonkwo et al. (2014) and Armitage et al. (2015). Consequently, the interannual variability of wind speeds in Chad is primarily influenced by major climatic factors, particularly the El Niño Southern Oscillation (ENSO), which alters regional atmospheric patterns. ...
This study provides a climatological assessment of wind resources in Chad using 100-m wind data from the ERA5 reanalysis. The results highlight spatio-temporal variations in wind potential, Enabling the identification of climate zones suitable for wind farm development. Wind speeds range from 1.5 to 11 m/s, with increasing gradients from south to north. Maximum wind speeds are observed between autumn and spring, particularly from October to April, while minimum speeds, below 5 m/s, occur during the wet season. A distinct diurnal cycle is noted, with nighttime wind speeds averaging 26% higher than daytime speeds. Winds predominantly blow in favorable directions, primarily toward the northeast. The study indicates that almost the entire northern region of Chad (north of 15°N latitude) is particularly favorable for wind farm installations, except for areas around the Tibesti mountain range, where wind speeds remain low due to the presence of a permanent anticyclone. Three zones are identified as particularly promising: the region around Faya, the far northeastern part of the country, and the area around Amjarass. Additionally, long-term analysis reveals a slight decrease in 100-m wind speeds, possibly linked to the increased frequency of La Niña events over the past decade. In conclusion, this study provides a comprehensive assessment of Chad's wind potential, emphasizing that the most suitable areas are located between the Saharan and Sahelian zones.
... The mid-Holocene (MH) period, approximately 6,000 years before the present, saw a massive transformation of the present-day arid landscapes like the Sahel and Sahara into shrubs, grasslands, rivers, and lakes (Claussen and Gayler 1997;Demenocal et al. 2000;Kohfeld and Harrison 2000;Holmes 2008;Hargreaves et al. 2013;Armitage et al. 2015;Tierney et al. 2017;Claussen et al. 2017). For this wet and humid period with significantly amplified rainfall, numerous studies based on reconstructions and climate model simulations tried to explain the reasons for the dramatic climate change (Kutzbach and Liu 1997;Claussen et al. 1999;Braconnot et al. 1999;Harrison et al. 2014;Tierney et al. 2017). ...
Changes in the global and annual Intertropical Convergence Zone (ITCZ) during the mid-Holocene are quantified in this study, using simulations from the Paleoclimate Modelling Intercomparison Project Phase 4. We find an ensemble mean northward migration of ITCZ position by 0.2 ∘ , 0.3 ∘ , and 0.8 ∘ using three different metrics, which is consistent with proxy evidence of a slight (within one degree) northward shift during the mid-Holocene. The width of the ITCZ exhibits no changes during this period, while the strength has a minor weakening by 0.9% and 0.6% based on precipitation and streamfunction metrics, respectively. Moreover, there is an ensemble mean increase in northern-to-southern hemisphere precipitation (4.6%) and ω ratio indices (12%) during the mid-Holocene, suggesting a more uneven and right-skewed meridional structure of the ITCZ with a greater asymmetry. Additionally, the moisture budget analysis indicates the eddy flux term and dynamic term primarily enhance the inter-hemispheric asymmetry of precipitation and poleward migration of both the northern and the southern rain belts within the ITCZ. Further investigation reveals that the northward migration of annual rainfall within the ITCZ predominantly occurs over terrestrial regions, which therefore dominate the change towards more asymmetric structure of the zonally averaged ITCZ. Moisture budgets for land and ocean separately indicate that the land-sea difference in the annual rainfall changes is primarily due to different changes in evaporation.
... A substantial fraction of atmospheric dust exported from the Sahara comes from the West African El Djouf and the Bodélé Depression (e.g., Koren et al., 2006;Yu et al., 2020;Barkley et al., 2022;Fig. 1), which corresponds to the remnants of ancient Lake Mega-Chad, once the largest lake in Africa, until it dried out a few thousand years ago at the end of the African Humid Period (Armitage et al., 2015). Mineral dust eroded from the Bodélé Depression and/or from other source regions in the Western Sahara are blown over thousands of kilometres by northeasterly and easterly trade winds, respectively (Kalu, 1979;Prospero et al., 2002;Yu et al., 2020;Barkley et al., 2022). ...
Huge amounts of mineral dust are produced in northern Africa, representing the largest source of aerosols worldwide. Transatlantic dust transport is known to fertilize soils as far as in the Amazon Basin. Yet, the influence of Saharan dust on chemical weathering fluxes and associated nutrient release in West Africa remains largely overlooked. To address this issue, we analysed clay fractions (<2 µm) of river sediments (n = 37) from across the Niger River basin - the largest river system in West Africa - using neodymium and hafnium isotope compositions as proxies for provenance (εNd) and chemical weathering (ΔεHf CLAY).
Compared to previously published data for corresponding sand fractions, measured εNd values indicate significant size-dependent decoupling for Nd isotopes in most samples, with εNd differences between clay and sand fractions yielding values as great as ∼26 ε-units. Using mixing models, we show that this discrepancy reflects the overwhelming presence in the studied clay fractions of Harmattan dust blown from the Bodélé Depression in Chad, which we estimate to account for about 40 % of the fine-grained sediment load exported to the Gulf of Guinea. Additionally, significant ΔεHf CLAY variability occurs across the Niger catchment, partly explained by the presence of zircon in clay-size fractions, but also by preferential alteration of dust-borne accessory phosphate minerals in the subtropical regions of the watershed.
Based on these results, we propose that Saharan dust plays a major role in controlling regional patterns of chemical weathering in West Africa, suggesting that enhanced wet deposition of mineral dust in shield areas dominated by transport-limited weathering regime can result in a large increase in weatherability and associated release of phosphorus. These findings have general implications for the importance of mineral aerosols in controlling sediment yield and the supply of weathering-derived nutrients to continental areas bordering large subtropical deserts worldwide.
... To the west and North of Lake Fitri (in particular for quarries 1-3) lays the former mega-Chad lake (Schuster et al. 2005). During the 3-14 ka period in Holocene, the above-mentioned outcrops were totally or partly drown into the lake, whose level peaked at 329 m above sea level (Armitage et al. 2015). One may wonder about the relationship between resistance to degradation and the preservation of Neoproterozoic outcrops in these specific Cross indicates existing quarries, in particular, 1 Alfallé, 2 N'Goura, 3 Moyto, 4 Birni erosional conditions. ...
This study focusses on the scarce resources of high-quality silicated rock aggregates in Central Chad, a wide area almost devoid of hard rock outcrops. We provide an original study of the petrographic and mechanical characteristics of Alfallé massif together with a review of quarries in activity or potentially exploitable. The resistance to degradation of the prepared aggregates were evaluated using the standard test of Los Angeles and micro-Deval. The Alfallé massif is made up of three lithologies: alkali feldspar coarse granite, biotite-rich fine-grained quartz monzonite and biotite-rich fine-grained gneiss. The mechanical test results obtained indicate that the materials studied have excellent to satisfactory properties, and that the Alfallé massif provides the best characteristics among six different quarrying areas in Central Chad. Moreover, the three quarries that were once isolated islands in the mega-Chad paleolake yield on average higher resistance. We thus propose that these rare residual reliefs within the mega-Chad paleolake area reached optimal mechanical properties through a natural selection process, specific of the paleolake erosional conditions.
... Stud. 2024, 2, FOR PEER REVIEW 7 great fluctuations in water levels and climate at millennial scales typical of marginal marine and semi-arid lake basins such as those of Lake Cariaco in northern Venezuela and Lake Chad in the central Sahara [51,52]. Throughout the Patrick Burn Formation, and in the Jamoytius bed itself, there is a complete absence of burrowing organisms and there are no tracks or trails on the bedding plane surfaces [37] The undisturbed nature of the sediments, together with abundant pyrite and organic matter, indicates anaerobic bottom conditions in very quiet water subject to periodic underflows [30]; in keeping with the abundance of Ceratiocaris. ...
... The bounding sandstones are low-angle tabular cross-bedded fine-grained mature muddy quartz-and felspathic-sandstones [49], deposited by sand waves in shallow water by storms [37,50] (Figure 8C,D). The deeper water Jamoytius bed with its turbidite sandstones and laminated siltstone contrasts markedly with the enclosing shallow water cross-bedded fine sandstones and testify to great fluctuations in water levels and climate at millennial scales typical of marginal marine and semi-arid lake basins such as those of Lake Cariaco in northern Venezuela and Lake Chad in the central Sahara [51,52]. Both the Jamoytius bed, and higher fish-bearing beds at Lesmahagow, have alternations of fine silt-clay couplets (varves) which contain the biota, and olive-grey massive mudstones which are barren [53]. ...
Jamoytius kerwoodi, is a primitive, eel-like jawless vertebrate found uniquely in an Early Silurian (Llandovery epoch; 444–433 Ma) horizon near Lesmahagow, Scotland. This species is a rare component of a low-diversity dominantly nektonic detritus-feeding and herbivorous fauna living over an anoxic bottom and is found at the transition from a marine-influenced, probably brackish-water, deep-water basin to a shallower-water, less saline and likely freshwater basin. In the absence of true teeth, Jamoytius was probably a detritivore or herbivore feeding on Dictyocaris. Jamoytius may have a common ancestor with living lampreys, especially as their ectoparasitic mode of life might have evolved from ancestral detritivores or herbivores.
... The area has regularly experienced wetter episodes during the Quaternary, including recently (early to middle Holocene), and was occupied by the wide Lake Mega-Chad and its associated river drainages (e.g. Ghienne et al., 2002;Schuster et al., 2005;Bouchette et al., 2010;Armitage et al., 2015). The contribution of a potential fluvial sediment input from these river systems has not been investigated. ...
... We focused our sampling around an approximate NE-SW transect following the direction of Harmattan winds, from the desertic Erdi Plateau (mean annual precipitation <50 mm) to the Sahelian parts of Chad and northern Cameroon (annual precipitation >200 mm; Fig. 1a). All samples are located in today's Lake Chad drainage basin, with some falling within the maximum extent of the last Lake Mega-Chad episode (early to mid-Holocene; Armitage et al., 2015). Two to three kg of sand were collected on modern aeolian dunes, including, from north to south: five samples at the southern edge of the Erdi Plateau; seven samples in the Djourab Desert; four samples in the Bahr el Ghazal area, south of the Djourab Desert; and six samples in the Kanem Desert. ...
Sahara sands have been proposed to result from the extensive and repetitive recycling of much older sedimentary rocks-a necessary mechanism to explain their petrographic maturity and the similarity of their detrital zircon populations at continental scale. Where and how this recycling occurs today remain poorly understood. This study investigates the source of modern sands from the southeastern Sahara by leveraging on a large (n > 7800) new dataset of detrital zircon ages from source rocks, modern and ancient dune fields in Chad and Cameroon. We show that zircon age populations show noticeable regional differences when analyzing a large n amount of ages, questioning the similarity of detrital zircon populations in Saharan sands. Dune fields from the driest parts of our sampling area have distinct age distributions that imply discrete sources with differences in bedrock zircon age populations at regional (several 100 km) scale. In the wetter, Sahelian part of our sampling area, the zircon age distribution of dune fields is best-explained by a significant contribution of recent alluvium from local wadis and rivers to the aeolian sedimentary budget. The origin of aeolian sands in the southeastern Sahara is thus local and polygenetic. Recycling of older sedimentary rocks via physical abrasion is only prominent in the driest parts of our sampling area and does not result in the homogenization of Saharan sands.
... To the west and North of Lake Fitri (in particular for quarries 1-3) lays the former mega-Chad lake [5]. During the 3-14 ka period in Holocene the above-mentioned outcrops were totally or partly drown into the lake, whose level peaked at 329 m above sea level [10]. One may wonder about the relationship between resistance to digradation and the preservation of Neoproterozoic outcrops in these specific erosional conditions. ...
This study focusses on the geological formations of the Alfallé massif located approximately 75km east of the town of N'Goura, in the Hadjer-Lamis Province of Chad (13°N, 17°E), and their ability to provide good quality aggregates in an area with poor hard rock ressources. We investigated the petrographic and mechanical characteristics of these formations in a resource perspective. The massif is made up of three lithologies: alkali feldspar megacrystalline granite, biotite-rich fine grained quartz monzonite and biotite-rich fine grained gneiss. Alkali feldspar megacrystalline granite is composed of quartz (30%), orthoclase (30%), plagioclase (20%), biotite (15%) and opaque minerals (≈5%). Micro-monzonite is composed of quartz (15-20%), orthoclase (20-25%), plagioclase (18-25%), biotite (20-25%) and opaque minerals (≈4%). The biotite gneiss is composed of quartz (25-30%), alkali feldspars (25-27%), plagioclase (17%), biotite (25-27%) and opaque minerals (≈5%). The mechanical characteristics of the aggregates show that the aggregates studied have specific density values of between 2.58 and 2.75g/cm3 with an average value of 2.66g/cm3, a Los-Angeles coefficient of 11.2% to 28.9% with an average value of 20.1% and a wet Micro-Deval coefficient of 5.2% to 10.8% with an average value of 7.6%. The values obtained indicate that the materials studied have excellent to satisfactory properties, and that Alfallé massif provide the best characteristics among six different quarrying area in Central Chad. We propose that such rare residual reliefs within the mega-Chad paleolake area reach optimal mechanical properties through a natural selection process, specific of the paleolake erosional conditions.
... Firstly, northern Africa (NA) is defined as the African continent area within the region 20 • W to 40 • E and 10 to 30 • N. The other two regions are related to the paleo-waterbodies during MH where the dust emissions were reduced the most. One is the Lake Chad (LC) area within 10-20 • E and 10-20 • N and the other is the western Sahara catchments (WSC), centered around the paleo-Lake Timbuktu area within 12 • W-3 • E and 17-29 • N. The two paleo-lakes are selected because the Lake Chad has been suggested to have thrived between 11 and 5 ka (Armitage et al., 2015), and Lake Timbuktu is considered to have experienced a wet period between 9.5 and 3.5 ka (Drake et al., 2022). The regions are shown on the map in Fig. 1 and the regional-average and summed values in Table 2 refer to the regions defined here. ...
Paleo-proxy data indicate that a “Green Sahara” thrived in northern Africa during the early- to mid-Holocene (MH; 11 000 to 5000 years before present), characterized by more vegetation cover and reduced dust emissions. Utilizing a state-of-the-art atmospheric chemical transport model, TM5-MP, we assessed the changes in biogenic volatile organic compound (BVOC) emissions, dust emissions and secondary organic aerosol (SOA) concentrations in northern Africa during this period relative to the pre-industrial (PI) period. Our simulations show that dust emissions reduced from 280.6 Tg a-1 in the PI to 26.8 Tg a-1 in the MH, agreeing with indications from eight marine sediment records in the Atlantic Ocean. The northward expansion in northern Africa resulted in an increase in annual emissions of isoprene and monoterpenes during the MH, around 4.3 and 3.5 times higher than that in the PI period, respectively, causing a 1.9-times increase in the SOA surface concentration. Concurrently, enhanced BVOC emissions consumed more hydroxyl radical (OH), resulting in less sulfate formation. This effect counteracted the enhanced SOA surface concentration, altogether leading to a 17 % increase in the cloud condensation nuclei at 0.2 % super saturation over northern Africa. Our simulations provide consistent emission datasets of BVOCs, dust and the SOA formation aligned with the northward shift of vegetation during the “Green Sahara” period, which could serve as a benchmark for MH aerosol input in future Earth system model simulation experiments.
... Arid conditions in the Chadian Sahara were already present 7 Myr ago, with alternation of numerous humid and arid periods since (Schuster et al., 2006). However, the former Chad mega-lake, whose last large extent dates from 5 ka (Armitage et al., 2015), covers an important part of the arid zone of Chad (Fig. 1), strongly diminishing the potential for finding Beauvilain, 1996), main geological units and position of the meteorites (stars), proven and proposed impact structures (yellow and white circles, respectively). Numbering is indicated in Table 1. ...
The large surface of Chad (fifth largest country in Africa) as well as a desert climate over much of its area, makes the country a potentially prolific place to recover meteorites and identify impact structures. However, only two meteorite finds have so far been reported, from Tibesti, and two probable falls have been recovered in the vicinity of the capital N’Djamena in the early 1960s. We report on two more recent meteorite falls, Andila and Boutel-Fil, occurring in 2014 and 2023, respectively. All these meteorites are ordinary chondrites, one LL, one L and four H. Chad has two proven eroded impact structures circa 15-20 km in diameter: Aorounga and Gweni-Fada. A number of potential impact structures have also been proposed. The most promising ones are the Faya, Lac Iro and Mousso structures, with diameters of 2, 13 and 3.5 km, respectively. Other previously proposed structures, near the Libyan border, are most probably of magmatic origin. Based on geomorphological and geological arguments, the presence of the Iro lake cannot be readily explained by any process other than impact and its investigation should be a priority based on its size and potential interest as a paleoclimatic recorder. We also suggest that a 35 km diameter structure named Ehi Gidaha, buried under the Devonian sandstones from south Tibesti, may be of impact origin. All proven or potential impact structures are located in the Sahara part of northern Chad, except Lac Iro.
... Nevertheless, these records are consistent with a regionally time-transgressive but progressive aridification of northern Africa through the late Holocene (Gasse 2000;Hoelzmann et al. 2004;Cremaschi et al. 2006;Kröpelin et al. 2008;Francus et al. 2013;Shanahan et al. 2015), as shown by the simulations. Indications of abrupt drying at individual sites, such as the drying of Lake Mega-Chad at ca 5 ka (Armitage et al. 2015), probably reflect the fact that multi-decadal to multicentennial droughts are superimposed on this long term aridification trend and the progressive southward positioning of the rain belt means there is no subsequent recovery. Certainly, lake and pollen records from further south show a progressive aridification through the late Holocene (Shanahan et al. 2015;Lemmonier and Lézine 2022). ...
Recent variability in West African monsoon rainfall (WAMR) has been shown to be influenced by multiple ocean–atmosphere modes, including the El Niño Southern Oscillation, Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation. How these modes will change in response to long term forcing is less well understood. Here we use four transient simulations driven by changes in orbital forcing and greenhouse gas concentrations over the past 6000 years to examine the relationship between West African monsoon rainfall multiscale variability and changes in the modes associated with this variability. All four models show a near linear decline in monsoon rainfall over the past 6000 years in response to the gradual weakening of the interhemispheric gradient in sea surface temperatures. The only indices that show a long-term trend are those associated with the strengthening of the El Niño Southern Oscillation from the mid-Holocene onwards. At the interannual-to-decadal timescale, WAMR variability is largely influenced by Pacific–Atlantic – Mediterranean Sea teleconnections in all simulations; the exact configurations are model sensitive. The WAMR interannual-to-decadal variability depicts marked multi-centennial oscillations, with La Niña/negative Pacific Decadal Oscillation and a weakening and/or poleward shift of subtropical high-pressure systems over the Atlantic favoring wet WAMR anomalies. The WAMR interannual-to-decadal variability also depicts an overall decreasing trend throughout the Holocene that is consistent among the simulations. This decreasing trend relates to changes in the North Atlantic and Gulf of Guinea Sea Surface Temperature variability.
Supplementary Information
The online version contains supplementary material available at 10.1007/s00382-023-07023-y.
