STABLE-ISOTOPE CONSTRAINTS ON PALEOENVIRONMENTS OF HOMINID OCCUPATION, WESTERN DESERT, EGYPT

The fossil-spring deposits (tufas) of the Western Desert of Egypt record times of increased precipitation in what is now a hyperarid region. The lithic artifacts associated with these tufas indicate human exploitation of the water resources provided by active spring runoff. To constrain climatic conditions during humid (pluvial) phases, we performed stable-oxygen and carbon-isotope analyses on tufa samples (dated by U-series methods) from Dakhleh and Kharga Oases, as well as on gastropods (Melanoides tuberculata) from lacustrine silts in Kharga. The amplitude of oxygen-isotope variability within individual stratigraphic units suggests that tufa-depositing waters were subjected to substantial evaporation upon issuing from their spring sources. Relatively high d¹³C values indicate little input of light organic carbon to tufa-depositing waters. This suggests that, despite increased precipitation during pluvial phases, the aquifer-recharge area (the Libyan Plateau) was not covered by a thick soil. However, abundant plant-stem and leaf casts within the tufas indicate significant plant growth in the areas surrounding the springs. The small variation in oxygen-isotope values within individual gastropod shells (>98% aragonite) from lacustrine carbonate deposits indicates that lake water was relatively unaffected by seasonal variation in rainfall or evaporation, despite the basin’s relatively small size (surface area <3 square km). Despite probable seasonality of rainfall, aquifer recharge during pluvial events would have been great enough to support year-round spring flow. The mineralogy of the lacustrine deposits and of the gastropod fauna recovered from the lacustrine silts suggest that the lake water was fresh to slightly saline. The Khargan lake would have represented a stable, potable water source for occupants of the region during the ~130 ka pluvial event. Despite a lack of evidence for significant tufa deposition after 100 ka, Epipalaeolithic and Neolithic artifacts associated with small playa basins record increased precipitation and coincident occupation of the region during the early and middle Holocene. The Holocene pluvial event was less intense than the events at 130 ka or 215 ka. This observation is consistent with a weaker African monsoon during the Holocene.