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International audience; Phaistos was one of the most important Minoan palaces in Crete and previous studies have addressed its relationship with the paleo-seashore position during historical times. Here, we reconstruct the environmental evolution of Phaistos from Early Minoan to Roman times. Study of two stratigraphic sections and nine boreholes drilled in the westernmost part of the Messara Plain has revealed the stratigraphy of the Mid-to Late-Holocene sediments. Laboratory analyses comprise granulometry, magnetic susceptibility measurements and identification of mollusks, diatoms and pollen grains. Eighteen radiocarbon dates provide a robust chronostratigraphy. In addition, a geophysical survey (electric resistivity tomography (ERT) method) was conducted to reveal the sub-surface morphology in the coring area. The results reveal that a freshwater lake existed from ca. 2100-2000 BC to ca. 1200-1100 BC, which subsequently became swampland until ca. 700 BC. A lake retreat is identified at ca. 1200-1000 BC and can be interpreted as resulting from the 3.2 cal kyr BP rapid climate change (RCC) dry event, observed elsewhere in the Eastern Mediterranean. Subsequently, from the 7th to the 5th century BC, there was the input of detrital material and fluvial dynamics prevailed until at least Roman times. The origin of the lake and its disappearance are discussed in the context of regional climate change and local tectonic activity, without excluding possible human influences. We also reconstruct the vegetation history for the period from the Late Minoan to the Early Archaic period. Pollen analysis reveals a Mediterranean maquis landscape dominated by Olea, together with hygrophilous vegetation, and highlights a clear transition from limnic to swampy environmental conditions around 1100 BC. The pollen sequence is also important for assessing the impact of the 3.2 cal. kyr BP RCC event and for assessing the possibility of an abrupt discontinuity in human activity around Phaistos after the demise of the Minoan Civilization.

Following the IPS-2012 Symposium where I received an award medal for « my lifetime achievements », this is a short note where I explained when, how and why I became interested in paleolimnology. I am grateful to the many people with whom I have worked.

We studied mineral magnetic properties of a 6-m-long, late Pleistocene through Holocene sediment sequence from Lake Aibi in Dzungaria (Zunggary, Junggar), northern Xinjiang, China. Results were used to infer environmental changes and are compared with previously studied cores from Lake Manas. Both water bodies occupy the deepest parts of the Dzungarian Basin and are remnants of large Holocene lakes. During the Late Pleistocene, the magnetic mineralogy in both lakes was dominated by detrital, iron oxide minerals. Oxic conditions, which dominated during sedimentation and early diagenesis, persisted over the Pleistocene–Holocene transition. Later, during the middle Holocene, lake bottom conditions enabled authigenic formation of iron sulphide minerals such as pyrite (FeS2) in Lake Aibi, and pyrite and greigite (Fe3S4) in Lake Manas. This iron sulphide mineralogy suggests increased biological activity in stagnant, anoxic bottom waters. Anoxic bottom conditions started about 9.8 cal kyr BP in Lake Manas and at about 7.2 cal kyr BP in Lake Aibi. A short dry event recorded in Lake Manas between 6.8 and 5.2 cal kyr BP is not clearly observed in Lake Aibi. In the late Holocene, i.e. the last 2.8 cal kyr, sediments of both lakes are again characterised by iron oxides, suggesting well-mixed, shallow water bodies. For this recent period, it seems that the detrital material in the two lakes had a common origin. Magnetic properties of sediments in Lakes Aibi and Manas show broadly similar environmental evolution during the late Pleistocene and Holocene. Nevertheless, despite the close proximity of the two lakes (~200 km) in the same basin, they display some different magnetic properties and record environmental changes at different times.

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.

This paper reports new multi-proxy palaeoenvironmental data on the late Glacial-Holocene transition (c. 14.8–9.3 ka) in equatorial East Africa, in the form of microfossil assemblages (chironomids, ...

Although the dramatic climate disruptions of the last glacial period have received considerable attention, relatively little has been directed toward climate variability in the Holocene (11,500 cal yr B.P. to the present). Examination of similar to50 globally distributed paleoclimate records reveals as many as six periods of significant rapid climate change during the time periods 9000-8000, 6000-5000, 4200-3800, 3500-2500, 1200-1000, and 600-150 cal yr B.P. Most of the climate change events in these globally distributed records are characterized by polar cooling, tropical aridity, and major atmospheric circulation changes, although in the most recent interval (600-150 cal yr B.P.), polar cooling was accompanied by increased moisture in some parts of the tropics. Several intervals coincide with major disruptions of civilization, illustrating the human significance of Holocene climate variability. (C) 2004 University of Washington. All rights reserved.

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.

A pollen and diatom study of a 40 m-sedimentary sequence from Lake Tritrivakely in the central highlands of Madagascar provides evidence for climatic and environmental changes during the late Pleistocene and the Holocene. The major features of the modern regional climate and vegetation distribution and the hydrological characteristics of the lake basin are first summarized. Lithological units and specific methods used to establish the pollen and the diatom biozones are then presented. The chronology of the upper 13 m of the sequence (0–41 kyr) is supported by AMS radiocarbon dates. It has not been possible to acquire absolute ages for the lower 27 m of the core. However, a tentative age model based on comparisons between the terrestrial pollen and the Vostok δD isotopic temperature records suggests an age of about 150 kyr for the core base. The vegetation primarily responded to variations in temperature. Six major cold/warm cycles are identified. During the cold phases, Ericaceous bush surrounded the site. Assuming that the age model is correct, cold phases (prior to 143, 115–110, 94–88, 75–69, and 22–17 kyr) coincide with periods of high local summer insolation. The warm phases are of different amplitude. They show first the development of a wooded grassland under a dry climate, which may coincide with periods of low summer insolation (around 125, 100, 83, 60 and 10–5 kyr), and then the growth of mid-latitude trees under wetter conditions. The diatom record primarily documents changes in the precipitation minus evaporation balance (P−E) in the lake catchment, which appear consistent with orbitally-induced changes in P and seasonal contrasts as suggested by our pollen-based age model and the 14C-dated section of the record. An overall trend from a relatively deep, nutrient-rich lake to a shallow, oligotrophic peat-marsh, is partly attributed to the basin infilling. However, wet–dry oscillations are superimposed over this trend. The two first lake high stands (before 143 and around 115 kyr if the age model is correct) and low stands (around 125 and 105 kyr) match pollen-inferred cold and warming phases, respectively. Later, periods of low P−E also fit cold phases, and occur apparently every 12–10 kyr. Summer rain during phases of high summer insolation was not heavy enough to compensate for the large evaporation–evapotranspiration losses during warm summers and dry winters, especially during the cold Last Glacial Maximum, which was drier than today.

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.

Links between southern and northern hemisphere climates during the Late Quaternary are poorly known, partly due to the scarcity of continuous climatic records in the southern tropics. Pollen and diatom evidence from Lake Tritrivakely (19°47′S) provides information on vegetational and hydrological changes in the central highlands of Madagascar over the past 40,000 yr. Most of the record reflects natural environmental variability since humans arrived on the island ca. 2000 yr B.P. During glacial times, the migration of mountain plants toward lower altitudes is consistent with a temperature decrease and with reduced atmospheric CO2levels. In the lake, a positive mean annual hydrologic balance, from 38,000 to 36,000 and from 17,500 to 9800 cal yr B.P., coincided with periods of decreasing summer insolation and preceded by several millennia lake rises in the northern tropics. A negative hydrologic budget during periods of maximum seasonal contrast in solar radiation is partly attributed to high summer evaporation rate. The last glacial maximum was cool and dry. The deglacial warming occurred in two steps. The first step, accompanied by an increase in wetness, occurred abruptly at ca. 17,000 cal yr B.P., about two millennia earlier than in the northern hemisphere. It is abundantly documented in southern terrestrial data. The second step, at 15,000 cal yr B.P., was in phase with the first major temperature change in the northern hemisphere.

The species Thalassiosira faurii (Gasse) Hasle was found dominating the planktonic diatom community during summer 2012 in Lake Langano which is part of the Ziway–Shala alkaline lake system in Ethiopia. In a previous study, the species had been recorded in high abundance in 13.5–12 cal ka old sediments from the neighbor Lake Abiyata. Using a transfer function for conductivity, peaks in abundance had been interpreted as dry spells at the Pleistocene–Holocene transition since the species was considered strictly halophilous. In this study, a strain of Thalassiosira faurii was isolated from Lake Langano and grown in the laboratory. The observation of its morphology and reproduction, and comparison between modern and fossil populations helped to improve the taxonomic definition of the species. The autecology of the species was also investigated through a salinity tolerance test. The results revealed that the species was much more sensitive to salinity than previously believed. A new conductivity optimum was derived from the experiment and incorporated into the transfer function. The reassessment of lake conductivity variations along the core of Lake Abiyata showed that the intensity of the dry periods preceding the onset of the Holocene had been probably over-estimated. The reconsideration of the species relationship to conductivity may have important consequences for paleoclimatic reconstructions in tropical Africa where Thalassiosira faurii has been reported in abundance. The use of experimentation on local diatom strains allows paleoecologists to solve the problems of low representation of some species in modern dataset and of regional autecological variability within morphotypes.

Paleo-hydrologic and -vegetation proxy data from the Tanganyika basin are integrated in energy and water balance equations to infer past evaporation and precipitation during the last glacial maximum (LGM). Our approach is first validated on the modern system. Large variations are assigned to input variables to simulate the interannual precipitation variability. Equations are then applied to the LGM. We first change those input parameters inferred from proxies (basin and lake surfaces, temperature, and land albedo). Our LGM simulation suggests (in percent of modern mean values) decreases in evaporation from the lake [El: −5% (between −13% and +3%)] and land [Ec: −8% (−19/+5)] bodies, in precipitation [P: −11% (−21/0)] and (P − Ec): −42% (−44/−40). Decreases in P and E are amplified [El: −8% (−16/0); Ec: −14% (−24/−2); P: −17% (−26/−6)] when including empirical changes in atmospheric transmission coefficient and Bowen ratio. Sensitivity runs suggest that even large changes in cloud cover and air humidity should not modify these trends. The results suggest that the Earth's glacial/interglacial boundary conditions play a significant role on climate of subequatorial southern Africa.

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.

Information on the Late Quaternary palaeoclimate in the Zunggar desert (northern Xinjiang) is deduced from the study of three 5-m long cores taken from Lake Manas. Analyses, including the mineralogical composition of bulk sediments, stable isotope contents of carbonates, organic matter, pollen and diatoms, were performed to help reconstruct the environmental evolution of the area. The chronology is provided by 9 AMS 14C dates done on carbonates or organic matter. The record suggests a Late Pleistocene humid episode radiocarbon-dated at 37,000–32,000 yr B.P. This episode is followed by a period of extreme aridity, attributed to the Last Glacial Maximum. The Early-Mid Holocene record indicates conditions generally wetter than today, as previously established for western China. However, our results show that the Holocene period is rather complex. After a minor wet/warm pulse estimated to be around 12,000 yr B.P., the most important environmental change is recorded at ≈ 10,000 yr B.P. It led to the establishment of a steppic vegetation cover in the catchment, and of a permanent lake which lasted ≈ 4000 years. Fluvial sedimentation took place between about 6000 and 4500 yr B.P., and it coincided with a decrease in moisture availability for vegetation growth. A second lacustrine episode is recorded from about 4500 to 2500 yr B.P., although the pollen record and a sudden influx in detrital organic matter suggest a short-term dry event around 3800-3500 yr B.P. The last 2500 years show climate unstability and the step-wise establishment of hypersaline conditions in Lake Manas. The major post glacial environmental changes appear to be roughly in phase with those observed in Tibet.

A new dataset of 282 modern diatom samples and associated environmental information has been created by merging existing regional datasets from North and East Africa and Niger. The relationships between diatom species distributions and hydrochemistry are examined using canonical correspondence analysis (CCA) and partial CCA. Variables reflecting water conductivity, pH, and cation and anion composition account for significant and independent components of the total variation in the diatom data. Predictive models (transfer functions) are developed using the method of weighted averaging for conductivity ( r 2 = 0.87), pH ( r 2 = 0.77), and ratios between alkali and alkaline earth metals ( r 2 = 0.81), and carbonate-bicarbonate and sulphate + chloride ions ( r 2 = 0.82). Prediction errors are estimated using the computer-intensive method of jackknifing. These transfer functions enlarge the potential domain for reconstruction of past hydrochemistry from fossil diatoms preserved in lake sediments.

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.

Postglacial expansion of deciduous oak woodlands of the Zagros—Anti-Taurus Mountains, a major biome of the Near East, was delayed until the middle Holocene at ~6300 cal. yr BP. The current hypotheses explain this delay as a consequence of a regional aridity during the early Holocene, slow migration rates of forest trees, and/or a long history of land use and agro-pastoralism in this region. In the present paper, support is given to a hypothesis that suggests different precipitation seasonalities during the early Holocene compared with the late Holocene. The oak species of the Zagros—Anti-Taurus Mts, particularly Quercus brantii Lindl., are strongly dependent on spring precipitation for regeneration and are sensitive to a long dry season. Detailed analysis of modern atmospheric circulation patterns in SW Asia during the late spring suggests that the Indian Summer Monsoon (ISM) intensification can modify the amount of late spring and/or early summer rainfall in western/northwestern Iran and eastern Anatolia, which could in turn have controlled the development of the Zagros—Anti-Taurus deciduous oak woodlands. During the early Holocene, the northwestward shift of the Inter-Tropical Convergence Zone (ITCZ) could have displaced the subtropical anticyclonic belt or associated high pressure ridges to the northwest. The latter could, in turn, have prevented the southeastward penetration of low pressure systems originating from the North Atlantic and Black Sea regions. Such atmospheric configuration could have reduced or eliminated the spring precipitation creating a typical Mediterranean continental climate characterized by winter-dominated precipitation. This scenario highlights the complexity of biome response to climate system interactions in transitional climatic and biogeographical regions.

Diatom study of a 12.6-m long sediment core (AB95II) from Lake Abiyata (Ethiopian Rift, ∼8°N) revealed large amplitude changes in the water level and chemistry over the past 13.4 cal kyr BP. Past water conductivity, pH and water chemical facies, reflected by ionic ratios, are quantitatively reconstructed, based on diatom transfer functions previously established. Conductivity has varied from 400 to 21 500 μS cm −1 , pH from 8 to 10.5; the anion ([HCO 3 − +CO 3 2− ]/[Cl − +SO 4 2− ]) and cation ([Na + +K + ]/[Ca 2+ +Mg 2+ ]) ratios ranged between ∼0.9–4, and 0.5–275, respectively. A high water salinity oscillation is recorded at the base of the sequence; the extrapolated age of this interval (12.7–12.0 cal kyr BP) corresponds to the Younger Dryas Chronozone. The Early–Middle Holocene registers a high-stand freshwater lake between 12 and 5.4 cal kyr BP. Changes in diatom assemblages suggest minor changes in water depth, lake mixing and/or nutrient content, during this interval. Post-5.4 cal kyr BP, shallow, saline conditions generally predominate. Short-term regressive events are recorded at 5.6 and 3.0 cal kyr BP. Our results are roughly consistent with previous works in the region. They refine the timing of the hydrochemical changes and quantify their amplitude, thus providing inputs for modelling approaches.

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.

a UMR CNRS 6538 ‘‘Domaines Oceaniques’’, Institut Universaire Europeen de la Mer, Universite de Bretagne Occidentale, place Nicolas Copernic, 29280 Plouzane, France b EP 1748 CNRS, Equipe Hydrologie, Paleohydrologie et Paleoenvironnement, Universite Paris-Sud, Bâtiment 504, 91405 Orsay Cedex, France c Laboratoire d’Hydrogeologie, Universite d’Avignon et des Pays du Vaucluse, 33 rue Louis Pasteur, 8400 Avignon Cedex, France d 3 Rue des Ajoncs, 64160 Morlaas, France e CEREGE, B.P. 80, 13545 Aix-en-Provence Cedex 04, France f Laboratoire ‘‘Environnements sedimentaires et Stratigraphie’’, Universite de Perpignan, 52 avenue de Villeneuve, 66860 Perpignan, France g Department of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia h Royal Belgian Institute of Natural Sciences, Freshwater Biology, Vautierstraat 29, B-1000 Brussels, Belgium i Department of Earth Sciences, Via Universita 4, 41100 Modena, Italy

Discussion sur l'article de Rognon P. paru en 1987 dans Palaeogeogr., Palaeoclimatol., Palaeoecol. vol 58, p 11-34

The Afar, in the horn of Africa, is at present a desert region. During the Holocene, it has undergone two major lacustrine phases (whose maxima are dated respectively 9.400-8.400 B.P. and 7.000-6.000 B.P.) and a smaller transgression around 2.500-1.000 B.P. The water level fluctuations of these lakes can be traced from strandline sequences, while the changing palaeoenvironments can be reconstructed from their sedimentary fades and diatom floras. The evolution of the Afar lakes is compared with other lakes in intertropical Africa., L'Afar, situé au Nord-Est de l'Afrique, est actuellement désertique. Il a subi, au cours de l'Holocène, deux phases lacustres de grande ampleur (dont les maximums sont datés respectivement de 9.400 à 8.400 ans B.P. et de 7.000 à 6.000 ans B.P.) et une transgression d'ampleur modérée (2.500-1 .000 ans B.P.). Les fluctuations de niveau des divers poléolacs sont retracées à partir de l'étude des anciens littoraux ; les paléoenvironnements successifs sont reconstruits à partir des faciès sédimentaires et des flores de Diatomées. L'évolution des lacs de l'Afar est comparée à celle de quelques autres lacs d'Afrique intertropicale., Gasse Françoise. Utilisation des milieux lacustres en milieu désertique pour la reconstitution des oscillations climatiques holocènes. In: Bulletin de l'Association de géographes français, N°433-434, 53e année, Mars-avril 1976. pp. 69-76.

Diatom assemblages from modern West African rivers and from lacustrine sediments subjected to deflation represent the present-day sources of continental diatoms to the sea. Diatom productivity in large rivers is high, especially for the genus Melosira. Windblown diatoms derive mainly from the central and northern Sahara (summer dust plume) where saline chloride-water assemblages are widespread, or from the southern edge of the Sahara (winter dust plume) where Melosira-rich assemblages of dilute water predominate. Freshwater diatom peaks in Atlantic cores may reflect (1) phases of increased river influx, correlated with humid episodes on the continent or (2) phases of enhanced deflation and wind transport during arid episodes (the single hypothesis for fine sediments from mid-ocean sites). Genus Melosira dominates the freshwater assemblages of many modern and fossil marine samples, whatever the transport agent is. Therefore, it is not an accurate paleoclimate indicator by itself, but associated taxa may provide information on the environmental and geographical origin of the displaced diatoms. This tentative approach shows that freshwater diatoms in Atlantic cores may be a good tool for reconstructing paleoclimates and for establishing continent-ocean correlations if species analyses are made and if the continental distribution of the taxa encountered is considered.

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.