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Regardless of the timescale, a distinctive characteristic of the tropics is a high variability in the water cycle, in particular in the P-E water balance (precipitation minus [evaporation + evapotranspiration]). This variability has more impact on life in the tropics than the variability in temperature which only has a moderate influence compared to the high and middle latitudes.

The Levant, a transition zone between the temperate Mediterranean domain and subtropical deserts, is a key area to study the latitudinal migrations of zonal climatic belts in response to glacial-interglacial conditions. The region underwent large climatic shifts during the Late Quaternary, with dramatic impacts on water resources and human populations. This paper presents the unique long-term hydro-climatic record (ca. 400 ka) from Northern Levant, derived from a sediment core taken from a small tectonic basin of northern Lebanon, the currently wettest area of the Levant. Combined biotic (pollen, biologic aquatic remains) and abiotic proxies (sediment properties, carbonate oxygen isotopes) reveal relatively high water availability during interglacials and generally drier glacials periods during the past four climate cycles. These general trends are in line with pollen records from southeastern and near east Mediterranean areas, but differ from some paleohydrological records from the southern Levant. Southward migrations of the rain-bearing mid-latitude westerlies during glacial periods and regional land topography are likely to explain the observed regional climate signatures. In addition, distinctive climate signatures of the successive wetter/drier intervals reflect interactions between global forcing (insolation, atmospheric greenhouse gas concentration), eastern Mediterranean Sea surface conditions intimately linked to North Atlantic climate and to ice-sheet and sea-ice extent at high northern latitudes, land topography, and local hydrogeological processes. Our record brings new benchmarks for understanding the spatial heterogeneity of eastern Mediterranean responses to global climate changes.

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.

Late Quaternary paleoclimatic changes in the Levant are difficult to extract from carbonate oxygen isotope records partly because the factors controlling the terrestrial δ18O signals are not fully understood in this region characterized by sharp climatic gradients. Here, we address this purpose by presenting the first 14C-dated isotope record from the northern Levant. The record is based on oxygen isotopes from ostracod shells from lacustrine-palustrine deposits accumulated in a small karstic, hydrologically open basin (Yammouneh), located on the eastern flank of Mount Lebanon. We have first constructed a composite isotopic record obtained from three different ostracod taxa. This yields an oxygen isotope record of ostracods (δost) related to the most widespread species (Ilyocypris inermis), and converted to δ18O values coeval with calcite precipitated in equilibrium (δC) with the lake water (δL). As with other records from the Mediterranean region, the Yammouneh profile shows maximum and minimum δC values during the LGM and the early Holocene, respectively, and a slight late Holocene increase. In order to discuss the potential causes of the observed δ18O values fluctuations (e.g., changes in the isotopic composition of the moisture source, temperature, precipitation minus evaporation (P-E) balance, or atmospheric circulation), we tentatively reconstruct the lake water isotope composition by correcting the δC values for lake water temperature using regional paleotemperature estimates. Inferred δL values were then corrected for the isotopic composition of the Eastern Mediterranean sea surface water (the moisture source) derived from planktonic foraminifera δ18O values corrected for alkenone-based sea surface temperature. Our study suggests that Holocene δL fluctuations are primarily linked to changes in the sea surface water composition (source effect) amplified by enhanced inland rainfall during the early Holocene. During the LGM, low δL values at Yammouneh are likely due to the ground-temperature effect on the rainfall isotope composition, possibly associated with a steeper altitudinal thermal gradient inland, and with changes in air mass trajectories over the Mediterranean Sea.

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.

We present the first synchronously coupled transient simulation of the evolution of the northern Africa climate-ecosystem for the last 6500 years in a global general circulation ocean–atmosphere–terrestrial ecosystem model. The model simulated the major abrupt vegetation collapse in the southern Sahara at about 5 ka, consistent with the proxy records. Local precipitation, however, shows a much more gradual decline with time, implying a lack of strong positive vegetation feedback on annual rainfall during the collapse. The vegetation change in northern Africa is driven by local precipitation decline and strong precipitation variability. In contrast, the change of precipitation is dominated by internal climate variability and a gradual monsoonal climate response to orbital forcing. In addition, some minor vegetation changes are also simulated in different regions across northern Africa The model also simulated a gradual annual mean surface cooling in the subtropical North Atlantic towards the latest Holocene, as well as a reduced seasonal cycle of SST. The SST response is caused largely by the insolation forcing, while the annual mean cooling is also reinforced by the increased coastal upwelling near the east boundary. The increased upwelling results from a southward retreat of the North Africa monsoon system, and, in turn, an increased northeasterly trade wind. The simulated changes of SST and upwelling are also largely consistent with marine proxy records, albeit with a weaker magnitude in the model. The mismatch between the collapse of vegetation and gradual transition of rainfall suggests that the vegetation collapse is not caused by a strong positive vegetation feedback. Instead, it is suggested that the Mid-Holocene collapse of North African vegetation is caused mainly by a nonlinear response of the vegetation to a precipitation threshold in the presence of strong climate variability. The implication to the modeling and observations is also discussed.

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, ...

Tropical African climate has oscillated between markedly wetter and drier conditions on all timescales in response to global climate disturbances. A step-like increase in aridity over the past 3 Ma has been primarily paced by orbital cycles coupled with the onset and amplification of high-latitude glacial cycles. On the 10 4 –10 3 -ka timescales, observed changes imply interactions between insolation, sea-surface conditions and vegetation. High-frequency variations could be linked to oscillations in major atmospheric circulation modes, in solar output, or to major volcanic events. To cite this article: F. Gasse, C. R. Palevol 5 (2006) .

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).

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.

The KaraBogazGol (KBG) water level oscillations were reconstructed in the last 200 years using the geochemical evolution of the uppermost meter of its sedimentary infill. High-resolution studies of the mineralogical composition of the KBG sediments show alternating periods of high concentration brines followed by periods of more dilute waters. The relative water level reconstruction was based on statistical models (factor analysis (FA), correspondence analyses (CA) and detrended correspondence analyses (DCA)) whilst the chronological framework was established using the 210 Pb technique. This reconstruction was compared with the instrumental water level record of the Caspian Sea, showing a high degree of correlation ( r 2 =0.83). The agreement between the reconstructed and the measured water level oscillations of the Caspian Sea indicates that environmental changes can be reconstructed from closed lakes provided that an accurate chronological control of sedimentary processes is available. The water level reconstruction shows that the Caspian Sea water level fluctuations follow a cyclical pattern rather than an ‘erratic’ one, as has been suggested in the literature. Two main periodicities of 62.5 and 38.46 years have been found.

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.

Thirteen Holocene palaeolakes in the western Sahara and Sahel have provided diatom records, with carbonate oxygen isotope profiles available from eight of them. Most of these palaeolakes were groundwater-fed. Lake water chemistry is reconstructed using diatom transfer functions. Lake water salinity and 18 O records are assembled with some isotopic and chemical groundwater data to better understand the response of the hydrological systems to climate changes over the past 15,000 yr. Data are in general agreement with climate simulations using coupled atmosphere–ocean–vegetation models which show a mid-Holocene wetting over the whole of northwest Africa, and a rapid drying by 6–4 ka. The lake record also shows that at many sites the major lake infilling lags the end of the Younger Dryas by 1–2 ka. Regional differences also appear in the timing of the lake hydrological optimum: ca. 10.5–8.5 and 7.5–4.5 ka in the northern Sahara, 10–8.5 ka in the Air–Tenere, 10–5.7 or 4.5 in the Sahel, and 7.5 ka in Lake Chad. The whole of the Holocene is punctuated by short-term drying events. Changes in water isotopic composition through time are partly explained by changes in rainfall amount and air humidity. During the wet Holocene period however, the very low δ values in the southern Sahara also imply changes in the moisture transport pattern or rainfall mechanisms. Data suggest an apparent decrease in 18 O content of precipitation along the monsoon flow, in contrast with modern patterns. Changes in water availability and quality have driven population migrations in and out of the Sahara-Sahel, but relationships between climate and cultures are complex. Short-term dry events might have driven inventive adaptations. In the Sahara, drying at 5–4.5 ka coincides with both the collapse of the classical Neolithic civilization and the settlement of new cultures.

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.

Paleohydrological data from the African tropics and subtropics, including lake, groundwater and speleothem records, are reviewed to show how environments and climates from both hemispheres are inter-related. Although orbitally induced changes in the monsoon strength account for a large part of long-term climatic changes in tropical Africa, the Late Pleistocene–Holocene hydrological fluctuations rather appear to have been a series of abrupt events that reflect complex interactions between orbital forcing, atmosphere, ocean and land surface conditions. During the Last Glacial Maximum (23–18 ka BP), most records indicate that generally dry conditions have prevailed in both hemispheres, associated with lower tropical land- and sea-surface temperatures. This agrees with simulations using coupled ocean–atmosphere models, which predict cooling and reduced summer precipitation in tropical Africa; the global hydrological cycle was weaker than today when the extent of large polar ice-sheets and sea-ice was a prominent forcing factor of the Earth's climate. Glacial-interglacial climatic changes started early: a first wetting/warming phase at ca. 17–16 ka BP took place during a period of rapid temperature increase in Antarctica. Next, two drastic arid-humid transitions in equatorial and northern Africa occurred around 15–14.5 ka BP and 11.5–11 ka BP. Both are thought to match the major Greenland warming events, in concert with the switching of the oceanic thermohaline circulation to modern mode. However, part of the climatic signal after 15 ka BP also seems related to the Antarctica climate. During the Holocene, Africa has also experienced rapid hydrological fluctuations of dramatic magnitude compared to the climatic changes at high latitudes. In particular, major dry spells occurred around 8.4–8 ka and 4.2–4 ka BP in the northern monsoon domain. Comparison with other parts of the world indicates that these events have a worldwide distribution but different regional expressions. In the absence of large polar ice sheets, changes in the continental hydrological cycles in the tropics may have a significant impact on the global climate system. Climate information gathered here allows to identify geographical and methodological gaps, and raise some scientific questions that remain to be solved to better understand how the tropics respond to changes in major climate-forcing factors, and how they influence climate globally.

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.

The δ18O and 87Sr/86Sr values of thermal waters from the western coast of the Caspian sea, which contribute to its total budget, result from interactions with host rocks; they even suggest a supply by ‘deep’ waters. In fact, the Sr contents and the 87Sr/86Sr values of the river and thermal waters are not sufficient for the calculation of an equilibrated budget, implying that other sources contribute to the budget of the Caspian waters. The 206Pb/207Pb ratio of the waters from the Sulak river also suggests local anthropogenic pollution.

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.

A detailed and well-dated proxy record of summer rainfall variation in arid Central Asia is lacking. Here, we report a long-term, high resolution record of summer rainfall extracted from a peat bog in arid eastern-Central Asia (AECA). The record indicates a slowly but steadily increasing trend of summer rainfall in the AECA over the past 8500 years. On this long-term trend are superimposed several abrupt increases in rainfall on millennial timescales that correspond to rapid cooling events in the North Atlantic. During the last millennium, the hydrological climate pattern of the AECA underwent a major change. The rainfall in the past century has reached its highest level over the 8500-year history, highlighting the significant impact of the human-induced greenhouse effect on the hydrological climate in the AECA. Our results demonstrate that even in very dry eastern-Central Asia, the climate can become wetter under global warming.

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.

This article has been made available through Open Access by the Brunel Open Access Publishing Fund. Late Pleistocene and/or Holocene high-resolution palynological studies are available for the south basin of the Caspian Sea (CS), the world's largest lake. However, the north and middle basins have not been the object of high-resolution palynological reconstructions. This new study presents the pollen, spores and dinoflagellate cysts records obtained from a 10m-long sediment core recovered in the middle basin, which currently has brackish waters and is surrounded by arid and semi-arid vegetation.An age-depth model built based on six radiocarbon dates on ostracod shells indicates that the sequence spans the period from 14.47 to 2.43cal.kaBP. The present palaeoenvironmental study focuses on the top 666cm, or from 12.44 to 2.43cal.kaBP.At the vegetation level, the Younger Dryas is characterised by an open landscape dominated by desert vegetation composed by Amaranthaceae with shrubs and salt-tolerant plants. However, although the Early Holocene is also characterised by desert vegetation, it is enriched in various shrubs such as Ephedra and Calligonum, but tree expansion is not important at the Holocene onset. After a major shift at 8.19cal.kaBP, the Middle Holocene displays now both the character of desert and of steppe, although some trees such as Quercus and Corylus slightly spread. The Late Holocene records steppe vegetation as dominant, with more tree diversity.Regarding the lacustrine signal, the dinocyst assemblage record fluctuates between slightly brackish conditions highlighted by Pyxidinopsis psilata and Spiniferites cruciformis, and more brackish ones - similar to the present day - with the dominance of Impagidinium caspienense. The Late Pleistocene is characterised by low salinities, related to the Khvalynian highstand. From 11.56cal.kaBP, slightly more saline waters are reconstructed with an increase of I.caspienense for a period of 1000 years, which could be attributed to the Mangyshlak lowstand. From 10.55cal.kaBP, low salinity conditions return with remains such as Anabaena and Botryococcus abundant until 8.83cal.kaBP, followed by a slow, progressive decrease of P. psilata and S. cruciformis until 4.11cal.kaBP, which is the main assemblage change at lacustrine scale. Since then, higher salinities, similar to the present one, are reconstructed. Finally, Lingulodinium machaerophorum starts its development only at 2.75cal.kaBP, in the Late Holocene.The present research revealed fundamental differences from previously published sea-level curves, in that a 6000yr-long highstand suggested by low salinities is shown between 10.55 and 4.11cal.kaBP. Amongst other arguments, using a comparison to a similar palynological regard but in the south basin, a N-S salinity gradient that is the reverse of the present one across the CS, suggests that the Amu Darya was flowing in the CS. Hence the CS levels during the Late Pleistocene and Holocene were influenced by a combination of precipitation over the high European latitudes and the indirect influence of the Indian summer monsoon over the Pamirs. © 2014. This study has been conducted within the European Contract INCO-Copernicus “Understanding the Caspian Sea erratic fluctuations” n IC15-CT96-0112. This was funded by the Centre National de la Recherche Scientifique within the framework of the INSUDYTEC (DYnamique de la Terre et du Climat) Program (France).

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.

Resume Le lac Tele, vaste lentille d'eau ellipsoidale situee au cœur de la foret equatoriale du Nord-Congo, presente des echanges hydriques essentiellement verticaux qui conferent une tres faible mineralisation a ses eaux, par ailleurs tres riches en carbone organique et tres acides (pH

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.

Lakes in arid and semi-arid regions respond to climatic change through shifts in lake water volume and ionic concentration. Because diatom distribution is highly correlated with lake hydrochemistry, diatoms can be used to infer changes in salinity and brine composition and thus to infer past climates. Here we critically examine the use of diatom-inferred salinity as a climate proxy, with examples taken from both modern waterbodies and sedimentary profiles. Sediment records may contain assemblages mixed from periods or sites of differing hydrochemistries because of the high degree of spatial and temporal heterogeneity in saline systems. Dissolution and diagenesis in saline brines may further complicate interpretation of sedimentary assemblages. Furthermore threshold effects, salinity regulation via groundwater seepage, antecedent conditions, and other aspects of local hydrology may modify the relationship between salinity change and climatic forcing. These complexities necessitate critical examination of the ecology and taphonomy of sedimentary assemblages, as well as evaluation of potential non-linearities in the salinity/climate relationship through comparison of diatom-inferred salinity with other proxy records of hydrochemical change, historical documents where they exist. and with other regional lake systems.

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.

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This article has been made available through the Brunel Open Access Publishing Fund. Copyright @ The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. Pollen and dinoflagellate cysts have been analysed in a core from the south basin of the Caspian Sea, providing a picture of respectively past vegetation and water salinity for the Late Pleistocene to middle Holocene. A relatively sharp lithological change at 0.86 m depth reflects a shift from detrital silts to carbonates-rich fine silts. From this depth upwards, a Holocene chronology is built based on ten radiocarbon dates on ostracod shells and bulk carbonates. From the vegetation point of view, the Late Pleistocene deserts and steppes were partially replaced in the most sheltered areas by an open woodland with Pinus, Juniperus-Hippophae-Elaeagnus and even Alnus-Quercus-Pterocarya and Fraxinus, related to the Allerød palynozone. This was interrupted by the Younger Dryas palynozone when Artemisia reaches a maximum in a first instance followed by a very dry phase with only a slight return of Pinus and Quercus and the rare presence of Ulmus-Zelkova. From 11.5 to 8.4 cal. ka BP, an open landscape dominated by shrubs such as Ephedra and progressively increasing Quercus appeared. The final spread of diverse evergreen and deciduous trees is delayed and occurs after 8.4 cal. ka BP. It is suggested that this delay is caused by an arid climate in the Early Holocene linked to high insolation and perhaps to a lake effect. The dinocyst assemblages fluctuate between slightly brackish (Pyxidinopsis psilata and Spiniferites cruciformis, 7 psu and lower) and more brackish (Impagidinium caspienense, ∼13 psu). In the Lateglacial (Khvalynian highstand), the assemblages remained dominated by relative low salinity taxa. A late and brief increase of salinity occurred prior to 11.2 cal. ka BP associated with the Mangyshlak lowstand. It is suggested that it was caused by a brief drop in meltwater flow from both the north and the southeast (Uzboy) and a likely evaporation increase. This lowstand occurs quasi at the same time as the end of a longer lowstand in the Black Sea. The freshest waters are then inferred as having occurred between 8.4 and ≤4.4 cal. ka BP, linked to a connection with the Amu Darya and the melting glaciers on the Pamir Mountains. The Caspian Sea is a sensitive environment, easily perturbed by global climatic changes, such as the Allerød and Holocene warming, and the Lateglacial and Younger Dryas cooling, as well as by regional changes in its hydrography, such as shifts in the Eurasian meltwater and the Volga and Amu Darya inflows. Centre National de la Recherche Scientifique, France

To obtain more accurate reconstructions of past salinities and climate it is necessary to merge regional datasets of saline lake diatoms and water chemistry; a prerequisite of which is to standardize taxonomic practice. To illustrate this process, an investigation of the identity of a small Cyclotella species, frequently observed in the diatom community of saline lakes, is described. This taxon, which has been referred to in earlier literature as Cyclotella caspia, is of interest because it is present in fossil African assemblages, yet is absent from the modem flora of this region. Uncertainties over its correct identification prevented material in a North American dataset from being used as an analogue in salinity reconstructions. LM and SEM investigations reveal that there are no significant differences between Recent North American and Holocene sub-fossil North American and African material which suggests that the salinity optimum calculated from the North American water chemistry data can be used for ...

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.

A study of magnetic parameters was carried out on a Late Pleistocene and Holocene sedimentary sequence (5 m) from Lake Manas (northern Xinjiang, China), that has already been ana- lysed for bulk sediment mineralogy, stables isotopes from carbon- ates and biogenic material. Magnetic minerals confirm major changes in palaeoenvironmental and palaeohydrological conditions that were deduced from other methods. They also provide complementary information on changes in the lake system during the early and middle Holocene. The most important mineral mag- netic change results from the authigenic formation of ferrimagnetic greigite, Fe3S 4 during the lacustrine episodes dated from-8.7 kyr B.P. to-5.5 kyr B.P. (380-335 cm) and from -4.5 kyr B.P. to -3.5 kyr B.P. (320-285 cm) respectively. Our data also indicate an abrupt break in sedimentary conditions which coincides with a short-term retmxtowards dry climatic conditions around 3.5 kyr B.P., which was only suggested from other proxy data.

A molecular phylogeny of the diatoms has been constructed from the nucleotide sequence of two domains of the nuclear-encoded large subunit ribosomal RNA in four centric and four pennate species. Distance and parsimony analyses support the monophyly of the diatoms and their closeness to the chrysophytes. The Centrales (radially symmetrical) appear to be paraphyletic: the centric Lithodesmium undulatum Ehrenberg (Lithodesmiales, Lithodesmiaceae) is associated with the pen-nate lineage. In the Pennales (bilaterally symmetrical), the araphids also appear to be paraphyletic. These phylogenetic inferences are in close congruence with those previously obtained from 18S rRNA data (Bhattacharya et al. 1992; Medlin et al. 1993). The evolutionary relationships of diatom families inferred from the ribosomal RNA data are discussed in relation to the fossil record.

The Nile valley accommodates the world's longest river and shaped the development of numerous complex societies, providing a reliable source of water for farming and linking populations to sub-Saharan Africa and the Mediterranean Sea. Its fertile delta lay at the heart of ancient Egyptian civilization, however little is known of its morpho-sedimentary response to basin-wide changes in Holocene hydrology. Here, we present two well-resolved records from the Nile delta (based on ∼320 radiocarbon dates) to reconstruct the timing and rhythm of catchment-scale modifications during the past 8000 years. On the orbital timescale, we demonstrate that Nilotic hydrology and sedimentation have responded to low-latitude insolation forcing while, on sub-millennial timescales, many of the major phases of deltaic modification were mediated by climate events linked to El Nino Southern Oscillation-type (ENSO) variability.

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.

Diatoms, unicellular eukaryotic algae with a siliceous skeleton, offer the rare advantage of displaying both an extensive fossil record and numerous extant species, thus providing the opportunity of confronting molecular and paleontological data in a protist group. A portion of the 28S ribosomal RNA was sequenced from 5 diatoms, the divergence times of which are well known. The nucleotide substitution rate was estimated in these unicellular eukaryotes and compared with the rate of multicellular eukaryotes, using a broad data base comprising metazoans and metaphytes. When using fossil record derived divergence times, our results show that the nucleotide substitution rate is about 5 times faster in diatoms than in chordates. But, when using the relative rate test, it is observed that, over a long time period, the nucleotide substitution rate may in fact have been slightly slower in diatoms than in chordates. For this contradiction, two possible explanations are proposed: (i) a failure of the relative rate test, (ii) a gap in the pre-Jurassic diatom fossil record. We have checked that our results concerning the relative rate test were valid. Thus, the second hypothesis, which implies pre-Jurassic diatom evolution, in fact already suggested by some non-molecular evidences, is favoured. Decoupling of morphological differentiation from genetic speciation also appears to have occurred and may account in part for the underestimation of the dates of recent cladogenesis events.

Two diatom frustule dissolution experiments (DE1 and DE2) were performed on material from Lakes Geneva and Nantua, SE France. In DEl, the dissolution of diatom silica from sediments was measured over a 92-d period at room temperature in six 3 M salt solutions (NaCl, Na,CO,, KN09, CaCl,, LiN03, MgCl,), in 0.6 M NaCl, and in distilled water. As expected, dissolution was greatest in Na,CO, where high pH leads to ionic dissociation. A similar process was also apparent in solutions of KNO, and LiNO, which were only slightly alkaline. In contrast, weakly acid solutions of MgCl, and CaCl, produced less dissolution than distilled water. Between 0.6 and 3 M NaCI, silica solubility appeared to reach an undetermined optimum which could correspond to minimum values of ion activity coefficients. In a second experiment (DE2), the dissolution of living diatoms from Lake Geneva was monitored over 46 d in 3 M solutions of Na,CO, and NaCl and in distilled water. The results were not in accordance with the expected theoretical levels predicted by Pitzer’s equations when applied to silica solubility in conccntrated solutions. Clear interspecific differences in diatom dissolution rates occurred in both experiments. The gross valve surface area : volume ratio was found to bc a useful first-order approximation of a particular diatom species’ propensity to dissolve. Only minor differences in diatom assemblage composition were observed in NaCl solutions over the experimental period whereas major changes were caused by Na,CO, solutions.

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.

Hydrologic and climatic changes in western Tibet since 13,000 BP are inferred from mineralogical and isotopic analyses of lacustrine sediments from the Sumxi-Longmu Co basin. The interpretation is based on isotope data from the modern system. Carbon isotope chemistry is complex because of the presence of old limestones on the watershed. Sediments contain detrital and authigenic calcites, the proportions of which are quantitatively estimated by X-ray diffractometry. Stable isotope contents of the authigenic calcite are reconstructed from mass-balance equations. The high 13C contents (up to +6‰) are attributed to an enhancement of calcite precipitation from water surface during freezing periods. The 18O content is related to residence time and evaporation. The validity of 14C dates obtained from different types of material is discussed. Our record, in good agreement with paleobiological data from the same material, shows the sudden establishment of wet/warm conditions at ≈10,000 BP which culminated at ≈7500-6000 BP after a brief reversal event at ≈7900 BP. Maximum aridity for the whole Holocene is recorded at ≈4300 BP.

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.