SEASONALITY DURING THE ~130 KA PLUVIAL EVENT IN THE WESTERN DESERT OF EGYPT, AS INFERRED FROM STABLE ISOTOPE AND MINOR ELEMENT PROFILES OF THE FRESHWATER GASTROPOD, MELANOIDES TUBERCULATA

Stable isotope and minor element analyses of authigenic carbonate silts and gastropods from a small (~1.5 km²) paleolake in Kharga Oasis, Egypt, indicate the prolonged presence of perennial, isotopically light surface water during the ~130 ka pluvial event in the Western Desert. Stable isotope analyses of the low-Mg calcite silts imply precipitation in equilibrium with water isotopically equivalent to ‘fossil’ groundwater at Kharga Oasis (~-11‰_SMOW), and with a dissolved inorganic carbon reservoir exhibiting little influence of respired carbon (average: -0.5‰_PDB). Mg/Ca and Sr/Ca profiles do not follow the d¹⁸O profile for the silts, suggesting that the lake remained relatively fresh and unaffected by substantial evaporation. Sequential intrashell stable isotope analyses of the freshwater gastropod Melanoides tuberculata from the silts and nearby tufa deposits generally have less than 1‰ variability in d¹⁸O, implying perennial, isotopically stable water sources. The isotopic and elemental variability in the shells can be attributed to either the interplay of seasonal temperature changes and modest evaporation, or a steady supply of isotopically light water in hydrologically open environments. As the carbonate-precipitating environments do not exhibit the extreme seasonal evaporation seen in closed basin lakes in modern monsoonal climates, a non-seasonal water source (such as the input of a spring/groundwater), or a significantly less seasonal climate then that exclusively controlled by the Atlantic Monsoon can explain their apparent isotopic steady states. With a total silt thickness exceeding five meters, the lake at Wadi Midauwara was likely stable for several millennia. As it appears to have been relatively insensitive to seasonal changes in local water balance, the lake would have been a year-round water resource for hominids occupying the region at ~130 ka, as is suggested by the presence of Middle Stone Age artifacts in the vicinity of the lacustrine deposits. Reliable water resources would have facilitated migration out of sub-Saharan Africa to the Mediterranean and the Levant.