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Fezzan Project - Palaeoclimate and environment

One of the aims of the Fezzan project is to determine how the physical environment and human populations have responded to dramatic changes in climate. Many such changes have taken place in the Sahara over the late Quaternary, generally consisting of oscillations between humid and arid phases (follow the links below or scroll down for a summary of Late Quaternary climatic change). The Fezzan is poorly represented in the Saharan palaeoenvironmental record, despite the rich variety and high density of palaeoenvironmental indicators in the region. Such indicators can also tell us much about the latest phase of Saharan desiccation, which commenced some 5000 years ago. This period is particularly poorly understood, due to a paucity of data throughout the Sahara, and also to the complexity of the environmental response to changes in rainfall, which is determined principally by the dynamics of groundwater. The continued human occupation of the Fezzan, and the infered relatively high population density of the Wadi al-Ajal until at least around 500 AD (see archaeology), suggests a lagged (on millennial timescales) environmental response to climate change. See also Geomorphology.

  • Late Pleistocene climate change
  • First Holocene wet phase
  • Early Holocene arid interval
  • Second Holocene wet phase
  • Late Holocence Saharan desiccation
  • References
  • Palaeolake deposits in the Ubari Sand Sea

    Late Pleistocene climate change

    Over the past few hundred thousand years, the Fezzan has been subject to the dramatic changes in climate that have affected the Sahara as a whole (Petit-Maire et al., 1978; Graven et al., 1981; Szabo et al., 1995). These have generally taken the form of hyper-arid periods associated with large-scale ice cover in the northern hemisphere during glacial episodes and wet episodes associated interglacial conditions (Nicholson and Flohn, 1980). Some 135 thousand years ago (ka), large lakes covered much of the Sahara as a result of massively increased rainfall with respect to conditions today (Causse et al., 1988). Dating of gastropod shells and geomorphological and topographic studies undertaken as part of the Fezzan Project indicate the presence of extensive freshwater lakes in the Fezzan at around 85 ka (White et al., 2000 - see also Geomorphology), which are likely to have been contemporaneous with lakes dated to around 90 ka in southern Tunisia (Causse et al., 1988, 1989). After the last glacial maximum at 21 ka, climatic conditions in the Sahara fluctuated in response to the episodic collapse of the northern hemisphere ice sheets and the resulting impact on northern hemisphere atmospheric and oceanic circulation (Maley, 1977; Nicholson and Flohn, 1980; Alley, 1997, Barber, 1999). Top

    First Holocence wet phase

    The most important wet periods in terms of archaeology and the development of human society occured during the Holocene (the last 10,000 years). By about 10 ka, rainfall was plentiful and most of the Sahara was vegetated; in the south, vegetation zones were displaced some 400 km north of their present-day positions, and fauna from the equatorial regions had migrated north into the Sahara (Lezine, 1989; Lioubimsteva, 1995; Ritchie and Haynes, 1995). Between about 10 ka and 8 ka, it is believed that rainfall in the Sahara was generated by the interaction between mid-latitude weather systems and the inter-tropical convergence zone, where warm dry Saharan air meets cooler moist air originating over the eastern tropical Atlantic (Maley, 1977; Nicholson and Flohn, 1980). It is the northwards migration of this moist oceanic air in the form of the West African Monsoon that today brings rainfall to the Sahel (the semi-arid transition zone between the hyper-arid Sahara and the humid equatorial regions) in summer. Significantly, it is not thought that the West African Monsoon penetrated any further north than today between 10 ka and 8 ka . The rainfall-generating systems during this period were the result of semi-permanent low pressure regions sustained by remnant ice-sheets over North America and northern Europe, and resulted in precipitation throughout the Sahara from south to north (Nicholson and Flohn, 1980). The monsoon would have remained active over the Sahel. Top

    Early Holocence arid interval

    The wet episode described above was interrupted by century-scale arid episode sometime around 8 ka, which was most probably due to the collapse of the remnants of the Laurentide Ice Sheet in North America (Nicholson and Flohn, 1980; Alley et al., 1997; Barber et al., 1999; deMenocal et al., 2000). This would have caused a massive injection of cold fresh water into the North Atlantic, altering oceanic and atmospheric circulation and lowering sea surface temperatures, which would have reduced the moisture content of the atmosphere by suppressing evaporation from the ocean surface. Reduced surface temperatures would have reduced the intensity of atmospheric convection and hence its capacity to sustain rainfall-generating weather systems. These changes represented the transition to full interglacial conditions in the northern hemisphere. Top

    Second Holocene wet phase

    Sometime after 8 ka, wetter conditions returned to much of the Sahara as the northern hemisphere warmed, and were certainly fully established by 6.5 ka. However, the northernmost parts of the Sahara remained dry. The most likely explanation for this situation is that the transition to full interglacial conditions was associated with increased solar heating of the northern hemisphere due to changes in the tilt of the Earth (Beck, 1998). This would have intensified the West African Monsoon, which may have penetrated to some 30 degrees north, some 10 degrees further north than at present (Maley, 1977; Nicholson and Flohn, 1980). However, the disappearance of the ice sheets vastly reduced the interaction between mid-latitude and tropical weather systems that previously had generated rainfall in the northern Sahara (as well as much of the central and southern Sahara). The two Holocene wet phases thus represented very different climatic regimes; it is probably that the perennial vegetation of the first phase gave way to semi-arid seasonal savannah in the second, and that in the latter the survival of human populations required greater ingenuity. Top

    Late Holocene Saharan Desiccation

    There is widespread evidence that the onset of the hyper-arid conditions that characterise the Sahara today occurred at around 5 ka (Nicholson and Flohn, 1980; Claussen et al, 1999; Cremaschi et al., 1999). It is believed that the desiccation occurred in two phases, and was the result of changes in the Earth's orbital parameters which resulted in reduced solar heating of the northern African landmass that caused a weakening of the West African Monsoon.Groundwater levels remained high in some areas after the onset of hyper-aridity, and lakes are likely to have persisted in some regions. Open water bodies and near-surface groundwater would have sustained a reduced human population in many regions; Cremaschi (1999) has found evidence of human activity and open water bodies in the Fezzan outside of the Wadi al-Ajal as late as about 3 ka. The Garamantian civilisation in the Wadi el-Agial appears to have developed soon after this time (Mattingly et al., 1998, 2000), and it is likely that people settled in the wadi in increasing numbers as access to water became more and more difficult in other parts of the Fezzan. Where such oasis refuges did not exist, people would have migrated to the the Saharan margins and the Nile Valley; it is plausible that a large influx of Saharan refugees was one of the factors that led to the development of Egyptian Dynastic civilisation; certainly a knowledge of astronomy and various religious themes appear to have been common to Pharaonic Egypt and pre-Dynastic Saharan cultures (Malvill et al., 1998). Top

    References

  • Alley, R. B., Mayewski, P. A., Sowers, T., Stuiver, M., Taylor, K. C. and Clark, P. U. (1997) Holocene climatic instability: A prominent, widespread event 8200 years ago, Geology 25, 483-486
  • Barber, D. C., Dyke, A., Hillaire-Marcel, C., Jennings, A. E., Andrews, J. T., Kerwin, M. W. and Bilodeau, G., McNeely, R., Southon, J., Morehead, M. D. and Gagnon, J. M. (1999) Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes, Nature 400, 344-348.
  • Beck, W. (1998) Warmer and wetter 6000 years ago?, Science 279, 1003-1004.
  • Cause, C., Conrad, G., Fontes, J-C., Gasse, F., Gibert, E. and Kassir, A. (1988) Le dernier "Humide" Pléistocène du Sahara nord-occidental daterait de 80-100 000 ans, Geochimie et Géochronologie Isotopiques 306, 1459-1464.
  • Causse, C., Coque, C., Fontes, J. Ch., Gasse, F., Gibert, E., Ben Ouezdou, H., Zouari, K. (1989) Two high levels of continental waters in the southern Tunisian chotts at about 90 and 150 ka, Geology 17, 922-925.
  • Claussen, M., Kubatzki, C., Brovkin, V., Ganopolski, A., Hoelzmann, P. and Pachur, H. J. (1999) Simulation of an abrupt change in Saharan vegetation in the mid- Holocene, Geophysical Research Letters 26, 2037-2040
  • Cremashi, M. (1999) Late Quaternary geological evidence for environmental changes in south-western Fezzan (Libyan Sahara), in Cremashi, M. and Di Lernia, S. (eds.) Wadi Teshuinat: Palaeoenvironment and prehistory in south-western Fezzan (Libyan Sahara), Centro Interuniversitario di Ricerca per le Civiltà e l'Ambiente del Sahara Antico, 13-47.
  • Gasse, F., Tehet, R., Durand, A., Gibert, E. and Fontes, J. C. (1990) The arid-humid transition in the Sahara and the Sahel during the last deglaciation, Nature 346, 141-146.
  • Gasse, F. and van Campo, E. (1994) Abrupt postglacial climate events in West Asia and North Africa monsoon dynamics, Earth and Planetary Science Letters 126, 435-456.
  • Girod, A. (1999) Molluscs and palaeoenvironment of Holocene lacustrine deposits in the Erg Uan Kasa and in the Edeyen of Murquq (Libyan Sahara), in Cremashi, M. and Di Lernia, S. (eds.) Wadi Teshuinat: Palaeoenvironment and prehistory in south-western Fezzan (Libyan Sahara), Centro Interuniversitario di Ricerca per le Civiltà e l'Ambiente del Sahara Antico, 73-88.
  • von Grafenstein, U., Erlenkeuser, H., Muller, J., Jouzel, J. and Johnsen, S. (1998) The cold event 8200 years ago documented in oxygen isotope records of precipitation in Europe and Greenland, Climate Dynamics 14, 73-81.
  • Grandi, G. T., Lippi, M. M. and Mercuri, A. M. (1999) Pollen in dung layers from rockshelters and caves of Wadi Teshuinat (Libyan Sahara), in Cremashi, M. and Di Lernia, S. (eds.) Wadi Teshuinat: Palaeoenvironment and prehistory in south-western Fezzan (Libyan Sahara), Centro Interuniversitario di Ricerca per le Civiltà e l'Ambiente del Sahara Antico, 95-106.
  • Graven, C., Hillaire-Marcel, C. and Petit-Maire, N. (1981) A Pleistocence lacustrine episode in southeastern Libya, Nature 290, 131-133
  • Lezine, A-M. (1989) Late Quaternary vegetation and climate of the Sahel, Quaternary Research 32, 317-334.
  • Lioubimsteva, E. U. (1995) Landscape changes in the Saharo-Arabian area during the last glacial cycle, Journal of Arid Environments 30, 1-17.
  • Maley, J. (1977) Palaeoclimates of the Central Sahara during the early Holocene, Nature 269, 573-577.
  • Malvill, J. M., Wendorf, F., Mazar, A. A. and Schild, R. (1998) Megaliths and Neolithic astronomy in southern Egypt,Nature 392, 499-491.
  • Mattingly, D. J., al-Mashai, M., Aburgheba, H., Balcombe, P., Eastaugh, E., Gillings, M., Leone, A., McLaren, S., Owen, P., Pelling, R., Reynolds, T., Stirling, L., Thomas, D., Watson, D., Wilson, A. and White, K. (1998) The FezzanProject 2000: Preliminary report on the second season of work, Journal of Libyan Studies 29, 115-14.
  • Mattingly, D. J., al-Mashai, M., Balcombe, P., Barnett, T., Brooks, N., Cole, F., Dore, J., Drake, N., Edwards, D., Hawthorne, J., Helm, R., Leone, A., McLaren, S., Pelling, R., Preston, J., Reynolds, T., Townsend, A., Wilson, A. and White, K. (2000) The Fezzan Project 2000: Preliminary report on the fourth season of work, Journal of Libyan Studies 31,
  • deMenocal, P, Oritz, J., Guilderson, T. and Sarnthein, M. (2000) Coherent high- and low latitude climate variability during the Holocene warm period, Science 288, 2198-2202.
  • Nicholson, S. E. and Flohn, H. (1980) African environmental and climatic changes and the general atmospheric circulation in Late Pleistocene and Holocene, Climatic Change 2, 313-348.
  • Petit-Maire, N., Casta, L, Delibrias, G., Gaven, CH., Testud, A-M. (1978?) Preliminary data on Quaternary palaeolacustrine deposits in the Wadi ash Shati area, Libya, Proceedings of the 2nd Symposium on the Geology of Libya?, 797-807.
  • Ritchie, J. C. and Haynes, C. V. (1987) Holocene vegetation in the eastern Sahara, Nature 330, 645-647.
  • Szabo, B. J., Haynes, C. V. and Maxwell, T. A. (1995) Ages of Quaternary pluvial episodes determined by uranium-series and radiocarbon dating of lacustrine deposits of Eastern Sahara, Palaeogeography, Palaeoclimatology, Palaeoecology 113, 227-242.
  • White, K., McLaren, S., Black, S. and Parker, A. (2000) Evaporite minerals and organic horizons in sedimentary sequences in the Libyan Fezzan: implactions for palaeoenvironmental reconstruction, in S. J. McLaren and D. R. Kniveton (eds.), Linking Climate Change to Land Surface Change, 193-208.
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