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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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... During the early-to-middle Holocene period 11,000 to 5000 years before the present, known as the "Green Sahara", the currently arid landscapes over the Sahel and Sahara were widely replaced by shrubs, grasslands, rivers, and lakes (Claussen and Gayler 1997;Demenocal et al. 2000;Kohfeld and Harrison 2000;Holmes 2008;Armitage et al. 2015;Claussen et al. 2017). For this humid and green Sahara period with significantly amplified rainfall, multiple studies based on reconstructions and climate model simulations tried to explain the reasons for the dramatic climate change. ...
The mid-Holocene was a warm period with significantly amplified precipitation in North Africa, and a northward shifted Western African Monsoon during boreal summer. We conduct simulations for the pre-industrial and mid-Holocene periods to investigate the connection between summer rainfall variability and changes of African easterly waves (AEWs) during the mid-Holocene. Summer rainfall increases and migrates northward during the mid-Holocene, but the magnitude of change fails to reconcile the discrepancy with mid-Holocene proxy evidence, possibly due to no prescribed vegetation change in our simulations. The spectrum of summer rainfall over the Sahel and West Africa reveals enhanced synoptic time scale (3-to-6 days) variability during the mid-Holocene, which is consistent with the enhanced AEW activity influence. Specifically, the southern AEW track strengthens and migrates poleward during the mid-Holocene period, which modulates summer rainfall over the Sahel and West Africa. By comparison, the northern AEW track changes less and produces a minor contribution to rainfall changes in those regions. We find enhanced baroclinic and barotropic instabilities to promote the AEW activity during the mid-Holocene, with a doubling of the eddy kinetic energy of the meridional wind from that in PI, and baroclinic energy conversion plays a more important role. Stronger low-level meridional thermal gradients increase moisture flux from the Atlantic Ocean to inland.The amplified AEW activity, together with promoted moist convection and increased precipitation, results in a northern shift of the summer rainfall band during the mid-Holocene.
... In the same sense, Moeyersons et al. (2006) report increasing humidity from ca. 15 ka onwards based on numerical dating of tufa dams near Mekele (Tigray), which indicate a raised groundwater table during the late Pleistocene. Wetter conditions than today prevailed from ca. 15 ka until the mid-Holocene (ca. 5 ka; Armitage et al., 2015), owing to orbitally forced increased monsoon strength (Umer et al., 2004;Williams et al., 2006). ...
... Climatic reconstructions for the area based on palaeopedological findings indicate that precipitation rates might have decreased in the course of the Holocene, which is in agreement with the declining monsoon strength after 5 ka (Armitage et al., 2015). Based on samples obtained close to Yeha, palaeoprecipitation rates of 870 mm a −1 were determined for the Early Holocene, which decreased to 780 mm a −1 in the first millennium BCE (Pietsch and Machado, 2014). ...
The sites of Hawelti–Melazo in the Tigray region of the northern Ethiopian Highlands is an archaeological hotspot related to the D'mt kingdom (ca. 800–400 BCE). The existence of several monumental buildings, which have been excavated since the 1950s, underline the importance of this area in the Ethio-Sabaean period. We investigated the geomorphological and geological characteristics of the site and its surroundings and carried out sedimentological analyses, as well as direct (luminescence) and indirect (radiocarbon) sediment dating, to reconstruct the palaeoenvironmental conditions, which we integrated into the wider context of Tigray. Luminescence dating of feldspar grains from the May Agazin catchment indicate enhanced fluvial activity in the late Pleistocene, likely connected to the re-occurring monsoon after the Last Glacial Maximum (LGM). The abundance of trap basalt on the Melazo plateau, which provides the basis for the development of fertile soils, and the presumably higher groundwater level during the Ethio-Sabaean Period, provided favourable settlement conditions. The peninsula-like shape of the Melazo plateau was easily accessible only from the east and northeast, while relatively steep scarps enclose the other edges of the plateau. This adds a possible natural protective function to this site.
... We ought not ignore that a large lake 350,000 km 2 in area (e.g., Schuster et al., 2005), and therefore roughly half the size of the Miocene megalake, occupied a large fraction of the Lake Chad Basin in Holocene time. That Holocene lake, however, drained southward into the Niger River and then into the South Atlantic (e.g., Armitage et al., 2015;Burke, 1976;Ghienne et al., 2002;Leblanc et al., 2006). Thus, the hypsometry of the Chad Basin has changed since Pliocene time. ...
Wetter conditions in the African Sahel during the Pliocene (5.3–2.6 Ma) likely played an important role in hominid evolution and may be explained by warm North Atlantic sea‐surface temperatures (SSTs), similar to the modern warm phase of the Atlantic multidecadal oscillation (AMO). We reconstruct Pliocene North Atlantic (2°S to 60°N) SSTs through a multiproxy reduced‐dimension methodology that combines two new foraminiferal (Trilobatus sacculifer) Mg/Ca‐based SST records from Ocean Drilling Program sites in the northwestern Atlantic (Site 997) and Gulf of Mexico (Site 625) with previously published multi‐proxy SST records from seven additional Atlantic sites. Our Pliocene reconstructions indicate that the North Atlantic was overall warmer than the comparison period (0–0.5 Ma) with the most extreme warming occurring in the tropical and eastern subtropical North Atlantic and in the Labrador Sea. Furthermore, the mean state of North Atlantic warming at 5 Ma resembles that of the modern warm phase of the AMO with average SST anomalies of 3.8°C ± 0.5°C in the AMO region. With that reconstructed index, along with reconstructed El Niño‐like eastern Tropical Pacific SSTs from our previous research, we reconstruct rainfall over the Pliocene North Africa, covering the Sahara‐Sahel region and the Megalake Chad Basin and find up to twice the rainfall of that today. We surmise that this increased precipitation rate, along with other aiding land and sub‐surface processes, sustained Megalake Chad.
... Climate change is the ideal illustration of punctuated equilibria with different periodicities. High-resolution paleoenvironmental research on the Holocene has now allowed the detection of many abrupt climate change events in the past 10,000 years (Armitage et al. 2015;Phelps et al. 2019). These studies have demonstrated the selective pressures that climate change has exerted on long-term cultural adaptation. ...
... Soon after, "the lake level fell dramatically, and dunes of the Erg of Djourab within the northern Bodélé catchment became active" (Armitage et al. 2015, p. 8545). Data from Mega-Chad, partly summarized in Armitage et al. (2015), point to an abrupt end of the African Humid Period (AHP), a phenomenon that is also documented in East and North Africa. The data indicate rapid, centennial-scale dry conditions all over the large paleolake (Mega-Chad basin) after ca. ...
High-resolution paleoenvironmental research allows us to pinpoint the tempo and amplitude of past climate changes. Abrupt climate events have axiomatically triggered cascades of adjustments, in vegetation, fauna, humans, and pathogens. This essay focuses on the abrupt end of the African Humid Episode (9000– 6000 cal BP), ca. 5000 cal BP in the Sahara. Neolithic pastoralists, practicing transhumance between sandy lowlands and Saharan mountains, adopted new cultural practices: cattle burials and livestock bone deposits in built installations. Their ritual nature is indisputable. But ritual for what? If considered from the perspective of livestock zoonoses, such practices may point to the “dark side” of cultural adjustments—strategies to counter human and livestock diseases. Livestock zoonoses are constant sources of emerging infectious diseases (EID) in the present, as they were in the past. Sustained research on livestock and human health are of paramount importance given the accelerating rate of world urbanization. Résumé La haute résolution des recherches paléoenvironnementales actuelles permet de préciser la chronologie et l’amplitude précises des changements climatiques passés. Axiomatiquement, de brusques changements climatiques ont déclenché des cascades d’ajustements, de la végétation, la faune, les humains et les agents pathogènes. Cet article porte sur la fin abrupte de l’Episode Humide Africain (9000-6000 Cal yr BP), autour de 5000 calBP dans le Sahara. Les pasteurs néolithiques, transhumant entre les basses terres sablonneuses et les montagnes sahariennes, ont adopté de nouvelles pratiques culturelles: enterrements de bétail et dépôts de leurs os dans des installations construites. Leur nature rituelle est incontestable. Mais ritual pour Quoi? Si elles sont prises en considération du point de vue des zoonoses du bétail, de telles pratiques peuvent indiquer le « côté obscur » des ajustements culturels, des stratégies de lutte contre les maladies humaines et du bétail. Les zoonoses sont des sources constantes de maladies infectieuses émergentes [EID] dans le présent, ainsi que dans le passé, en effet. En raison de l’urbanisation acceleree, les recherches soutenues sur la santé humaine et du bétail sont d’une importance capitale.KeywordsMid-HoloceneAbrupt climate changeBiosphere adjustmentsPastoralismLivestock zoonosesCultural adaptation
... Archaeological excavations found evidence of the presence of herders, especially cattle remains, in a great number of archaeological sites in the Sahara (Jesse et al. 2013;Kuper and Riemer 2013). Around 5.5 ka, a rapid climate change in this region led to intensive aridification with a serious impact on all life forms (Manning and Timpson 2014;Armitage et al. 2015). Along with the fauna and flora, human populations gradually emigrated from the newly formed steppe to less hostile environments situated southwards. ...
The Sahel/Savannah belt harbors diverse populations with different demographic histories and different subsistence patterns. However, populations from this large African region are notably under-represented in genomic research. To investigate the population structure and adaptation history of populations from the Sahel/Savannah space, we generated dense genome-wide genotype data of 327 individuals-comprising 14 ethnolinguistic groups, including ten previously unsampled populations. Our results highlight fine-scale population structure and complex patterns of admixture, particularly in Fulani groups and Arabic-speaking populations. Among all studied Sahelian populations, only the Rashaayda Arabic-speaking population from eastern Sudan shows a lack of gene flow from African groups, which is consistent with the short history of this population in the African continent. They are recent migrants from Saudi Arabia with evidence of strong genetic isolation during the last few generations and a strong demographic bottleneck. This population also presents a strong selection signal in a genomic region around the CNR1 gene associated with substance dependence and chronic stress. In Western Sahelian populations, signatures of selection were detected in several other genetic regions, including pathways associated with lactase persistence, immune response, and malaria resistance. Taken together, these findings refine our current knowledge of genetic diversity, population structure, migration, admixture and adaptation of human populations in the Sahel/Savannah belt and contribute to our understanding of human history and health.
... i) The Quiescent model: Deprived of its major inflows, the basin dried out and remained largely inactive over the late Quaternary, with the exception of neotectonic modifications to its existing geomorphology; ii) The Dynamic model: Driven by large-scale climate variability, lake levels continued to rise and fall with similar frequency and amplitude to transgressions and regressions that occurred elsewhere across Africa (e.g. Armitage et al., 2015;Drake et al., 2018;Moernaut et al., 2010;Scholz et al., 2011). ...
The intersection of archaeological material with the landscape is investigated using OSL dating of landforms associated with Middle Stone Age (MSA) archaeology in the Makgadikgadi basin, Botswana. In this study, MSA archaeological sites on the Makgadikgadi pan floor date to two dry periods in the basin during the late Quaternary. Site formation at one site occurred during dry, or seasonally dry conditions that followed a period of high lake levels between 128 ± 18 ka and 81 ± 6 ka. The site was buried by sediments from a subsequent period of high lake levels dating to between 72 ± 5 ka and 57 ± 8 ka. At other investigated sites, the archaeological material was most likely deposited during a second dry period sometime after this. Overlying dunes are much younger (late Holocene) than the late Pleistocene lakebed sediments associated with the archaeological sites. Rapid burial of the archaeological sites by clayey sands has resulted in limited disturbance and weathering of archaeological material which appears to have only been exposed very recently, perhaps in the last 350 years when conditions have been particularly dry and susceptible to deflation. The spatial patterning of both sediment accumulation and deflation strongly influences archaeological visibility both within and around the Makgadikgadi basin.
... The continuous increase of humid air from the Atlantic Ocean characterized the monsoonal regime and it flows into the continent up to about 11°N in May, this is associated with seasonal migration of the Intertropical Convergence Zone (ITCZ) from its southern position in the boreal winter to its northern position in the boreal summer [18]. Paleo-climate records from the LCB was used to back up the fact that the African monsoon responds to insolation forcing in no definite pattern and that Lake Mega-Chad influenced biogeochemical cycles on a global scale [19]. According to Jury, decade-long rainfall variability in the region has been attributed to the interaction of Hadley Walker cells over Africa at decade-long frequency through anomalous north-south displacement of the near-equatorial trough [20], while Chaminade linked it to atmospheric variability [5], Paeth attributed it to global warming [21] while Zeng attributed it to processes related to vegetation feedback [22] .Time series of river discharge incorporates some important hydrological parameters such as precipitation, temperature, and changes in land cover [23]. ...
Periodic assessment of trend and climatic variability is important for better planning and water resource management in the LCB catchment. This study assessed the impact of climatic variables on the LCB (LCB) using rainfall data derived from TAMSAT, run off and evapotranspiration data derived from climate engine over a period of twenty years (2001-2020). Non-parametric Mann Kendall (MK) and Spearman rho tests were applied to detect the presence of rainfall trends and trend magnitude was calculated using Sen's Slope Estimator (SSE). With an average annual rainfall of 523.62mm, trend analysis showed an annual positive trend for precipitation at 5% significance level. Analysis of seasonal rainfall using both tests revealed an increasing trend during the dry season. However, the late dry season experienced a slight increasing trend and for wet seasons. There was 2.7% runoff and 4.6% evapotranspiration which are the lowest for the time series.
... The climate history of the Sahara, the world's largest warm desert, is a subject of considerable interest as much as it is difficult to achieve adequate documentation. Much of what we know of past climate dynamics in the Sahara during the Holocene (starting 11,700 years ago; 11.7 ka to present) is deduced from natural climate archives situated along its fringes (1)(2)(3)(4), as archives with adequate continuity and age control are extremely rare in the continental interior (3,5). The last 3000 years, in particular, remain largely blank, because progressive aridification of North Africa throughout the Middle and Late Holocene (6) eventually led to complete desiccation of the lakes that would have accumulated climate proxy information in their sediments (7,8). ...
... This result supports the hypothesis, based on mollusk assemblages, that the basin's aquatic habitats have experienced temporal discontinuity (11,24), thereby countering the idea that the modern-day Serir oasis contains relict aquatic habitats dating back to the AHP (15). The timing of Lake Teli's most recent complete desiccation notably aligns with the well-known 4.2-ka event, a possibly near-global climate anomaly (33) that is widely expressed as a dry spell at other Sahara oases, at lakes across the Sahel, and at other tropical areas of North Africa (2,5,(34)(35)(36)(37)(38). The transgression of Lake Teli immediately following this dry spell corresponds with a temporary return to wetter conditions across the Sahara and Sahel (5,8,36), which has been broadly dated to the period 4.2 to 3.2 ka. ...
... The timing of Lake Teli's most recent complete desiccation notably aligns with the well-known 4.2-ka event, a possibly near-global climate anomaly (33) that is widely expressed as a dry spell at other Sahara oases, at lakes across the Sahel, and at other tropical areas of North Africa (2,5,(34)(35)(36)(37)(38). The transgression of Lake Teli immediately following this dry spell corresponds with a temporary return to wetter conditions across the Sahara and Sahel (5,8,36), which has been broadly dated to the period 4.2 to 3.2 ka. The Late Holocene high stand of Lake Chad dated to ca 3 ka (2,38) is also an expression of this modestly wetter episode. Combined with the available evidence of Saharan climate evolution through the Holocene (8,36), our data indicate that the 4.2-ka event may represent the most extreme drought that the Sahara desert has experienced in the last 11,000 years. ...
The climate history of the Sahara desert during recent millennia is obscured by the near absence of natural climate archives, hampering insight in the relative importance of southerly (tropical) and northerly (midlatitude) weather systems at submillennial time scales. A new lake sediment record from Ounianga Serir oasis in northern Chad, spanning the Late Holocene without interruption, confirms that immediately before ca 4200 years ago, the Sahara experienced an episode of hyperaridity even more extreme than today's desert climate. The hypersaline terminal lake which formed afterwards never desiccated during the late Holocene due to continuous inflow of fossil groundwater, yet its water balance was sensitive to temporal variation in local rainfall and lake surface evaporation. Our in-lake geochemical proxies show that, during the last 3000 years, century-scale hydroclimate variation in the central Sahara primarily tracked the intensity of the tropical West African monsoon, modulated at shorter time scales by weather patterns linked to shifts in midlatitude Atlantic Ocean circulation.
... Changes in lake level provide a wealth of information about the hydrological cycle of the entire basin (Anderson et al., 2005(Anderson et al., , 2011Luoto and Sarmaja-Korjonen, 2011), which can be used to indicate past changes in lake water volume and further evaluate changes in local water resources. The analysis of millennial-scale lake level evolution in closed basins has focused on typical lakes, including Lake Lahontan (Briggs et al., 2005), Lake Titicaca (Rowe et al., 2002), Lake Mega-Chad (Armitage et al., 2015), Lake Malawi (Gasse et al., 2002), Lake Mega-Frome (Cohen et al., 2011), etc. Based on the synchronous fluctuation of large-scaled lake levels, paleolake studies have summarized some regularities which indicate that high lake levels were attained during glacial maxima at high latitudes, and during interglacials or interstadials in low-latitude tropics (Street and Grove, 1979). ...
... (c) Lake level curve based on core-top δ 13 C org from Lake Titicaca (Rowe et al., 2002). (d) Lake level history of Lake Mega-Chad (Armitage et al., 2015). (e) Periphyton diatoms in Lake Malawi (Gasse et al., 2002). ...
Mostly concurring with arid and semi-arid regions, closed basins are faced with water scarcity, prominent imbalance between water resource supply and demand, and ecological degradation. Spatial-temporal patterns of water resource change in closed basins have received increasing attention in recent years, but it is still unclear whether there is a connection between the present patterns and those under the millennial-scale natural state. According to lake records, lake models and paleoclimate simulations, we provided a preliminary impression of water volume changes in the long history of different closed basins, which clarified the linkages and differences between the past and present water volume changes. We also evaluated possible water volume changes in the near future. We found that there has been a declining trend of water volume in closed basins of mid-latitude since the Last Glacial Maximum in general, whereas there has been a rising trend of water volume in closed basins of low-latitude over the same period. Over the last few decades, the declining trend of water storage in Western North America, Sahara and Arabia and Central Eurasia possibly has inherited the decreased water volume conditions across the millennial-scale under the natural state, while the increasing water storage in Great Rift Valley and Southern Africa and the decreasing water storage in Dry Andes and Patagonia may be temporary. The assessment of near future water volume changes from a paleoclimatological perspective indicates that the water volume in parts of Western North America, northern Dry Andes, northern Sahara, and southwestern Central Eurasia will continue to decline, while in parts of southern Sahara, southern Great Rift Valley, southeastern Central Eurasia, and eastern Australia it will continue to rise.
... In areas that are currently arid/hyper-arid the presence of sediment/landform features such as spring mounds, travertine/tufa, lake shorelines, lacustrine sediments and fluvial deposits all provide evidence for hydrological processes being more active in the past (Grove and Warren, 1968;Kropelin et al., 2008). The dating of these features by 14 C, luminescence techniques and U/Th disequilibria, allows the timing of these humid phases to be reconstructed (Causse et al., 1989;Armitage et al., 2015). ...
... However, it is increasingly recognised that in some humid phases, the terrestrial record indicates that the landscape of the Sahara was not characterised by the existence of isolated and small-scale water bodies. Instead the record has been interpreted as indicating the existence of regionalscale integrated hydrological networks that occurred across the Sahara, terminating in a series of megalakes; megalakes Chad (Schneider, 1967;Ghienne et al., 2002;Schuster et al., 2003Schuster et al., , 2005Leblanc et al., 2006;Drake and Bristow, 2006;Armitage et al., 2015), Chotts (Causse et al., 1988(Causse et al., , 1989(Causse et al., , 2003Zouari et al., 1998), Fezzan (Thiedig et al., 2000;Armitage et al., 2007;Geyh and Thiedig, 2008;Drake et al., 2008), Darfur (Ghoneim and El-Baz, 2007); White Nile (Barrows et al., 2014;Williams et al., 2003), Tushka (Maxwell et al., 2010) and Ahnet-Mouydir (Conrad, 1970;Conrad and Lappartient, 1991;Drake et al., 2011). The proposed existence of these megalakes has wide-ranging implications for both palaeoclimate, as their formation requires large-scale increases in mean annual precipitation, and human dispersal, as it has been suggested that in concert they acted as humid corridors across the desert aiding human migration (Drake et al., 2008(Drake et al., , 2011. ...
... These include evidence for palaeoshorelines eroded into the landscape, littoral sediment accumulations (deltas, spits and berms, Drake and Bristow, 2006;Schuster et al., 2005Schuster et al., , 2014 and fine-grained deposits within the deeper parts of the basin that contain diatoms (Gasse, 2002), ostracoda and molluscs (Bristow et al., 2018) indicative of freshwater conditions. Dating of numerous shorelines suggest megalake Chad existed between 5 and 11 ka (Armitage et al., 2015) and during MIS5, with two beach ridges dated to 114.2 ± 14 and 125.4 ± 11.6 ka (Drake et al., 2011). The evidence preserved in the Chad basin is, therefore, extensive and well-documented. ...
It has long been recognised that the Sahara Desert contains sediment, landform and palaeoecological evidence for phases of increased humidity during the Quaternary period. Many authors have also suggested that during some of these humid periods very large lakes, termed megalakes, developed in several basins within the Sahara. Recent work has questioned their existence. In particular it has been argued that the lack of well-developed and spatially extensive shorelines in these basins suggests that discrete groundwater and spring deposits have been misinterpreted as evidence for megalakes. In this paper we re-evaluate the evidence used to identify megalakes. Firstly, we apply a comprehensive remote sensing and GIS analyses to the megalake shorelines, their catchments and the wider Sahara. This not only supports the previously proposed existence of numerous megalakes, but also indicates a previously unrecognised megalake in the Niger Inland Delta region, here named Megalake Timbuktu. Secondly, we review the geomorphic and sedimentary evidence for the megalakes, highlighting the importance of the sedimentary record in identifying lake highstands, particularly through the example of the Chotts Megalake in southern Tunisia where we provide new sedimentary information on lake shorelines. This analysis demonstrates that in much of the Sahara the dynamic aeolian systems preclude the preservation of well-developed shorelines, but the distribution of fragmented geomorphic features and localised lake deposits provide robust evidence for Quaternary megalake formation. The paper concludes by highlighting that although extensive evidence for Saharan megalake formation exists, the current chronology of lake highstands indicates that the vast majority date to Marine Isotope Stage (MIS) 5 or earlier. Only megalakes Chad and Timbuktu, which derive much of their water from outside the desert, show evidence for Holocene (African Humid Period or AHP) shorelines. The AHP record of the other megalakes indicate the existence of much smaller water bodies than those that developed earlier in the Pleistocene indicating that it was significantly drier than these earlier humid phases.
