Françoise Gasse

Past climate variability through Europe and Africa

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

Diatom-based transfer functions for inferring past hydrochemical characteristics of African lakes

Abstract 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 Late Pleistocene-Holocene lacustrine record from Lake Manas, Zunggar (northern Xinjiang, western China)

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.

Palaeoenvironmental changes in the arid and sub arid belt (Sahara-Sahel-Arabian Peninsula) from 150 kyr to present

The PEP III Arid to Subarid Belt includes the largest hot desert in the world, the Sahara- Arabian desert and the Sahel zone. The region of interest extends south of the Atlas Mountains and south and east of the Mediterranean Sea to approximately 10 °N and shows a broadly zonal pattern with a varying seasonal distribution of precipitation. In the north (ca. 20–23 °N), rainfall results from the southward displacement of the midlatitude westerlies during winter whereas the south is governed by seasonal northward migration of the Intertropical Convergence Zone (ITCZ). Contraction and expansion phases of these presently semi-arid to hyper-arid desert areas result from significant changes in local precipitation. Palaeoenvironmental records from Northern Africa (north of 10 °N) and the surrounding seas document long-term changes in the magnitude and extent of the African monsoon in response to orbitally-forced changes in insolation. However, marine records as well as terrestrial palaeohydrological indicators (e.g., lakes, speleothems, rivers, pollen and charcoal) show that there have been changes in the hydrological cycle superimposed on the long-term waxing and waning of the monsoon which cannot be explained exclusively by changes in insolation. These fluctuations in space, time and magnitude were on a regional to continental scale. Here, we review available data on near-surface palaeohydrological indicators and vegetational changes in arid North Africa and the Arabian Peninsula as well as changes in the intensity of the South Asian Monsoon identified from marine sediments of the Arabian Sea. A comparison of regional environmental changes can clarify relations between the environment and changes in the Earth’s climate system. Each data-set is initially presented independently because they represent heteregeneous records from different regions and time periods and thereby emphasise their potential to provide evidence of continental chronostratigraphic palaeoenvironmental changes. Data-sets of lake status and vegetational change are complementary as they strongly reflect hydrological variation. Deep-sea sediments from the Arabian Sea were used to generate continuous records of oceanic upwelling, continental humidity, and dust and river discharge, that are closely related to palaeoenvironmental changes on the surrounding continents.After presenting the individual data-sets we compare the palaeoclimatic reconstructions derived from the different types of evidence.

Indian summer monsoon variations could have affected the early Holocene woodland expansion in the Near East

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.

Variations hydrologiques et extension des lacs holocenes du Desert Danakil

Abstract 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.

Taphonomy and diagenesis in diatom assemblages; a Late Pleistocene palaeoecological study from Lake Magadi, Kenya

Many fossil diatom assemblages do not possess a direct modern analogue as a result of taphonomic processes and diagenesis within the assemblage. Some of these problems are illustrated with reference to core material collected from hypersaline Lake Magadi, Kenya, which during the Late Pleistocene experienced major fluctuations in water chemistry and depth. Competing multiple hypotheses are proposed for no analogue assemblages, with selection between these hypotheses being based on the results of interdisciplinary research.

The CASPIA project: diatoms, salt lakes, and environmental change

The Climate and Salinity (CASPIA) Project is concerned with diatoms as environmental indicators in inland waters and their use in reconstructing salinity and major ion composition from fossil diatom assemblages in lake sediments. By comparing saline lake diatom floras from around the world the project aims to establish a common, harmonised approach to sample collection, diatom identification and nomenclature, and to develop techniques for numerical analysis and data storage.

STANDARDIZING THE TAXONOMY OF SALINE LAKE CYCLOTELLA SPP.

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 ...

Miocene non-marine diatoms from the western Cordillera basins of northern Peru

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...