Reconstructing Quaternary pluvial conditions in the Egyptian Western Desert through analyses of fossil-spring tufas
by
Jennifer Smith
Dept. of Earth and Environmental Science, University of Pennsylvania
Coauthors: Robert Giegengack (University of Pennsylvania), Henry Schwarcz (McMaster University), Alicia L. Hawkins (Hebrew University), Maxine R. Kleindienst (University of Toronto), Mary M. A. McDonald (University of Calgary)

Fossil-spring carbonate rocks, or tufas, that line the edge of the Libyan Plateau in the central Western Desert of Egypt record times in the past when precipitation was greater than at present; the modern hyper-arid climate cannot sustain substantial spring flow at significant elevations along the Libyan Escarpment. Stratigraphic studies of tufas in Dakhleh and Kharga (undertaken by members of the Dakhleh Oasis Project since 1992) have shown that only systematic examination of multiple areas of tufa deposition can provide a regionally relevant and reasonably comprehensive record of humid climatic phases, as the locations of spring activity have shifted along the escarpment with time. Furthermore, while the most areally extensive valley-filling tufa units in each area record major, long-term climatic events, the perched-springline or "slope veneer" tufas draped along the escarpments may provide a relatively high-resolution record of smaller scale climatic events.

To constrain pluvial climatic conditions, we performed stable-oxygen and carbon-isotope analyses on tufa samples from Dakhleh and Kharga Oases, as well as on gastropods (Melanoides tuberculata) from lacustrine silts at Wadi Midauwara. There was no significant isotopic distinction between tufas from different areas or different stratigraphic units, suggesting that the source of pluvial precipitation was the same across the region for all events that resulted in tufa formation. The relatively depleted oxygen-isotope values of the tufas are consistent with rainfall being brought by the westerly circulation pattern to which previous workers attributed significant Pleistocene rainfall events in the Eastern Sahara. The amount of oxygen-isotope variability within individual stratigraphic units (~4 ‰) suggests that tufa-depositing waters were subjected to substantial evaporation upon issuing from their spring sources. Conversely, the small variation (~1 ‰) in oxygen-isotope values within individual gastropod shells (>98% aragonite) from lacustrine carbonate deposits indicates that lake waters were relatively unaffected by seasonal variation in rainfall or evaporation.

Lithic artefacts indicate a long human presence in Kharga Oasis. Some aggregates are found in direct association with tufas, indicating deposition coincident with times of active spring discharge. Earlier Stone Age (ESA) implements tend to be associated with breccias, gravels, or basinal silts interbedded with the tufas, though ESA implements can also be found as lags on tufa surfaces. However, Middle Stone Age artefacts were found directly associated with tufas (i.e. overlain by or encased within tufas) at 7 different archaeological localities along the edge of the Libyan Plateau. However, neither we nor Caton-Thompson and Gardner have discovered late Middle Stone Age Aterian technocomplex material associated with tufa. Despite abundant evidence of Holocene occupations, we have not found any Holocene tufas; Holocene artefacts are generally associated with playa silts deposited in depressions on tufa surfaces. The absence of Holocene tufas suggests that the conditions of the Holocene and latest Pleistocene pluvial episodes in Kharga were significantly different from conditions during Pleistocene pluvial events.

Date received: April 30, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahi-65.