During the transition from the penultimate glacial to the last interglacial period (Isotope Stages 6-5e or Terminal II), large freshwater lakes abruptly infilled sandy depressions in arid regions of the eastern Sahara. Geochemical and mineralogical studies of samples from lacustrine sequences in the Western Desert of southern Egypt indicate: (1) The sediments are chalk composed of authigenic chemical precipitates, predominantly low-Mg calcite, with carbonate contents averaging about 80%. The low dilution of the chemical precipitate by detrital input implies that physical erosion and transport of detrital grains were minimized by extensive vegetation cover in the catchment area. The fauna and flora associated with the lake deposits and Middle Paleolithic occupation sites on the paleo-shorelines attest to a more amenable environment. (2) Vastly different rainfall or atmospheric circulation patterns than at present must have existed based on consistently negative deltaO-18PDB values for the chalk, as low as -9.5 parts per thousand, which would require precipitation from lake waters significantly more depleted in O-18 than the modern local spring water (deltaO-18SMOW= -1.8 parts per thousand). (3) The lakes were stable perennial water bodies with relatively long residence times sufficient to produce isotopic covariant signals in the chalks. Apparently, the lakes disappeared as abruptly as they appeared without going through a terminal phase of evaporative enrichment. Extremely arid conditions again prevailed. During the transition from the last glacial to the present interglacial (Terminal 1), a similar change from arid to pluvial conditions occurred. Thus, it is proposed that, during transitions from glacial to interglacial climates, rapid global warming can lead to a temporary alteration of atmospheric circulation with an intensification of the southwest monsoons, bringing large quantities of isotopically light moisture to the Eastern Sahara.
