Kharga Oasis, Egypt

Many lines of evidence, such as lake deposits, spring carbonates, and integrated drainage networks, indicate that the currently hyper-arid Western Desert of Egypt has experienced periods of markedly less arid climate (e.g. McCauley et al, 1982; McKenzie, 1993; Smith et al., 2004).  However, due to the arid conditions that have occurred between pluvial phases, few continuous records are present, which can give clues as to climatic variability within a pluvial phase either on the seasonal or millennial-scale. 

Groundwater discharge along the Libyan Plateau escarpment during several pluvial phases is recorded by extensive tufa deposits at Wadi Midauwara (Smith et al., 2004a,b).  These deposits provide an excellent opportunity to place constraints upon the timing and duration of pluvial phases.  In particular, the deposits of Wadi Midauwara are of interest, as they record at least 3 episodes of groundwater discharge.

In addition to the tufa deposits (right), thick sections of authigenic low-Mg calcite silts (below) at Midauwara record the prolonged presence of standing carbonate-rich water.  The authigenic calcite silts (above) can reach up to 7.8 meters thick, and probably record several millennia of standing water at Wadi Midauwara.  Thus, stable isotope and minor element analyses of the silts can provide insight into the long-term hydrologic and climatic dynamics of the lake at Wadi Midauwara.  The δ¹⁸O values for the silts suggest precipitation in equilibrium with water considerably heavier than Nubian aquifer water.  This, when coupled with the minor element analyses (Mg/Ca, Sr/Ca, Ba/Ca) has interesting implications for the climate of the Western Desert of Egypt at this time, and the influences of the Atlantic and Indian Ocean monsoons.

The  freshwater gastropod, Melanoides tuberculata (below) is found in abundance in the silts, and  can be used as a proxy for seasonality of water chemistry, as it grows its shell in approximate oxygen isotopic equilibrium with the ambient water.  By sampling incrementally along the whorl of the shell, one can get an idea of how the isotopic and minor elemental chemistry of the water changes throughout the year (e.g. Leng et al., 2000; Rosenthal and Katz, 1989).  Our results indicate that the lake at Wadi Midauwara was most likely perennially fresh, apparently buffered from the intense evaporation expected for a monsoonal climate. 

A critical component of this research includes considering the interactions between local hydrology and climate.  For example: The tufas and silts likely precipitated from water derived from local limestone units, which act as a perched aquifer, separated from the Nubian aquifer by relatively impermeable shale.  Thus, the question arises: Was the water in the limestone aquifers derived from meteoric precipitation, or did it travel up faults from the Nubian aquifer?  How could we tell?  Additionally, I consider how the relative influences of Atlantic-derived rainfall and Indian Ocean-derived rainfall would affect the isotopic and elemental records of the silts.  (There's a story here, can you tell?)

This research is also of interest to the archaeological community, as Late Early Stone Age and Middle Stone Age artifacts have been found in association with, and occasionally in situ in the spring carbonates at Wadi Midauwara, and other localities along the Libyan Plateau escarpment in Kharga Oasis, attesting to the past habitability of these environments.  Thus, our palaeoclimatic data can improve an understanding of the environment in which these early people lived...

References/Further Reading:

Image Credit for location map: Descloitres et al., 2000, NASA Visible Earth

Other images taken by Johanna Kieniewicz, 2004/2005 field season.

The Washington University Record Reports on our work!

Kieniewicz, J.M., and Smith, J.R., 2004, Seasonality during the ~130 ka pluvial event in the Western Desert of Egypt as derived from stable isotope and minor element analyses of the freshwater gastropod Melanoides tuberculata: Geological Society of America Abstracts with Program.  Read the Abstract here!

Kieniewicz, J.M., and Smith, J.R., 2005, Hydrology of a small Pleistocene pluvial lake in the Western Desert of Egypt, Eos Transactions 86(52).  Read the Abstract here!

McCauley, J.F., Schaber, G.G., Breed, C.S., Grolier, M.J., Haynes, C.V., Issawi, B., Elachi, C., and Blom, R., 1982, Subsurface valleys and geoarcheology of the Eastern Sahara revealed by Shuttle radar: Science, v. 218, p. 1004-1020.

McKenzie, J.A., 1993, Pluvial conditions in the eastern Sahara following the penultimate deglaciation; implications for changes in atmospheric circulation patterns with global warming: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 103, p. 95-105.

Leng, M.J., Lamb, A.L., Lamb, H.F., and Telford, R.J., 1999, Palaeoclimatic implications of isotopic data from modern and early Holocene shells of the freshwater snail Melanoides tuberculata from lakes in the Ethiopian Rift valley: Journal of Paleolimnology, v. 21, p. 97-106.

Rosenthal, Y., and Katz, A., 1989, The applicability of trace elements in freshwater shells for paleogeochemical studies: Chemical Geology, v. 78, p. 65-76.

Smith, J.R., Geigengack, R., and Schwarcz, H.P., 2004, Constraints on Pleistocene Pluvial Climates through Stable-Isotope Analysis of Fossil-Spring Tufas and Associated Gastropods, Kharga Oasis, Egypt: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 206, p. 157-175.

Smith, J.R., Geigengack, R., Schwarcz, H.P., McDonald, M.M.A., Kleindienst, M.R., Hawkins, A.L., and Churcher, C.S., 2004, Reconstructing pluvial environments and human occupation through study of the stratigraphy and geochronology of fossil-spring tufas, Kharga Oasis, Egypt: Geoarchaeology: An International Journal, v. 19, p. 407-439.