HYDROCLIMATIC CHANGE IN THE EASTERN SAHARA: THE LITHOSTRATIGRAPHIC AND GEOCHRONOLOGIC RECORD FROM UMM AKHTAR PLAYA, SOUTHERN EGYPT

Integrated lithostratigraphic and geochronologic studies of Umm Akhtar Playa, a dry lake located at N 22 deg 36’ E 30 deg 18’ in southern Egypt, indicate the accumulation and rapid desiccation of a sizeable (>65 sq km) standing water body during the Late Quaternary. More than twenty radiocarbon dates on hearths and incorporated organic materials bracket ‘wet’ phases from ~8915–8580 14C yr BP and ~7105–5955 14C yr BP. During these intervals, rapid incursions of sediment-laden flows discharged into the basin after seasonal or periodic floods, depositing lithic gravels (Qal) and massive muds (Qp) which are locally interstratified with ribbons of aeolian sand (Qd) along the paleoshore. The ponded water was formerly deep and persistent enough to create a beach berm of rolled pebbles, and to sustain cultural activities. Artifacts of the Early Neolithic-Neolithic tradition, including ostrich eggshell vessels, beads, lithics, and grinding tools, provide a context for reconstructing human occupation of the site through the close of the 6th millennium BP. Cultural abandonment, as well as increasing amounts of sand up-section, evaporite precipitation, and the formation of large (5 m) polygonal cracks, marked the final desiccation of the playa; optical dates on sands constrain the timing of rapid sedimentological transitions forced by hydroclimatic change. Comparison of the AMS dates from Umm Akhtar Playa to a compilation of 536 published radiocarbon dates from Egypt and northern Sudan corroborate a period of enhanced surface water storage from 8100-6000 BP. This ‘wet’ phase appears to lag the Northern Hemisphere seasonal insolation maximum centered at 10,400 BP, and the greatest frequency of African lake highstands (9500-8500 BP).