Search

International audience; During the ‘Green Sahara event’, water bodies developed throughout the Sahara and Sahel, reflecting the enhanced influence of the Atlantic monsoon rainfall. Major lakes then dried out between 6.5 and 3.5 ka. This study investigates land cover change and lacustrine environment during the Holocene at I-n-Atei, Southern Algeria, a desert region lying in the hyperarid core of the Sahara. This site is remarkable by its extent (up to 80 km2) and by the exceptional preservation and thickness of the lacustrine deposits (7.2 m). I-n-Atei was a lake from 11 to 7.4 ka, then it dried out and left place to a swampy environment. Charcoal concentrations show that the surroundings of the lake were vegetated throughout the wet period with two short phases of possible vegetation deterioration associated with a lowering of the lake level at 9.3 and 8.2 ka, coeval with well-known dry events in the tropics. The stable carbon isotope record reflects the penetration of C4 herbaceous populations in replacement of the original C3, typical of the regional vegetation at the time of the maximum lake expansion. The δ13C of charcoals increase non-linearly with the 14C-based ages from − 24.5‰ to − 13.0‰ (V-PDB). Assuming that these extreme values sample both C3 and C4 plant end-members, mass balance calculations suggest that C3 were replaced by C4 plants according to an exponential decay law with a half-life (t1/2) of 850 ± 110 years. The replacement of C3 by C4 plants occurred in two main steps: a mixed C3–C4 vegetation of “wooded grassland” type was present from 10 ka to 8.4 ka while a C4 exclusive vegetation developed after 8.4 ka. After the end of the lacustrine phase a catastrophic event (flooding?) provoked the lifting of most of the lacustrine deposits and their re-deposition above the lacustrine sequence.

The Holocene continental hydrology, as revealed by a number of archives and proxies, fluctuates at all time scales, due to factors external or internal to the Earth system that control local, regional and global climates. Palaeohydrological reconstructions shed light on climate changes, on the history of human societies and on hydrological risk assessment. To cite this article: F. Gasse, C. R. Geoscience 337 (2005).

The interactions between climatic and volcanic forcing on diatom communities contained in a 50,000-year sedimentary sequence from Lake Massoko, Tanzania, were examined. At the century scale, 19 discrete tephra inputs to the lake isolated the sedimentary nutrient supply and shifted the diatom communities to those tolerant of low phosphorus levels, whereas at the millennial scale, diatom-inferred shifts in precipitation–evaporation based on conductivity optima and diatom life-form ratios were broadly similar to lake-level reconstructions from Lake Rukwa, Lake Malawi, and others in the region. Some fluctuations of Lake Massoko are consistent with the precession-driven changes in insolation, but the major climate shifts do not relate directly to orbital forcing of summer insolation south of the equator and show more consistency with records from the equatorial and northern tropics that receive rainfall from the passing of the intertropical convergence zone. Sea surface temperatures are strongly correlated to multimillennial-scale climate patterns over this region of Africa.

Lake Abiyata is a small, closed, saline-alkaline lake located in the central part of the Ethiopian Rift Valley, East Africa. A multi-proxy study of a sediment core, 116 cm long and with undisturbed mud-water interface, was performed to test the sensitivity of the lake system and of different proxies to the changes in climate and human activities that occurred in the catchment during the past few centuries. The 210Pb analyses suggest that the upper 80 cm of the core represent the past 200 years. This study complements millennial-scale environmental records available in the region. The main freshwater-climatic and biological features of the modern lake system and their variations over the past decades, as known from observations, are first summarised. Results derived from individual proxies analysed along the core are then presented (successively, major physical and chemical properties of bulk sediments, diatoms and pollen). Uncertainties on the chronological framework are discussed. Major limnological stages are finally identified based on the multi-proxy interpretation of our record. Our record shows large variations in the lake water and salt balances, in the sediment sources, and in the vegetation distribution in the basin. Using our 210Pb chronology, major changes observed in the core are tentatively compared with environmental events known from instrumental and historical records. The upper 41 cm of the core (210Pb age: 1940-1998 AD) reveal several fluctuations in diatom-inferred water depth and salinity which seem to be consistent with known changes in water level. Human impact on vegetation clearly appears since about 30 years. The interval 85-41 cm suggests a period of overall water deficit. Lake Abiyata experienced episodes shallower and more saline than over the past decades, especially around 68-66 cm, 210Pb dated at ca. 1890 AD. This level may coincide with one of the worst droughts known in the Ethiopian history during 1888-1892. The lower part of the core includes a stage (108-85 cm) of lake level much higher than today and which ended before 1800 AD. Although its base is undated so far, this stage suggests that conditions much wetter than today have prevailed in the region during at least part of the 18th century. Lake Abiyata appears to be a suitable site for a detailed environmental reconstruction over the recent past, although further work is needed to reduce the uncertainties on our record, as discussed in the conclusions.

Five lakes in the Qinghai–Tibet Plateau and North Xinjiang have provided oxygen isotope records covering the past 17–11 cal. kyr BP. The modern lakes are first presented within the major atmospheric circulation and climate patterns of China, with the climate-related isotope composition of input water. Equations used to extract hydrological and climatic information from δ18O in carbonates are then given. After a discussion on potential causes for δ18O changes, each of the five isotope profiles is interpreted in the light of its specific environmental data. The most significant event observed in all records occurred at 12–11 cal. kyr BP: an abrupt intensification of summer rainfall associated with an extremely rapid 1000–1500 km westward migration of the monsoon fronts. The monsoon regime weakened in stages after 8–7 cal. kyr BP. Holocene maximal aridity is recorded at all sites between about 4.5 and 3.5 cal. kyr BP. Long-term changes in monsoon strength as inferred from our isotopic interpretation are in good agreement with climate model reconstructions in the Tibet–Qinghai Plateau, while observed hydrological changes in North Xinjiang are not simulated by the models. Also, the non-linearity of the Holocene monsoon variations cannot be accounted for by orbital forcing.

In the central sector of the Main Ethiopian Rift, the Ziway–Shala lake basin system includes four present-day residual lakes, from north to south, lakes Ziway, Langano, Abijata, and Shala. This region of East Africa is under the influence of the Intertropical Convergence Zone seasonal migration. Thus it has been designated as a potential core site by the ERICA Project (`Environmental Research for Intertropical Climate in Africa'). The four lakes have been subjected to strong changes in water level and water salinity at least during the Late Pleistocene. The purpose of this study is to produce a model of basin formation and sediment accumulation for this system of lakes, in order to separate the effects of climatic change from environmental variations induced by local or regional factors such as volcano-tectonic forcings. In addition to an exhaustive synthesis of available data, various investigations have been used to develop this model: 3D remote sensing, high-resolution seismics, coring, and structural, sedimentological, and hydrological field studies. New AMS radiocarbon dating helped to refine the pre-existing stratigraphic framework for this region, and basin age estimations were calculated using mean sediment accumulation rates. The history of the Ziway–Shala lake basin system has been reconstructed from the Late Pliocene–Early Pleistocene period (106 yr), mainly characterized by catastrophic explosive volcanic eruptions. The early-middle Pleistocene–Late Pleistocene period (104–106 yr) was marked by a regional volcano-tectonic paroxysm, resulting in major changes in the morphology of the area, with the formation of the Abijata, Ziway and Shala lake basins. From ∼0.20 Ma, the Ziway–Shala basin history is marked by the eastward migration of volcano-tectonic activity, resulting in the development of the youngest basin of the Ziway–Shala system, the Langano Basin. The joint history of sedimentation in the Ziway, Langano, Abijata, and Shala lake basins started during the early-Late Pleistocene period (101–104 yr) and is characterized from this period up to the present-day by a series of climatically controlled rises and falls of lake level.

Given the uncertainties associated with GCM modelling of aerosol effects on regional precipitation, a simple model is used to make a preliminary assessment of the influence of aerosols on rainfall in the Lake Tanganyika region of tropical southern Africa. No attempt will be made to consider the effects of changing carbon dioxide and moisture contents of the atmosphere that might have occurred at the same time. Atmospheric transport and recirculation of air and aerosols are considered and shown to occur on a large scale. Size distributions, residence times and concentrations of aerosols are examined and air volume and aerosol mass transports are estimated. South African data from stations on the interior plateau are used to approximate present‒day seasonal and interannual variations in transmissivity due to changing aerosol loading in the atmosphere. A lake‒catchment hydrological model is used to determine rainfall over Lake Tanganyika and its environs and from this a sensitivity analysis of rainfall receipt to aerosol‒induced changes in atmospheric transmissivity is carried out. It is shown that a decrease in transmissivity commensurate with the present‒day interannual range of variability of 10 per cent produces, in the absence of other changes, about a 15 per cent diminution in rainfall. The analysis is extended to show that in the past, during the sixteenth‒century Little Ice Age, the transmissivity induced rainfall decrease could have been about 25 per cent and that at 18 000 years BP it would have been more. Aerosol loading is shown to have important consequences for the occurrence of droughts through its modification of the surface radiation balance. © 1997 Royal Meteorological Society.

A 12.4 m core taken in Lake Bangong provides a continuous Holocene climatic record. We summarize information on changes in stable isotope and radiocarbon balances in the lake, hydrobiology and vegetal cover in the catchment, deduced from detailed analytical results given in the three preceding papers. The Bangong record is then compared with the environmental history of the neighbouring Lake Sumxi also constructed from multidisciplinary analyses. The two records show a major environmental change at ≈ 10-9.5 ka B.P., attributed to a rapid strengthening of the summer monsoonal circulation which led to wet-warm conditions. This event was followed by a long-term trend toward aridity which culminated around 4-3 ka B.P.. In Western Tibet, maximum monsoon rainfall seems to have occurred from ≈ 9.5 to 8.7 ka B.P. and from ≈ 7.2 to 6.3 ka B.P., as two wet pulses separated by a reversal event centered on 8.0–7.7 ka B.P. Our results broadly agree with the records from Lake Seling in Central Tibet, and Lake Qinghai at the plateau's northeastern margin, and with palaeoclimatic studies in western China which document conditions wetter and warmer than those of today during the early-middle Holocene. The environmental fluctuations recorded in western Tibet appear in phase with climatic changes recognized in tropical North Africa, suggesting that the 8.0–7.7 ka B.P. and the 4.0–3.0 ka B.P. dry events were caused by abrupt disequilibrium in the climatic system.

A 12.4 m core collected from the eastern part of Lake Bangong provided a continuous Holocene record. This core was analyzed for diatoms, ostracods, charophytes, content in total organic carbon and identification of organic particles contained in the sediments. The record is remarkable for the abundance and diversity of its microorganisms. An oligo- to mesosaline marsh occupied the core site before 9.5 ka B.P. Bangong was most likely closed. A sudden filling of the lake basin by dilute, nutrient-poor water occurs around 9.4 ka B.P. Maximum lake extent seems to have occurred around 9.4–8.7 ka B.P. Generally freshwater, relatively deep conditions were maintained up to 6.3 ka B.P., although interrupted by short-term regressive events between 8.6 and 7.5 ka B.P. A trend towards palustral conditions begins after 6.3 ka B.P. and the interval 3.9-3.2 ka B.P., rich in detrital organic material, coincides with an extremely low lake level. A last lacustrine episode of minor amplitude is recorded from about 3 to 2 ka B.P., before the closure of the system leading again to slightly saline conditions. The resemblances and synchronism between environmental changes recorded at Bangong and in Lake Sumxi, situated 200 km northwards and 800 m higher in elevation, indicate that regional climatic changes have been the major driving factor on ecological and hydrobiological changes in lakes of western Tibet.

Regions beyond the present or past penetration of the Indian and African monsoons have experienced several large and abrupt climatic fluctuations over the past 13 14C kyr. Pollen and lake records from West Asia (Western Tibet and Rajasthan), East Africa (Ethiopia) and West Africa (Western Sahara, Sahel and subequatorial Africa) were selected on the basis of chronological control, sensitivity of both site and environmental indicators to climate change, the continuity of the record, and interdisciplinary control of the palaeoclimatic interpretation. Conditions wetter than those of today prevailed during the early-mid-Holocene period, but major dry spells are recorded at all sites during the intervals ∼ 11.0–9.5 kyr BP, ∼ 8–7 kyr BP and 3–4 kyr BP. Several records also suggest dry events of minor amplitude around 6 kyr BP. Potential boundary forcings of insolation and sea surface and tropical land surface conditions are discussed. The solar radiation accounts for the general envelop of the post-glacial monsoon fluctuations, but explains neither the timing nor the amplitude of the short-term changes. In spite of apparent covariation between fluctuations in sea surface conditions in the North Atlantic and the monsoon record, no direct mechanism could be found relating the intensity of the oceanic thermohaline conveyor belt to the monsoon strength. Changes in tropical land surface conditions (soil moisture negative feedback, and changes in CH4 production from wetlands) provide a more satisfactory hypothesis for explaining abrupt reversal events.

Diatom assemblages are documented from diatomite layers of two Miocene fluvio-lacustrine units from the basins of the western Cordillera of northern Peru: the Namora formation and the Cajabamba formation. Emphasis is given to taxa of particular stratigraphic interest such as Mesodictyon sp., Cyclotella stelligera Cleve & Grunow, and Aulacoseira agassizii (Ostenfeld) Simonsen and their ultra- structure is investigated with the SEM. The diatom assemblages indicate for the Namora formation the occurrence of swampy conditions with very dilute, low alkalinity water. The diatom assemblages of the Cajabamba formation reflect the occurrence of fresh, slightly alkaline, eutrophic lakes with deep water in some samples, and swampy conditions with relatively high salt content in other samples. The Namora formation is late Miocene in age based on the diatom assemblages and radiometric analyses (7.2 ± 0.6 Ma). The diatom layers of the Cajabamba formation are dated as late middle to early late Miocene (ca. 11–8 Ma) base...

Although the Tibetan Plateau greatly influences the atmospheric circulation of the Nortbern Hemisphere, few continuous paleoclimatic records are available from the plateau. A 13,000-yr pollen and diatom record from the Sumxi-Longmu Co basin in western Tibet gives information on major changes both in regional vegetation and in local hydrology. After the basin first filled ca. 13,000 yr B.P., a dry spell occurred about 10,500 yr B.P. within the interval spanned by the European Younger Dryas chronozone. A major environmental change occurred suddenly at ≈10,000 yr B.P., with the establishment of wet conditions, and was followed by a long-term trend toward maximum aridity, which lasted approximately 6000 yr. Short-term oscillations are superimposed on this general climatic change with a major reversal event about 8000 yr B.P. and a second wet pulse leading to a maximum lake volume ca. 7500-6000 yr B.P. Maximum aridity occurred 4300 yr B.P. The major environmental fluctuations recorded at Sumxi-Longmu Co appear in phase with climatic changes recognized in north tropical Africa, suggesting that the 8000 to 7000-yr-B.P. event was caused by an abrupt disequilibrium in the climatic system, as was the Younger Dryas and possibly the 4300-yr-B.P. event.

Integrated dynamic water and chloride balance models with a catchment-scale hydrological model (PRMS) are used to investigate the response of a terminal tropical lake, Lake Abiyata, to climate variability and water use practices in its catchment. The hydrological model is used to investigate the response of the catchment to different climate and land-use change scenarios that are incorporated into the lake model. Lake depth–area–volume relationships were established from lake bathymetries. Missing data in the time series were filled using statistical regression techniques. Based on mean monthly data, the lake water balance model produced a good agreement between the simulated and observed levels of Lake Abiyata for the period 1968–83. From 1984 onwards the simulated lake level is overestimated with respect to the observed one, while the chloride concentration is largely underestimated. This discrepancy is attributed to human use of water from the influent rivers or directly from the lake. The simulated lake level and chloride concentration are in better agreement with observed values (r2 = 0·96) when human water use for irrigation and salt exploitation are included in the model. A comparison of the simulation with and without human consumption indicates that climate variability controls the interannual fluctuations and that the human water use affects the equilibrium of the system by strongly reducing the lake level. Sensitivity analysis based on a mean climatic year showed that, after prolonged mean climatic conditions, Lake Abiyata reacts more rapidly to an abrupt shift to wetter conditions than to dry conditions. This study shows the significant sensitivity of the level and salinity of the terminal Lake Abiyata to small changes in climate or land use, making it a very good ‘recorder’ of environmental changes that may occur in the catchment at different time scales. Copyright © 2004 John Wiley & Sons, Ltd.

1. Introduction F. Gasse and R.W. Battarbee 2. Archives and Proxies along the PEP III Transect F. Oldfield and R. Thompson 3. Oceanic Climate Variability at Millennial Time-Scales: Models of Climate Connections L. Vidal and H. Arz 4. Between Agulhas and Benguela: responses of Southern African climates of the Late Pleistocene to current fluxes, orbital precession and the extent of the Circum-Antarctic vortex T.C. Partridge, L. Scott and R.R. Schneider 5. Holocene climatic trends and rhythms in southern Africa L. Scott and J.A. Lee-Thorp 6. Diatom productivity in Northern Lake Malawi during the past 25,000 years: implications for the position of the intertropical convergence zone at millennial and shorter time scales T.C. Johnson, E.T. Brown, and J. McManus 7. Late Quaternary climatic variability in intertropical Africa P. Barker, M.R. Talbot, F.A. Street-Perrott, F. Marret, J. Scourse and E. Odada 8. Decadal and century-scale climate variability in tropical Africa during the past 2000 years D. Verschuren 9. Late Quaternary climate changes in the Horn of Africa M. Umer, D. Legesse, F. Gasse, R. Bonnefille, H.F. Lamb, M.J. Leng and A. Lamb 10. Palaeoenvironments, palaeoclimates and landscape development in Atlantic Equatorial Africa: a review of key sites covering the last 25 kyrs H. Elenga, J. Maley, A. Vincens and I. Farrera 11. Aspects of Nigerian coastal vegetation in the Holocene: some recent insights M.A. Sowunmi 12. Palaeoenvironmental changes in the arid and sub arid belt (Sahara-Sahel-Arabian Peninsula) from 150 kyr to present P. Hoelzmann, F.Gasse, L.M. Dupont, U. Salzmann, M. Staubwasser, D C. Leuschner and F. Sirocko 13. Historical chronology of ENSO and the Nile flood record L. Ortlieb 14. Groundwater as an archive of climatic and environmental change: Europe to Africa W.M. Edmunds, A. Dodo, D. Djoret, F. Gasse, C.B. Gaye, I.B. Goni, Y. Travi, K. Zouari and G.M. Zuppi 15. Mediterranean Sea palaeohydrology and pluvial periods during the Late Quaternary N. Kallel, J. Duplessy, L. Labeyrie, M. Fontugne and M. Paterne 16. Palaeoenvironmental changes in the Mediterranean region 250-10 kyr BP D. Magri, N. Kallel and B. Narcisi 17. Holocene climate, environment and cultural change in the circum-Mediterranean region N. Roberts, A.C. Stevenson, B. Davis, R. Cheddadi, S. Brewer and A. Rosen 18. Speleothems as palaeoclimate indicators, a case study from Soreq Cave located in the Eastern Mediterranean Region, Israel M. Bar-Matthews and A. Ayalon 19. Climatic and environmental variability in the Mid-Latitude Europe sector during the last interglacial-glacial cycle J.J. Vandenberghe, J.J. Lowe, R. Coope, T. Litt and L. Zoller 20. Atlantic to Urals - the Holocene climatic record of Mid-Latitude Europe K. Barber, B. Zolitschka, P. Tarasov and A.F. Lotter 21. Climate variability during the last interglacial-glacial cycle in NW Eurasia M. Saarnisto and J.P. Lunkka 22. Holocene climate dynamics in Fennoscandia and the North Atlantic I. Snowball, A. Korhola, K.R. Briffa and N. Koc 23. Recent developments in Holocene climate modelling H. Renssen, P. Braconnot, S.F.B. Tett, H. Von Storch and S.L. Weber 24. Evaluation of PMIP coupled ocean-atmosphere simulations of the Mid-Holocene P. Braconnot, S.P. Harrison, S. Joussaume, C.D. Hewitt, A. Kitoch, J. E. Kutzback, Z. Liu, B. Otto-Bliesner, J. Syktus and S.L.N. Weber 25. Famine, climate and crisis in Western Uganda P. Robertshaw, D. Taylor, S. Doyle and R. Marchant 26. Palaeo-research in Africa: relevance to

The PEP III Europe-Africa transect extends from the arctic fringes of NW Eurasia to South Africa. It encompasses the presently temperate sector of mid-latitude Europe, the Mediterranean region, the arid and semi-arid lands of the Sahara, Sahel and the Arabian Peninsula, and the inter-tropical belt of Africa. The palaeoenvironmental evidence available from these regions, which has been summarised in earlier chapters of this volume and which collectively spans the last 250,000 years, clearly bears the stamp of long-term global climate forcing induced by variations in solar insolation. External forcing is ultimately the reason why the Eurasian continental ice sheets waxed and waned repeatedly during the late Quaternary, and why the southerly limit of permafrost migrated southwards across mid-latitude Europe, periodically becoming degraded during warmer episodes. At the same time, pronounced fluctuations in atmospheric and soil moisture have affected the Mediterranean, desert and Sahel regions, while there is abundant evidence from every sector of the PEP III transect for marked migrations of the principal vegetation belts, as well as for other major environmental changes, that are also considered to reflect long-term climate forcing. It is only in the last decade or so, however, that the full complexity of the history of climate changes during the last interglacial-glacial cycle, and their environmental impacts in continental Europe and Africa, have begun to be recognised. The discovery of evidence for the abrupt Dansgaard-Oeschger (D-O) and Heinrich (H) climatic oscillations in Greenland ice-core (Johnsen et al. 1992) and North Atlantic (Bond et al. 1993) records, have prompted a re-examination of the continental record. This, together with a number of technical improvements in field and laboratory equipment, greater access to sites in remote and difficult terrain, diversification in the range of available palaeoecological and geochronological tools, and closer inter-disciplinary collaboration, have led to a more penetrating examination of the field evidence, which has progressed the science considerably. We can now see that the stratigraphical record is much more complex than appreciated hitherto, and more detailed and refined models of past climatic and environmental models are beginning to emerge. There is, for example, a growing body of evidence which suggests that D-O and H events had significant impacts on the environment of Europe and Africa, as well as on the Mediterranean Sea.

A hydrological modelling at a catchment scale has been used to investigate the impact of climatic and land use change on water resources in data scarce Tropical Africa using a distributed precipitation-runoff modelling system. The model divides a catchment into homogeneous hydrological response units, providing the ability to impose changes in climate or land use spatially. Model parameters were either estimated from different existing data or by calibration against measured discharge data available over 11 years (1985-1995). The model simulation-period was divided into calibration (1986-1990) and validation (1991-1995) periods. The model provided relatively good fits between measured and simulated discharge both at a daily and monthly scales. Based on sensitivity analyses, a 10% decrease in rainfall produced a 30% reduction on the simulated hydrologic response of the catchment, while a 1.5 °C increase in air temperature would result in a decrease in the simulated discharge of about 15%. Converting the present day dominantly cultivated/grazing land in the studied river basin by woodland would decrease the discharge at the outlet by about 8%. In order to use the results of this kind of model for decision making and water resources management, the model should be tested under different environment and different scenario conditions. Rainfall measurement and stage-discharge rating curves should be given priority to improve model performance.

Lake Télé has a nearly perfect ellipsoidal shape and is surrounded by swamps and barely penetrable flooded forests in the heart of the Congo-Zaïre watershed basin, and has intrigued the international scientific community for decades. In June 1992, a first Franco-Congolese multidisciplinary scientific expedition was finally able to reach the lake which is vast (23 km2) and shallow (3 m). Its volume is estimated at 71 000 000 m3 and is 40% filled with an organic silt layer a metre thick. Its hydrological exchanges are almost exclusively vertical with very little lateral contribution from the surrounding swamp. This leads to the observation that the waters are very slightly mineralized (< 3 mg/l), but are very rich in organic carbon (44% of suspended matter and of total dissolved matter) and are very acidic (pH < 4). In addition, a magnetic anomaly of some hundred nanoTesla from a magnetic body at shallow depth has been detected in the lake's northern half, although its origin remains unknown. Botanical and palynological observations suggest the persistence of a strongly hydromorphic forest environment for at least the last 6600 years. Identified pollen taxa, whether ancient or recent, come mainly from pioneer and colonizing taxa such as #Macaranga$, which tends to indicate that the forest is continuing to gradually fill in the lake. (Résumé d'auteur)

The Levant, influenced by both the Mediterranean Sea and the Saharan–Arabian deserts, is a key region for understanding climatic changes in response to glacial/interglacial boundary conditions that have greatly affected regional hydrology. Here, we present the first long-term paleoenvironmental record from the northern Levant, presently much wetter than the southern Levant. Our record derives from the multi-proxy study of a sediment core (36 m long) retrieved from the small intra-mountainous, tectonic basin of Yammouneh (Lebanon), which is mainly supplied by karstic springs.Sediments consist of a thick accumulation of brownish to greenish clayey silts, interrupted by intervals of whitish marls composed of CaCO3 rich material. The core chronology is based on radiometric dating (14C and U/Th) and paleomagnetic techniques. The sedimentary sequence spans approximately the last two glacial–interglacial cycles (~ 250 ka). Carbonate content is relatively high throughout the profile due to a perennial input of detrital calcite from the watershed, but varies significantly. The marls are mostly composed of authigenic and biogenic calcite and reflect lacustrine environments with high carbonate productivity during peaks of interglacial periods. Their occurrence suggests low physical erosion of the basin slopes which were covered by arboreal vegetation, and intense karstic water circulation under warm and relatively wet conditions, in agreement with pollen data. The clayey silts are dominated by quartz and clay minerals, and a few amounts of K-feldspars and dolomite. They are characterized by high concentration in magnetic particles, and high relative concentrations of Si, Al, K and Fe considered as strictly of detrital origin. Due to the carbonated nature of the watershed, quartz and K-feldspars are attributed to eolian origin from a distal source. The clayey silt intervals generally suggest palustrine conditions with abrupt flows responsible for runoff-derived material, or subaerial environments and low local water availability in particular during the Last Glacial Maximum. High Si/Al and K/Al ratios, during interglacials, are attributed to relatively high eolian dust contribution due to the decrease of local detrital inputs.Our study reveals significant differences between Yammouneh and lake records from the Dead-Sea basin (southern Levant), the latter showing high/low levels during glacial/interglacial periods. These hydrological differences might reflect either changes in the North–South (NS) rainfall gradient in response to the development of the northern ice sheet and the southward migration of the westerly belt, or local factors, i.e., a decrease in efficient precipitation and water availability at Yammouneh during the coldest glacial periods (e.g., the Last Glacial Maximum), due to water storage by ice caps on the Mount Lebanon range and frozen soils in the Yammouneh basin.

The ice fields on Kilimanjaro, Africa's highest peak, are melting rapidly. In her Perspective, [Gasse][1] discusses the insights into past African climate gained from the report of [Thompson][2] et al ., whose Kilimanjaro ice core record may remain the first and last such record from Africa. Important questions remain with regard to temperature and hydrological changes in tropical Africa over the past 10,000 years. [1]: http://www.sciencemag.org/cgi/content/full/298/5593/548 [2]: http://www.sciencemag.org/cgi/content/short/298/5593/589

During the Holocene period, lake basins existed in some parts of the Danakil Desert. These basins were supplied either by the Awash River or by ground waters flowing through highly fissured basaltic flows. It is possible to reconstruct the areas and fluctuations of lake levels because of the preservation of old shore lines. Calcareous and diatomaceous bottom deposits give information about lacustrine paleo-environments (isotopic analyses, diatoms). It is suggested that the Holocene period started with an extensive lacustrine episode between 11,000 and 7,000 years B.P. At least one other transgressive stage, of lesser importance, took place between about 2,000 and 1,000 years B.P. The lakes mainly supplied with ground waters (Afrera) exhibit evidence of chemical and biochemical sedimentation. The depostion continued after 7,000 years B.P., and indicates a buffering effect owing to the aquifers. Hence, these basins do not have the simple behaviour of a rain gauge recording the local climate fluctuations of the Danakil Desert. The Dobi Graben can be partly related to this kind of basin. Because of the strong turbidity of the flood waters, the lakes with both shallow and underground supplies are quickly filled up in the head of the alluvial plain of the Awash River. The lakes at the downward part of the plain (Gamari and terminal Lake Abhe) have been preserved from the solid supply during the two humid episodes. During the intermediate stage, the lacustrine regression allowed the transport of silts and muds as far as Lake Abhe. The Awash River has not ceased flowing and the progression of the alluvial deposits isolates some area of retention on the margins of the plain (lacustrine relicts at about 4,000 and 5,000 years B.P.). After subtracting the local hydrological influences, by means of comparison with the recent phenomena, this succession fits into the general pattern of the Holocene climatic events of the southern margin of the Sahara.