Geoarchaeology, stable -isotope geochemistry, and geochronology of fossil -spring tufas, Western Desert, Egypt
The study of past climates has implications for Homo sapiens not only as a context for the climatic changes we are experiencing now, but also as a framework for hominid evolution. In both contexts, the Sahara is an intriguing and critical region. Fossil-spring tufas with associated artifacts in Kharga Oasis, Egypt, are particularly suitable for developing a climatic and cultural history for the currently hyperarid core of the Sahara. I carried out a geologic and archaeological survey of four tufa localities to establish a relative chronologic framework for these deposits as context for uranium-series dating, and to reconstruct the landscapes of prehistoric occupation in tufa-depositing regions. Most of the tufas were deposited as part of a fluvial barrage system, characterized by terraced, vegetated pools, which were impounded by arcuate tufa dams and separated by small waterfalls. Lithic artifacts associated with the tufas indicate repeated occupation during times when increased precipitation caused spring flow at these localities.
Stable-isotope analyses of tufas and freshwater gastropods from Kharga Oasis indicate that this region received enough precipitation to support perennial lakes during the height of the oxygen isotope stage 6/5e pluvial event. However, conditions were variable over the course of each pluvial event, signified by lithologic changes and by significant oxygen-isotope variability within individual tufa stratigraphic units.
A compilation of Pleistocene dates on pluvial conditions in the Sahara reveals a number of time periods which are represented by humid conditions at multiple localities in North Africa. The timing of these events corresponds well to the peaks of a monsoon index calculated from orbital parameters. The relative magnitude of the Saharan pluvial events also matches what would be predicted from the peak values of monsoon index. As the northward expansion of monsoonal rainfall would have left dry tropics in its wake, the Saharan pluvial events not only allowed for hominid migration into North Africa, but also encouraged such migration. Based on monsoon-index values, the 210 ka pluvial event would have represented the largest “push” north for hominid populations.