Palaeoenvironmental reconstruction in the Sahara Desert
The Sahara Desert is the largest desert in the world occupying about one third of the African continent. It plays an important part in the global climate system, with persistent high pressure over Libya which keeps the core desert hyper-arid (just a few mm/yr, with rainstorms occurring only once every ten or twenty years). As a result, the Sahara is a major source of atmospheric dust.
But it has not always been thus.
I work with a number of archaeological missions and environmental research projects, seeking evidence of these past, more humid, climate phases, and their impact on the Saharan landscape and people
As a geographer, my job is to survey the landscape. Given the vast areas involved, I use image processing and Geographical Information System (GIS) software to process satellite imagery and digital elevation data, in order to identify candidate sites for detailed field survey, specifically ancient rivers and lakes which may contain buried environmental evidence in the form of pollen, chemical precipitates, or datable material.
Coring a contemporary Saharan lake
© Toby Savage (www.tobysavage.co.uk)
The synoptic overview provided by optical and radar remote sensing is particularly useful to overcome the limitations of fieldwork in this vast, inaccessible area. The two images above show the same area, the colour image is from the Landsat Thematic Mapper, which shows only the subdued dune topography. The bottom image is a Synthetic Aperture Radar (SAR) image from RADARSAT, which picks out the bright reflections from indurated ancient lake sediment.
To understand these SAR images better, we are using the state-of-the-art Ground Penetrating Radar (GPR) instruments of UltraGPR (http://groundradar.com/) to examine how radar pulses interact with buried palaeolake sediments. The GPR transect below runs across a palaeolake depression, showing how the palaeolake surface can be easily traced beneath the cross-bedded dune sands on either side.
UltrGPR transect across 'Turquoise' palaeolake basin flanked by dune sands. © Jan Franke
UltraGPR deployed on dune transect. © Toby Savage (www.tobysavage.co.uk)
'Turquoise' palaeolake basin. Note residual 'mesas' in the background
Five limestone/sand cycles are found throughout the Fazzan basin, but only the top cycle is young enough (420 ka) to fall within the range of OSL dating. © Dr. Nick Drake, KCL
Reconstruction of Lake PalaeoFezzan, superimposed on 3D map, viewed from northwest.
Probability density function of absolute ages for palaeolake sediments, evaporites and organic deposits in the Libyan Fezzan. © Dr. Nick Drake, KCL.
For more information, see the following references:
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.I. and White, K. 1998 The Fezzan Project 1998: preliminary report on the second season of work. Libyan Studies, 29, pp. 115-144.
Mattingly, D.J., al-Mashai, M., Balcombe, P., Drake, N.A., Knight, S., MacLaren, S., Pelling, R., Reynolds, T., Thomas, D., Wilson, A.I. and White, K. 1999 The Fezzan Project 1999: preliminary report on the third season of work. Libyan Studies, 30, pp. 129-145.
White, K., McLaren, S.J., Black, S. and Parker, A. 2000 Evaporite minerals and organic horizons in sedimentary sequences in the Libyan Fezzan: Implications for Palaeoenvironmental reconstruction. In McLaren, S.J. and Kniveton, D.R. (Eds.) Linking Climate Change to Land Surface Change, Amsterdam: Kluwer, pp. 193-208.
Mattingly, D.J., Al-Mashai, M., Balcombe, P., Barnett, T., Brooks, N.P.J., Cole, F., Dore, J., Drake, N.A., Edwards, D., Hawthorne, J., Helm, R., Leone, A., McLaren, S.J., Pelling, R., Preston, J., Reynolds, T., Townsend, A., Wilson, A.I. and White, K. 2000 The Fezzan Project 2000: Preliminary report on the fourth season of work. Libyan Studies, 31, pp. 103-120.
Brooks, N.P.J., White, K., Warr, B., Drake, N.A., McLaren, S.J. 2000 Remote Sensing for Dryland Geoarchaeological Investigations in Southern Libya: Preliminary Findings from the Fezzan Project. Adding Value to Remotely Sensed Data, Proceedings of the 26th Annual Conference of the Remote Sensing Society, 12-14 September 2000, University of Leicester, 17pp., ISBN 0 946226 30 X
Mattingly, D.J., Brooks, N., Cole, F., Dore, J., Drake, N., Leone, A., Hay, S., McLaren, S., Newson, P., Parton, H., Pelling, R., Preston, J., Reynolds, T., Schrüfer-Kolb, I., Thomas, D., Tindall, A., Townsend, A. and White, K. 2001 The Fezzan Project 2001: Preliminary report on the fifth season of work. Libyan Studies, 32, pp. 133-153.
Brooks, N.P.J., White, K., Drake, N. and MacLaren, S. 2001 Changes in vegetative cover in the Wadi al-Ajal, southern Libya, over the past half-century. Geomatics, Earth Observation and the Information Society, Proceedings of the First Annual Conference of the Remote Sensing & Photogrammetry Society, 12-14 September 2001, University of London, pp. 247‑259, ISBN 0 946226 31 8
White, K., Brooks, N.P.J., Drake, N.A. and McLaren, S.J. 2003 Studies in Geography, Geomorphology, Environment and Climate. Chapter 2 in Mattingly, D.J. (Ed.) The Archaeology of Fazzan, Volume 1, Synthesis. London: The Society for Libyan Studies/Tripoli: The Department of Antiquities, pp. 37-74.
White, K., Brooks, N.P.J., Drake, N.A., Charlton, M. and MacLaren, S.J. 2003 Monitoring vegetation change in desert oases by remote sensing; a case study in the Libyan Fazzān. Libyan Studies, 34, pp. 153-166.
Drake, N.A., Wilson, A., Pelling, R., White, K., Mattingly, D.J. and Black, S. 2004 Water table decline, springline dessication and the early development of irrigated agriculture in the Wadi al-Ajal, Libyan Fazzan. Libyan Studies, 35, pp. 95-112.
White, K. and Mattingly, D.J. 2006 Ancient lakes of the Sahara. American Scientist, 94, pp. 58-65.
White, K., Charlton, M., Drake, N., McLaren, S., Mattingly, D. and Brooks, N. 2006 Lakes of the Edeyen Ubari and the Wadi al-Hayat. In Mattingly, D., McLaren, S., Savage, E., al-Fasatwi, Y. and Gadgood, K. (Eds.) The Libyan Desert, Natural Resources and Cultural Heritage, London: The Society for Libyan Studies, pp. 123-130.
Drake, N., White, K. and McLaren, S. 2006 Quaternary climate change in the Jarma region of Fazzan, Libya. In Mattingly, D., McLaren, S., Savage, E., al-Fasatwi, Y. and Gadgood, K. (Eds.) The Libyan Desert, Natural Resources and Cultural Heritage, London: The Society for Libyan Studies, pp. 133-144.
McLaren, S., Drake, N. and White, K. 2006 Late Quaternary environmental change in the Fazzan, Southern Libya: evidence from sediments and duricrusts. In Mattingly, D., McLaren, S., Savage, E., al-Fasatwi, Y. and Gadgood, K. (Eds.) The Libyan Desert, Natural Resources and Cultural Heritage, London: The Society for Libyan Studies, pp. 157-166.
Charlton, M.B. and White, K. 2006 Sensitivity of radar backscatter to desert surface roughness. International Journal of Remote Sensing, 27, pp. 1641-1659.
Armitage, S.J., Drake, N.A., Stokes, S., El-Hawat, A., Salem, M.J., White, K., Turner, P. and McLaren, S.J. 2007 Multiple phases of North African humidity recorded in lacustrine sediments from the Fazzan Basin, Libyan Sahara. Quaternary Geochronology, 2, pp. 181-186.
Mattingly, D., Lahr, M., Armitage, S., Barton, H., Dore, J., Drake, N.A., Foley, R., Merlo, S., Salem, M., Stock, J. and White, K. 2007, Desert migrations: people, environment and culture in the Libyan Sahara. Libyan Studies, 38, pp. 115-156.
Drake, N.A., El-Hawat, A.S., Turner, P., Armitage, S.J., Salem, M.J., White, K. and McLaren, S. 2008 Palaeohydrology of the Fazzan Basin and surrounding regions: the last 7 million years. Palaeogeography Palaeoclimatology Palaeoecology, 263, pp. 131-145, doi: 10.1016/j.palaeo.2008.02.005
Parker, A.G., Harris, B., White, K. and Drake, N.A. 2008 Phytoliths as indicators of grassland dynamics during the Holocene from lake sediments in the Ubari sand sea, Fazzan Basin, Libya. Libyan Studies, 39, pp. 29-40.
Drake, N.A., Blench, R.M., Armitage, S.J., Bristow, C.S. and White, K. 2011 Ancient watercourses and biogeography of the Sahara explain the peopling of the Desert. P.N.A.S., 108, pp. 458-462. doi: 10.1073/pnas.1012231108